There are moments in the history of all sciences when remarkable progress is made in relatively short periods of time. Such leaps in knowledge result in great part from two factors: one is the presence of a creative mind—a mind sufficiently perceptive and original to discard hitherto accepted ideas and formulate new hypotheses; the second is the technological ability to test the hypotheses by appropriate experiments. The most original and inquiring mind is severely limited without the proper tools to conduct an investigation; conversely, the most-sophisticated technological equipment cannot of itself yield insights into any scientific process.

An example of the relationship between those two factors was the discovery of the cell. For hundreds of years there had been speculation concerning the basic structure of both plants and animals. Not until optical instruments were sufficiently developed to reveal cells, however, was it possible to formulate a general hypothesis, the cell theory, that satisfactorily explained how plants and animals are organized. Similarly, the significance of Gregor Mendel’s studies on the mode of inheritance in the garden pea remained neglected for many years until technological advances made possible the discovery of the chromosomes and the part they play in cell division and heredity. Moreover, as a result of the relatively recent development of extremely sophisticated instruments, such as the electron microscope, the ultracentrifuge, and automated DNA sequencing machines, biology has moved from being a largely descriptive science—one concerned with entire cells and organisms—to a discipline that increasingly emphasizes the subcellular and molecular aspects of organisms and attempts to equate structure with function at all levels of biological organization.

Although it is not known when the study of biology originated, early humans must have had some knowledge of the animals and plants around them. Human survival depended upon the accurate recognition of nonpoisonous food plants and upon an understanding of the habits of dangerous predators. Archaeological records indicate that even before the development of civilization, humans had domesticated virtually all the amenable animals available to them and had developed an agricultural system sufficiently stable and efficient to satisfy the needs of large numbers of people living together in communities. It is clear, therefore, that much of the history of biology predates the time at which humankind began to write and to keep records.

Much of the earliest recorded history of biology is derived from Assyrian and Babylonian bas-reliefs showing cultivated plants and from carvings depicting veterinary medicine. Illustrations on certain seals reveal that the Babylonians had learned that the date palm reproduces sexually and that pollen could be taken from the male plant and used to fertilize female plants. Although a precise dating of those early records is lacking, a Babylonian business contract of the Hammurabi period (c. 1800 BCE) mentions the male flower of the date palm as an article of commerce, and descriptions of date harvesting extend back to about 3500 BCE.

The Babylonians knew of the separate sexes of the date palm (Phoenix dactylifera) by the time of the Hammurabi period (c. 1792–1750 BCE). The species was one of the first plants in which sexual reproduction was recorded.

Another source of information concerning the extent of biological knowledge of these early peoples was the discovery of several papyri that pertain to medical subjects; one, believed to date to 1600 BCE, contains anatomical descriptions; another (c. 1500 BCE) indicates that the importance of the heart had been recognized. Because those ancient documents, which contained mixtures of fact and superstition, probably summarized then-current knowledge, it may be assumed that some of their contents had been known by earlier generations.

Papyri and artifacts found in tombs and pyramids indicate that the Egyptians also possessed considerable medical knowledge. Their well-preserved mummies demonstrate that they had a thorough understanding of the preservative properties of herbs required for embalming; plant necklaces and bas-reliefs from various sources also reveal that the ancient Egyptians were well aware of the medicinal value of certain plants. An Egyptian compilation known as the Ebers papyrus (c. 1550 BCE) is one of the oldest known medical texts.

Ebers papyrus prescription for asthma treatment.

In ancient China, three mythical emperors—Fu Xi, Shennong, and Huangdi—whose supposed ruling periods extended from the 29th to the 27th century BCE, were said to possess medical knowledge. According to legend, Shennong described the therapeutic powers of numerous medicinal plants and included descriptions of many important food plants, such as the soybean. The earliest known written record of medicine in China, however, is the Huangdi neijing (The Yellow Emperor’s Classic of Internal Medicine), which dates to the 3rd century BCE. In addition to medicine, the ancient Chinese possessed knowledge of other areas of biology. For example, they not only used the silkworm Bombyx mori to produce silk for commerce but also understood the principle of biological control, employing one type of insect, an entomophagous (insect-eating) ant, to destroy insects that bored into trees.

As early as 2500 BCE the people of northwestern India had a well-developed science of agriculture. The ruins at Mohenjo-daro have yielded seeds of wheat and barley that were cultivated at that time. Millet, dates, melons, and other fruits and vegetables, as well as cotton, were known to the civilization. Plants were not only a source of food, however. A document, believed to date to the 6th century BCE, described the use of about 960 medicinal plants and included information on topics such as anatomy, physiology, pathology, and obstetrics.

Although the Babylonians, Assyrians, Egyptians, Chinese, and Indians amassed much biological information, they lived in a world believed to be dominated by unpredictable demons and spirits. Hence, learned individuals in those early cultures directed their studies toward an understanding of the supernatural, rather than the natural, world. Anatomists, for example, dissected animals not to gain an understanding of their structure but to study their organs in order to predict the future. With the emergence of the Greek civilization, however, those mystical attitudes began to change. Around 600 BCE there arose a school of Greek philosophers who believed that every event has a cause and that a particular cause produces a particular effect. That concept, known as causality, had a profound effect on subsequent scientific investigation. Furthermore, those philosophers assumed the existence of a “natural law” that governs the universe and can be comprehended by humans through the use of their powers of observation and deduction. Although they established the science of biology, the greatest contribution the Greeks made to science was the idea of rational thought.

One of the earliest Greek philosophers, Thales of Miletus (c. 7th century BCE), maintained that the universe contained a creative force that he called physis, an early progenitor of the term physics; he also postulated that the world and all living things in it were made from water. Anaximander, a student of Thales, did not accept water as the only substance from which living things were derived; he believed that in addition to water, living things consisted of earth and a gaslike substance called apeiron, which could be divided into hot and cold. Various mixtures of those materials gave rise to the four elements: earth, air, fire, and water. Although he was one of the first to describe Earth as a sphere rather than as a flat plane, Anaximander proposed that life arose spontaneously in mud and that the first animals to emerge had been fishes covered with a spiny skin. The descendants of those fishes eventually left water and moved to dry land, where they gave rise to other animals by transmutation (the conversion of one form into another). Thus, an early evolutionary theory was formulated.

At Crotone in southern Italy, where an important school of natural philosophy was established by Pythagoras about 500 BCE, one of his students, Alcmaeon, investigated animal structure and described the difference between arteries and veins, discovered the optic nerve, and recognized the brain as the seat of the intellect. As a result of his studies of the development of the embryo, Alcmaeon may be considered the founder of embryology.

Although the Greek physician Hippocrates, who established a school of medicine on the Aegean island of Cos around 400 BCE, was not an investigator in the sense of Alcmaeon, he did recognize through observations of patients the complex interrelationships involved in the human body. He also contemplated the influence of environment on human nature and believed that sharply contrasting climates tended to produce a powerful type of inhabitant, whereas even, temperate climates were more conducive to indolence.

Hippocrates, undated bust.

Hippocrates and his predecessors were concerned with the central philosophical question of how the cosmos and its inhabitants were created. Although they accepted the physis as the creative force, they differed with regard to the importance of the roles played by earth, air, fire, water, and other elements. Although Anaximenes, for example, who may have been a student of Anaximander, adhered to the then-popular precept that life originated in a mass of mud, he postulated that the actual creative force was to be found in the air and that it was influenced by the heat of the Sun. Members of the Hippocratic school also believed that all living bodies were made up of four humours—blood, black bile, phlegm, and yellow bile—which supposedly originated in the heart, the spleen, the brain, and the liver, respectively. An imbalance of the humours was thought to cause an individual to be sanguine, melancholy, phlegmatic, or choleric. These words persisted in the medical literature for centuries, a testament to the lengthy popularity of the idea of humoral influences. For centuries it was also believed that an imbalance in the humours was the cause of disease, a belief that resulted in the common practice of bloodletting to rid the body of excessive humours.

Around the middle of the 4th century BCE, ancient Greek science reached a climax with Aristotle, who was interested in all branches of knowledge, including biology. Using his observations and theories, Aristotle was the first to attempt a system of animal classification, in which he contrasted animals containing blood with those that were bloodless. The animals with blood included those now grouped as mammals (except the whales, which he placed in a separate group), birds, amphibians, reptiles, and fishes. The bloodless animals were divided into the cephalopods, the higher crustaceans, the insects, and the testaceans, the last group being a collection of all the lower animals. His careful examination of animals led to the understanding that mammals have lungs, breathe air, are warm-blooded, and suckle their young. Aristotle was the first to show an understanding of an overall systematic taxonomy and to recognize units of different degrees within the system.

The most-important part of Aristotle’s work was that devoted to reproduction and the related subjects of heredity and descent. He identified four means of reproduction, including the abiogenetic origin of life from nonliving mud, a belief held by Greeks of that time. Other modes of reproduction recognized by him included budding (asexual reproduction), sexual reproduction without copulation, and sexual reproduction with copulation. Aristotle described sperm and ova and believed that the menstrual blood of viviparous organisms (those that give birth to living young) was the actual generative substance.

Although Aristotle recognized that species are not stable and unalterable and although he attempted to classify the animals he observed, he was far from developing any pre-Darwinian ideas concerning evolution. In fact, he rejected any suggestion of natural selection and sought teleological explanations (i.e., all phenomena in nature are shaped by a purpose) for any given observation. Nevertheless, many important scientific principles, some of which are often thought of as 20th-century concepts, can be ascribed to Aristotle. The following are a few such: (1) Using birds as an example, he formulated the principle that all organisms are structurally and functionally adapted to their habits and habitats. (2) Nature is parsimonious; it does not expend unnecessary energy. (3) In classifying animals, Aristotle rejected the idea of dividing them solely by their external structures (e.g., animals with wings and those without wings). He recognized instead a basic unity of plan among diverse organisms, a principle that is still conceptually and scientifically sound. Further, Aristotle also believed that the entire living world could be described as a unified organization rather than as a collection of diverse groups. (4) By his observations, Aristotle realized the importance of structural homology, basically similar organs in different animals, and functional analogy, different structures that serve somewhat the same function—e.g., the hand, the claw, and the hoof are analogous structures. Those principles constitute the basis for the biological field of study known as comparative anatomy. (5) Aristotle’s observations also led to the formulation of the principle that general structures appear before specialized ones and that tissues differentiate before organs.

Theophrastus, statue in the Botanic Garden, Palermo, Italy.

Of all the works of Aristotle that have survived, none deals with what was later differentiated as botany, although it is believed that he wrote at least two treatises on plants. Fortunately, however, the work of Theophrastus, one of Aristotle’s students, has been preserved to represent plant science of the Greek period. Like Aristotle, Theophrastus was a keen observer, although his works do not express the depth of original thought exemplified by his teacher. In his great work, De historia et causis plantarum ( The Calendar of Flora, 1761), in which the morphology, natural history, and therapeutic use of plants are described, Theophrastus distinguished between the external parts, which he called organs, and the internal parts, which he called tissues. This was an important achievement because Greek scientists of that period had no established scientific terminology for specific structures. For that reason, both Aristotle and Theophrastus were obliged to write very long descriptions of structures that can be described rapidly and simply today. Because of that difficulty, Theophrastus sought to develop a scientific nomenclature by giving special meaning to words that were then in more or less current use; for example, karpos for fruit and perikarpion for seed vessel.

Although he did not propose an overall classification system for plants, more than 500 of which are mentioned in his writings, Theophrastus did unite many species into what are now considered genera. In addition to writing the earliest detailed description of how to pollinate the date palm by hand and the first unambiguous account of sexual reproduction in flowering plants, he also recorded observations on seed germination and development.

Alexandria, Library of Illustration of the ancient Library of Alexandria, Egypt.

With Aristotle and Theophrastus, the great Greek period of scientific investigation came to an end. The most famous of the new centres of learning were the library and museum in Alexandria. From 300 BCE until around the time of Christ, all significant biological advances were made by physicians at Alexandria. One of the most outstanding of those individuals was Herophilus, who dissected human bodies and compared their structures with those of other large mammals. He recognized the brain, which he described in detail, as the centre of the nervous system and the seat of intelligence. On the basis of his knowledge, he wrote a general anatomical treatise, a special one on the eyes, and a handbook for midwives.

Erasistratus, a younger contemporary and reputed rival of Herophilus who also worked at the museum in Alexandria, studied the valves of the heart and the circulation of blood. Although he was wrong in supposing that blood flows from the veins into the arteries, he was correct in assuming that small interconnecting vessels exist. He thus suspected (but did not see) the presence of capillaries; he thought, however, that the blood changed into air, or pneuma, when it reached the arteries, to be pumped throughout the body.

Perhaps the last of the ancient biological scientists of note was Galen of Pergamum, a Greek physician who practiced in Rome during the middle of the 2nd century CE. His early years were spent as a surgeon at the gladiatorial arena, which gave him the opportunity to observe details of human anatomy. At that time in Rome, however, it was considered improper to dissect human bodies, and, as a result, a detailed study of human anatomy was not possible. Thus, though Galen’s research on animals was thorough, his knowledge of human anatomy was faulty. Because his work was extensive and clearly written, Galen’s writings, nevertheless, dominated medicine for centuries.

After Galen there were no significant biological investigations for many centuries. It is sometimes claimed that the rise of Christianity was the cause of the decline in science. However, while it is true that Christianity did not favour the questioning attitude of the Greeks, science had already receded significantly by the end of the 2nd century CE, a time when Christianity was still an obscure sect.

During the almost 1,000 years that science was dormant in Europe, the Arabs, who by the 9th century had extended their sphere of influence as far as Spain, became the custodians of science and dominated biology, as they did other disciplines. At the same time, as the result of a revival of learning in China, new technical inventions flowed from there to the West. The Chinese had discovered how to make paper and how to print from movable type, two achievements that were to have an inestimable effect upon learning. Another important advance that also occurred during that time was the introduction of the so-called Arabic numerals into Europe from India.

From the 3rd until the 11th century, biology was essentially an Arab science. Although the Arabic scholars themselves were not great innovators, they discovered the works of such men as Aristotle and Galen, translated those works into Arabic, studied them, and wrote commentaries about them. Of the Arab biologists, al-Jāḥiẓ, who died about 868, is particularly noteworthy. Among his biological writings is Kitāb al-ḥayawān (“Book of Animals”), which, although revealing some Greek influence, is primarily an Arabic work. In it the author emphasized the unity of nature and recognized relationships between different groups of organisms. Because al-Jāḥiẓ believed that earth contained both male and female elements, he found the Greek doctrine of spontaneous generation (life emerging from mud) to be quite reasonable.

Muslim physician Avicenna was an outstanding scientist who lived during the late 10th and early 11th centuries; he was the true successor to Aristotle. His writings on medicine and drugs, which were particularly authoritative and remained so until the Renaissance, did much to take the works of Aristotle back to Europe, where they were translated into Latin from Arabic.

During the 12th century the growth of biology was sporadic. Nevertheless, it was during that time that botany was developed from the study of plants with healing properties; similarly, from veterinary medicine and the pleasures of the hunt came zoology. Because of the interest in medicinal plants, herbs in general began to be described and illustrated in a realistic manner. Although Arabic science was well developed during the period and was far in advance of Latin, Byzantine, and Chinese cultures, it began to show signs of decline. Latin learning, on the other hand, rapidly increasing, was best exemplified perhaps by the mid-13th-century German scholar Albertus Magnus (Saint Albert the Great), who was probably the greatest naturalist of the Middle Ages. His biological writings (De vegetabilibus, seven books, and De animalibus, 26 books) were based on the classical Greek authorities, predominantly Aristotle. But in spite of that classical basis, a significant amount of his work contained new observations and facts; for example, he described with great accuracy the leaf anatomy and venation of the plants he studied.

Saint Albertus Magnus, who worked to meld theology and Aristotelianism and was probably the greatest naturalist of the Middle Ages.

Albertus was particularly interested in plant propagation and reproduction and discussed in some detail the sexuality of plants and animals. Like his Greek predecessors, he believed in spontaneous generation; he also believed that animals were more perfect than plants, because they required two individuals for the sexual act. Perhaps one of Albertus’s greatest contributions to medieval biology was the denial of many superstitions believed by his contemporaries, a skepticism that, together with the reintroduction of Aristotelian biology, was to have profound effects on subsequent European science.

One of Albertus’s pupils was Thomas Aquinas, who, like his mentor, endeavoured to reconcile Aristotelian philosophy and the teachings of the church. Because Aquinas was a rationalist, he declared that God created the reasoning mind; hence, by true intellectual processes of reasoning, man could not arrive at a conclusion that was in opposition to Christian thought. Acceptance of this philosophy made possible a revival of rational learning that was consistent with Christian belief.

Italy, during the Middle Ages, became the most-active scientific centre, although its major interests were concentrated on agriculture and medicine. A development of particular significance at that time was the introduction of dissection into medical schools, a step that revitalized the study of anatomy. Because of what it reveals about medieval anatomy in general, the work of Mondino de’ Luzzi, the most famous of the Italian anatomists at the beginning of the 14th century, is particularly important. It is thought that early in his career, contrary to the trend at the time, in which the teacher left the actual dissection to an underling, Mondino performed many dissections himself. Later, however, it is likely that he increasingly left the work to his assistants. Mondino adhered closely to the works of the Greeks and Arabs, and he thus repeated their errors.

Beginning in Italy during the 14th century, there was a general ferment within the culture itself, which, together with the rebirth of learning (partly as a result of the rediscovery of Greek work), is referred to as the Renaissance. Interestingly, it was the artists, rather than the professional anatomists, who were intent upon a true rendering of the bodies of animals, including humans, and thus were motivated to gain their knowledge firsthand by dissection. No individual better exemplifies the Renaissance than Leonardo da Vinci, whose anatomical studies of the human form during the late 1400s and early 1500s were so far in advance of the age that they included details not recognized until a century later. Furthermore, while dissecting animals and examining their structure, Leonardo compared them with the structure of humans. In doing so he was the first to indicate the homology between the arrangements of bones and joints in the leg of the human and that of the horse, despite the superficial differences. Homology was to become an important concept in uniting outwardly diverse groups of animals into distinct units, a factor that is of great significance in the study of evolution.

Other factors had a profound effect upon the course of biology in the 1500s, particularly the introduction of printing around the middle of the century, the increasing availability of paper, and the perfected art of the wood engraver, all of which meant that illustrations as well as letters could be transferred to paper. In addition, after the Turks conquered Byzantium in 1453, many Greek scholars took refuge in the West; the scholars of the West thus had direct access to the scientific works of antiquity rather than indirect access through Arabic translations.

Over the period 1530–40, German theologian and botanist Otto Brunfels published the two volumes of his Herbarum vivae eicones, a book about plants, which, with its fresh and vigorous illustrations, contrasted sharply with earlier texts, whose authors had been content merely to copy from old manuscripts. In addition to books on the same subject, Hieronymus Bock (Latinized to Tragus) and Leonhard Fuchs also published about the mid-1500s descriptive well-illustrated texts about common wild flowers. The books published by the three men, who are often referred to as the German fathers of botany, may be considered the forerunners of modern botanical floras (treatises on or lists of the plants of an area or period).

Throughout the 16th century, interest in botanical study also existed in other countries, including the Netherlands, Switzerland, Italy, and France. During that time there was a great improvement in the classification of plants, which had been described in ancient herbals merely as trees, shrubs, or plants and, in later books, were either listed alphabetically or arranged in some arbitrary grouping. The necessity for a systematic method to designate the increasing number of plants being described became obvious. Accordingly, using a binomial system very similar to modern biological nomenclature, the Swiss botanist Gaspard Bauhin designated plants by a generic and a specific name. Although affinities between plants were indicated by the use of common generic names, Bauhin did not speculate on their common kinship.

Pierre Belon, a French naturalist who traveled extensively in the Middle East, where he studied the flora, illustrates the wide interest of the 16th-century biologists. Although his botanical work was limited to two volumes, one on trees and one on horticulture, his books on travel included numerous biological entries. His two books on fishes reveal much about the state of systematics at the time, including that of not only fishes but also other aquatic creatures such as mammals, crustaceans, mollusks, and worms. In his L’Histoire de la nature des oyseaux (1555; “Natural History of Birds”), however, in which Belon’s taxonomy was remarkably similar to that used in the modern era, he showed a clear grasp of comparative anatomy, particularly of the skeleton, publishing the first picture of a bird skeleton beside a human skeleton to point out the homologies. Numerous other European naturalists who traveled extensively also brought back accounts of exotic animals and plants, and most of them wrote voluminous records of their excursions. Two other factors contributed significantly to the development of botany at the time: first was the establishment of botanical gardens by the universities, as distinct from the earlier gardens that had been established for medicinal plants; second was the collection of dried botanical specimens, or herbaria.

It is perhaps surprising that the great developments in botany during the 16th century had no parallel in zoology. Instead, there arose a group of biologists known as the Encyclopedists, best represented by Conrad Gesner, a 16th-century Swiss naturalist, who compiled books on animals that were illustrated by some of the finest artists of the day (Albrecht Dürer, for example). But because the descriptions of many of the animals were grossly inaccurate, in many cases continuing the legends of the Greeks, apart from their aesthetic value the books did little to advance zoological knowledge.

Vesalius, Andreas; anatomy Woodcut depicting Renaissance physician Andreas Vesalius teaching anatomy, from the title page of the first edition of De humani corporis fabrica libri septem (1543).

Like that of botany, the beginning of the modern scientific study of anatomy can be traced to a combination of humanistic learning, Renaissance art, and the craft of printing. Although Leonardo da Vinci initiated anatomical studies of human cadavers, his work was not known to his contemporaries. Rather, the appellation father of modern human anatomy generally is accorded to the Belgian anatomist Andreas Vesalius, who studied initially at the rather conservative schools in Leuven (Louvain) and Paris, where he became a successful teacher very familiar with Galen’s work. In 1537 he went to Padua, where he became noted for far-reaching teaching reforms. Most important, Vesalius abolished the practice of having someone else do the actual dissection; instead, he dissected his own cadavers and lectured to students from his findings. His text, De humani corporis fabrica libri septem (1543; “The Seven Books on the Structure of the Human Body”), was the most extensive and accurate work on the subject of anatomy at the time and, as such, constituted a foundation of great importance for biology. Perhaps Vesalius’s greatest contribution, however, was that he inspired a group of younger scientists to be critical and to accept a description only after they had verified it. Thus, as anatomists became more questioning and critical of the works of others, the errors of Galen were exposed. Of Vesalius’s successors, Michael Servetus, a Spanish theologian and physician, discovered the pulmonary circulation of the blood from the right chamber of the heart to the lungs and stated that the blood did not pass through the central septum (wall) of the heart, as had previously been believed.

Seventeenth-century advances in biology included the establishment of scientific societies for the dissemination of ideas and progress in the development of the microscope, through which scientists discovered a hitherto invisible world that had far-reaching effects on biology. Systematizing and classifying, however, dominated biology throughout much of the 17th and 18th centuries, and it was during that time that the importance of the comparative study of living organisms, including humans, was realized. During the 18th century the long-held idea that living organisms could originate from nonliving matter (spontaneous generation) began to crumble, but it was not until after the mid-19th century that it was finally disproved by the French chemist and microbiologist Louis Pasteur, who demonstrated the self-replicating ability of microorganisms.

Biological expeditions added to the growing body of knowledge of plant and animal forms and led to the 19th-century development of the theory of evolution. The 19th century was one of great progress in biology: in addition to the formulation of the theory of evolution, the cell theory was established, the foundations for modern embryology were laid, and the laws of heredity were discovered.

In the early 17th century, the English physician William Harvey, who studied at Padua with one of Vesalius’s students, became the first to describe the full circulation of the blood through the human body. Prior to Harvey, blood was supposed to be consumed by the body and produced anew rather than continually circulated. It had also been suggested that the blood flowed through pores between the two halves of the heart and that the heart produced a vital heat, which was tempered by the air from the lungs. In his own work, however, Harvey demonstrated that the heart expands passively and contracts actively. By measuring the amount of blood flowing from the heart, he concluded that the body could not continuously produce that amount. He also was able to show that blood is returned to the heart through the veins, postulating a connection (the capillaries) between the arteries and veins that was not to be discovered until later in the 17th century. Harvey was also interested in embryology, to which he made a significant contribution by suggesting that there is a stage (the egg) in the development of all animals during which they are undifferentiated living masses. A biological dictum, ex ovo omnia (“everything comes from the egg”), is a summation of that concept.

A development of great importance to science was the establishment in Europe of academies or societies; they consisted of small groups of men who met to discuss subjects of mutual interest. Although some of the groups enjoyed the financial patronage of princes and other wealthy members of society, the members’ interest in science was the sole sustaining force. The academies also provided freedom of expression, which, together with the stimulus of exchanging ideas, contributed greatly to the development of scientific thought. One of the earliest of these organizations was the Italian Accademia dei Lincei ( Academy of the Lynx-eyed), founded in Rome around 1603. Galileo Galilei made a microscope for the society; another of its members, Johannes Faber, an entomologist, gave the instrument its name. Other academies in Europe included the French Academy of Sciences (founded in 1666), a German Academy in Leipzig, and a number of small academies in England that in 1662 became incorporated under royal charter as the Royal Society of London, an organization that was to have considerable influence on scientific developments in England.

In addition to providing a forum for the discussion of scientific matters, another important aspect of those societies was their publications. Before the advent of printing there were no convenient means for the wide dissemination of scientific knowledge and ideas; hence, scientists were not well informed about the works of others. To correct that deficiency in communications, the early academies initiated several publications, the first of which, Journal des Savants (originally Journal des Sçavans), was published in 1665 in France. Three months later, the Royal Society of London originated its Philosophical Transactions. At first the publication was devoted to reviews of work completed and in progress; later, however, the emphasis gradually changed to accounts of original investigations that maintained a high level of scientific quality. Gradually, specialized journals of science made their appearance, though not until at least another century had passed.

The magnifying power of segments of glass spheres was known to the Assyrians before the time of Christ; during the 2nd century CE, Claudius Ptolemy, an astronomer, mathematician, and geographer at Alexandria, wrote a treatise on optics in which he discussed the phenomena of magnification and refraction as related to such lenses and to glass spheres filled with water. Despite that knowledge, however, glass lenses were not used extensively until around 1300 (an anonymous person invented spectacles for the improvement of vision probably in the late 1200s). That invention aroused curiosity concerning the property of lenses to magnify, and in the 16th century several papers were written about such devices. Then, in the late 16th century, the Dutch optician Hans Jansen and his son Zacharias invented the compound microscope. The utility of that instrument in the biological sciences, however, was not realized until the following century. Following subsequent technological improvements in the instrument and the development of a more-liberal attitude toward scientific research, five microscopists emerged who were to have a profound affect on biology: Marcello Malpighi, Antonie van Leeuwenhoek, Jan Swammerdam, Nehemiah Grew, and Robert Hooke.

The Italian biologist and physician Marcello Malpighi conducted extensive studies in animal anatomy and histology (the microscopic study of the structure, composition, and function of tissues). He was the first to describe the inner (malpighian) layer of the skin, the papillae of the tongue, the outer part (cortex) of the cerebral area of the brain, and the red blood cells. He wrote a detailed monograph on the silkworm; a further major contribution was a description of the development of the chick, beginning with the 24-hour stage. In addition to those and other animal studies, Malpighi made detailed investigations in plant anatomy. He systematically described the various parts of plants, such as bark, stem, roots, and seeds, and discussed processes such as germination and gall formation. Many of Malpighi’s drawings of plant anatomy remained unintelligible to botanists until the structures were rediscovered in the 19th century. Although Malpighi was not a technical innovator, he does exemplify the functioning of the educated 17th-century mind, which, together with curiosity and patience, resulted in many advances in biology.

Microscope made by Antonie van Leeuwenhoek.

Antonie van Leeuwenhoek, a Dutchman who spent most of his life in Delft, sold cloth for a living. As a young man, however, he became interested in grinding lenses, which he mounted in gold, silver, or copper plates. Indeed, he became so obsessed with the idea of making perfect lenses that he neglected his business and was ridiculed by his family and neighbours. Using single lenses rather than compound ones (a system of two or more), Leeuwenhoek achieved magnifications from 40 to 270 diameters, a remarkable feat for hand-ground lenses. Among his most-conspicuous observations was the discovery in 1675 of the existence in stagnant water and prepared infusions of many protozoans, which he called animalcules. He observed the connections between the arteries and veins; gave particularly fine accounts of the microscopic structure of muscle, the lens of the eye, the teeth, and other structures; and recognized bacteria of different shapes, postulating that they must be on the order of 25 times as small as the red blood cell. Because that is the approximate size of bacteria, it indicates that his observations were accurate.

Leeuwenhoek’s fame was consolidated when he confirmed the observations of a student that male seminal fluid contains spermatozoa. Furthermore, he discovered spermatozoa in other animals as well as in the female tract following copulation; the latter destroyed the idea held by others that the entire future development of an animal is centred in the egg, and that sperm merely induce a “vapour,” which penetrates the womb and effects fertilization. Although that theory of preformation, as it is called, continued to survive for some time longer, Leeuwenhoek initiated its eventual demise.

Leeuwenhoek’s animalcules raised some disquieting thoughts in the minds of his contemporaries. The theory of spontaneous generation, held by the ancient world and passed down unquestioned, was now being criticized. Christiaan Huygens, a scientific friend of Leeuwenhoek, hypothesized that the little animals might be small enough to float in the air and, on reaching water, reproduce themselves. At the time, however, criticism of spontaneous generation went no farther.

In contrast to Leeuwenhoek, who was virtually unschooled, his contemporary fellow countryman Jan Swammerdam was highly educated in medicine. However, similar to Leeuwenhoek, Swammerdam confined his attention to microscopical studies. He employed highly innovative techniques; for example, he injected wax into the circulatory system to hold the blood vessels firm, he dissected fragile structures under water to avoid destroying them, and he used micropipettes to inject and inflate organisms under the microscope. In 1669 Swammerdam published Algemeene Verhandeling van bloedeloose diertjens (The Natural History of Insects, 1792), in which he described the structure of a large number of insects as well as spiders, snails, scorpions, fishes, and worms. He regarded all of those animals as insects, distinguishing between them according to their mode of development. Although that classification was erroneous, Swammerdam discovered a great deal of information concerning insect development.

Swammerdam was subject to fits of mental instability, which, combined with financial difficulties, led to periods of depression. It was while in a state of mental disturbance that he produced his classic Ephemeri vita (“Life of the Ephemera”) in 1675, a book about the life of the mayfly noteworthy for its extremely detailed illustrations. Sometime after his death at age 43, Swammerdam’s works were published collectively as the Bijbel der Natuure (1737; “Bible of Nature”), which is considered by many authorities to be the finest collection of microscopic observations ever produced by one person.

Nehemiah Grew was educated at Cambridge and is regarded by some as one of the founders of plant anatomy. In 1672 he published the first of his great works, The Anatomy of Vegetables Begun, followed in 1682 by The Anatomy of Plants. Although Grew clearly recognized cells in plants, referring to them as vesicles, or bladders, their biological significance evaded him. He is best known for his recognition of flowers as the sexual organs of plants and for his description of their parts. He also described the individual pollen grains and observed that they are transported by bees, but he did not realize the significance of that observation. Twelve years after the publication of The Anatomy of Plants, a German physician utilized Grew’s anatomical studies in experiments to verify sexual reproduction in plants.

Of the five microscopists, Robert Hooke was perhaps the most intellectually preeminent. As curator of instruments at the Royal Society of London, he was in touch with all new scientific developments and exhibited interest in such disparate subjects as flying and the construction of clocks. In 1665 Hooke published his Micrographia, which was primarily a review of a series of observations that he had made while following the development and improvement of the microscope. Hooke described in detail the structure of feathers, the stinger of a bee, the radula, or “tongue,” of mollusks, and the foot of the fly. It is Hooke who coined the word cell; in a drawing of the microscopic structure of cork, he showed walls surrounding empty spaces and referred to the structures as cells. He described similar structures in the tissue of other trees and plants and discerned that in some tissues the cells were filled with a liquid while in others they were empty. He therefore supposed that the function of the cells was to transport substances through the plant.

Although the work of any of the classical microscopists seems to lack a definite objective, it should be remembered that these men embodied the concept that observation and experiment were of prime importance, that mere hypothetical, philosophical speculations were not sufficient. It is remarkable that so few men, working as individuals totally isolated from each other, should have recorded so many observations of such fundamental importance. The great significance of their work was that it revealed, for the first time, a world in which living organisms display an almost incredible complexity.

Work with the compound microscope languished for nearly 200 years, mainly because the early lenses tended to break up white light into its constituent parts. That technical problem was not solved until the invention of achromatic lenses, which were introduced about 1830. In 1878 a modern achromatic compound microscope was produced from the design of the German physicist Ernst Abbe. Abbe subsequently designed a substage illumination system, which, together with the introduction of a new substage condenser, paved the way for the biological discoveries of that era.

In 1687 the English mathematician, physicist, and astronomer Isaac Newton published his great work Principia, in which he described the universe as fixed, with Earth and other heavenly bodies moving harmoniously in accordance with mathematical laws. That approach of systematizing and classifying was to dominate biology in the 17th and 18th centuries. One reason was that the 16th-century “fathers of botany” had been content merely to describe and draw plants, assembling an enormous and diverse number that continued to increase as explorations of foreign countries made it evident that every country had its own native plants and animals.

Aristotle began the process of classification when he used mode of reproduction and habitat to distinguish groups of animals. Indeed, the words genus and species are translations of the Greek genos and eidos used by Aristotle. The Swiss botanist Bauhin had introduced a binomial system of classification, using a generic name and a specific name. Most classification schemes proposed before the 17th century were confused and unsatisfactory, however.

Two systematists of the 17th and 18th centuries were the English naturalist John Ray and the Swedish naturalist and explorer Carolus Linnaeus. Ray, who studied at Cambridge, was particularly interested in the work of the ancient compilers of herbals, especially those who had attempted to formulate some means of classification. Recognizing the need for a classification system that would apply to both plants and animals, Ray employed in his classification schemes extremely precise descriptions for genera and species. By basing his system on structures, such as the arrangement of toes and teeth in animals, rather than colour or habitat, Ray introduced a new and very important concept to taxonomic biology.

Prior to Linnaeus, most taxonomists started their classification systems by dividing all the known organisms into large groups and then subdividing them into progressively smaller groups. Unlike his predecessors, Linnaeus began with the species, organizing them into larger groups or genera, and then arranging analogous genera to form families and related families to form orders and classes. Probably utilizing the earlier work of Grew and others, Linnaeus chose the structure of the reproductive organs of the flower as a basis for grouping the higher plants. Thus, he distinguished between plants with real flowers and seeds (phanerogams) and those lacking real flowers and seeds (cryptogams), subdividing the former into hermaphroditic (bisexual) and unisexual forms. For animals, following Ray’s work, Linnaeus relied upon teeth and toes as the basic characteristics of mammals; he used the shape of the beak as the basis for bird classification. Having demonstrated that a binomial classification system based on concise and accurate descriptions could be used for the grouping of organisms, Linnaeus established taxonomic biology as a discipline.

Later developments in classification were initiated by the French biologists Comte de Buffon, Jean-Baptiste Lamarck, and Georges Cuvier, all of whom made lasting contributions to biological science, particularly in comparative studies. Subsequent systematists have been chiefly interested in the relationships between animals and have endeavoured to explain not only their similarities but also their differences in broad terms that encompass, in addition to structure, composition, function, genetics, evolution, and ecology.

Once the opprobrium attached to the dissection of human bodies had been dispelled in the 16th century, anatomists directed their efforts toward a better understanding of human structure. In doing so they generally ignored other animals, at least until the latter part of the 17th century, when biologists began to realize that important insights could be gained by comparative studies of all animals, including humans. One of the first of such anatomists was the English physician Edward Tyson, who studied the anatomy of an immature chimpanzee in detail and compared it with that of a human. In making further comparisons between the chimpanzee and other primates, Tyson clearly recognized points of similarity between those animals and humans. Not only was this a major contribution to physical anthropology, but it was also an indication—nearly two centuries before Darwin—of the existence of relationships between humans and other primates.

Among those who gave comparative studies their greatest impetus was Georges Cuvier, who utilized large collections of biological specimens sent to him from all over the world to work out a systematic organization of the animal kingdom. In addition to establishing a connection between systematic and comparative anatomy, he believed that there was a “correlation of parts” according to which a given type of structure (e.g., feathers) is related to a certain anatomical formation (e.g., a wing), which in turn is related to other specific formations (e.g., the clavicle), and so on. In other words, he felt that a great deal of anatomical information could be deduced about an organism even if the whole specimen was not available. That insight was to be of great practical importance in the study of fossils, in which Cuvier played a leading role. Indeed, the 1812 publication of Cuvier’s Recherches sur les ossemens fossiles de quadrupèdes (translated as Research on Fossil Bones in 1835) laid the foundation for the science of paleontology. But in order to reconcile his scientific findings with his personal religious beliefs, Cuvier postulated a series of catastrophic events that could account for both the presence of fossils and the immutability of existing species.

If a species can develop only from a preexisting species, then how did life originate? Among the many philosophical and religious ideas advanced to answer that question, one of the most popular was the theory of spontaneous generation, according to which, as already mentioned, living organisms could originate from nonliving matter. With the increasing tempo of discovery during the 17th and 18th centuries, however, investigators began to examine more critically the Greek belief that flies and other small animals arose from the mud at the bottom of streams and ponds by spontaneous generation. Then, when Harvey announced his biological dictum ex ovo omnia (“everything comes from the egg”), it appeared that he had solved the problem, at least insofar as it pertained to flowering plants and the higher animals, all of which develop from an egg. But Leeuwenhoek’s subsequent disquieting discovery of animalcules demonstrated the existence of a densely populated but previously invisible world of organisms that had to be explained.

The Italian physician and poet Francesco Redi was one of the first to question the spontaneous origin of living things. Having observed the development of maggots and flies on decaying meat, Redi in 1668 devised a number of experiments, all pointing to the same conclusion: if flies are excluded from rotten meat, maggots do not develop. On meat exposed to air, however, eggs laid by flies develop into maggots. Nonetheless, in 1745 support for spontaneous generation was renewed with the publication of An Account of Some New Microscopical Discoveries by the English naturalist and Roman Catholic divine John Turberville Needham. Needham found that large numbers of organisms subsequently developed in prepared infusions of many different substances that had been exposed to intense heat in sealed tubes for 30 minutes. Assuming that such heat treatment must have killed any previous organisms, Needham explained the presence of the new population on the grounds of spontaneous generation. The experiments appeared irrefutable until the Italian physiologist Lazzaro Spallanzani repeated them and obtained conflicting results. He published his findings around 1775, claiming that Needham had not heated his tubes long enough, nor had he sealed them in a satisfactory manner. Although Spallanzani’s results should have been convincing, Needham had the support of the influential French naturalist Buffon; hence, the matter of spontaneous generation remained unresolved.

experiments disproving spontaneous generation The hypothesis of spontaneous generation posited that living organisms develop from nonliving matter. This idea was disproved following experiments conducted in 1668 by Italian physician Francesco Redi and in 1859 by French chemist and microbiologist Louis Pasteur.

After a number of further investigations had failed to solve the problem, the French Academy of Sciences offered a prize for research that would “throw new light on the question of spontaneous generation.” In response to that challenge, Louis Pasteur, who at that time was a chemist, subjected flasks containing a sugared yeast solution to a variety of conditions. Pasteur was able to demonstrate conclusively that any microorganisms that developed in suitable media came from microorganisms in the air, not from the air itself, as Needham had suggested. Support for Pasteur’s findings came in 1876 from the English physicist John Tyndall, who devised an apparatus to demonstrate that air had the ability to carry particulate matter. Because such matter in air reflects light when the air is illuminated under special conditions, Tyndall’s apparatus could be used to indicate when air was pure. Tyndall found that no organisms were produced when pure air was introduced into media capable of supporting the growth of microorganisms. It was those results, together with Pasteur’s findings, that put an end to the doctrine of spontaneous generation.

When Pasteur later showed that parent microorganisms generate only their own kind, he thereby established the study of microbiology. Moreover, he not only succeeded in convincing the scientific world that microbes are living creatures, which come from preexisting forms, but also showed them to be an immense and varied component of the organic world, a concept that was to have important implications for the science of ecology. Further, by isolating various species of bacteria and yeasts in different chemical media, Pasteur was able to demonstrate that they brought about chemical change in a characteristic and predictable way, thus making a unique contribution to the study of fermentation and to biochemistry.

In the 1920s the Russian biochemist Aleksandr Oparin and other scientists suggested that life may have come from nonliving matter under conditions that existed on primitive Earth, when the atmosphere consisted of the gases methane, ammonia, water vapour, and hydrogen. According to that concept, energy supplied by electrical storms and ultraviolet light may have broken down the atmospheric gases into their constituent elements, and organic molecules may have been formed when the elements recombined.

Some of those ideas have been verified by advances in geochemistry and molecular genetics; experimental efforts have succeeded in producing amino acids and proteinoids (primitive protein compounds) from gases that may have been present on Earth at its inception, and amino acids have been detected in rocks that are more than three billion years old. With improved techniques it may be possible to produce precursors of or actual self-replicating living matter from nonliving substances. But whether it is possible to create the actual living heterotrophic forms from which autotrophs supposedly developed remains to be seen.

Although a number of 16th- and 17th-century travelers provided much valuable information about the plants and animals in Asia, America, and Africa, most of that information was collected by curious individuals rather than trained observers. In the 18th and 19th centuries, however, such information was collected increasingly in the course of organized scientific expeditions, usually under the auspices of a particular government. The most notable of those efforts were the voyages of the ships known as the HMS Endeavour, the HMS Investigator, the HMS Beagle, and the HMS Challenger, all sponsored by the English government.

Capt. James Cook sailed the Endeavour to the South Pacific islands, New Zealand, New Guinea, and Australia in 1768; the voyage provided the British naturalist and explorer Joseph Banks with the opportunity to make a very extensive collection of plants and notes, which helped establish him as a leading biologist. Another expedition to the same area in the Investigator in 1801 included the Scottish botanist Robert Brown, whose work on the plants of Australia and New Zealand became a classic; especially important were his descriptions of how certain plants adapt to different environmental conditions. Brown is also credited with discovering the cell nucleus and analyzing sexual processes in higher plants.

One of the most-famous biological expeditions of all time was that of the Beagle (1831–36), on which Charles Darwin served as naturalist. Although Darwin’s primary interest at the time was geology, his visit to the Galápagos Islands aroused his interest in biology and caused him to speculate about their curious insular animal life and the significance of isolation in space and time for the formation of species. During the Beagle voyage, Darwin collected specimens of and accumulated copious notes on the plants and animals of South America and Australia, for which he received great acclaim on his return to England.

The voyage of the Challenger (see Challenger Expedition) from 1872 to 1876 was organized by the British Admiralty to study oceanography, meteorology, and natural history. Under the leadership of the Scottish naturalist Charles Wyville Thomson, vast collections of plants and animals were made, the importance of plankton (minute free-floating aquatic organisms) as a source of food for larger marine organisms was recognized, and many new planktonic species were discovered. A particularly significant aspect of the Challenger voyage was the interest it stimulated in the new science of marine biology.

In spite of those expeditions, the contributions made by individuals were still very important. The British naturalist Alfred Russel Wallace, for example, undertook explorations of the Malay Archipelago from 1854 to 1862. In 1876 he published his book The Geographical Distribution of Animals, in which he divided the landmasses into six zoogeographical regions and described their characteristic fauna. Wallace also contributed to the theory of evolution, publishing in 1870 a book expressing his views, Contributions to the Theory of Natural Selection.

Although the microscopists of the 17th century had made detailed descriptions of plant and animal structure and though Hooke had coined the term cell to describe the compartments he had observed in cork tissue, their observations lacked an underlying theoretical unity. It was not until 1838 that the German botanist Matthias Jacob Schleiden, interested in plant anatomy, stated that “the lower plants all consist of one cell, while the higher ones are composed of (many) individual cells.” When the German physiologist Theodor Schwann, Schleiden’s friend, extended the cellular theory to include animals, he thereby brought about a rapprochement between botany and zoology. The formation of the cell theory—all plants and animals are made up of cells—marked a great conceptual advance in biology, and it resulted in renewed attention to the living processes that go on in cells.

In 1846, after several investigators had described the streaming movement of the cytoplasm in plant cells, the German botanist Hugo von Mohl coined the word protoplasm to designate the living substance of the cell. The concept of protoplasm as the physical basis of life led to the development of cell physiology.

A further extension of the cell theory was the development of cellular pathology by the German scientist Rudolf Virchow, who established the relationship between abnormal events in the body and unusual cellular activities. Virchow’s work gave a new direction to the study of pathology and resulted in advances in medicine.

The detailed description of cell division was contributed by the German plant cytologist Eduard Strasburger, who observed the mitotic process in plant cells and further demonstrated that nuclei arise only from preexisting nuclei. Parallel work in mammals was carried out by the German anatomist Walther Flemming, who published his most important findings in Zellsubstanz, Kern und Zelltheilung (“Cell Substance, Nucleus and Cell Division”) in 1882.

As knowledge of plant and animal forms accumulated during the 16th, 17th, and 18th centuries, a few biologists began to speculate about the ancestry of those organisms, though the prevailing view was that promulgated by Linnaeus—namely, the immutability of the species. Among the early speculations voiced during the 18th century, the British physician Erasmus Darwin (grandfather of Charles Darwin), concluded that species descend from common ancestors and that there is a struggle for existence among animals. The French biologist Jean-Baptiste Lamarck, among the most important of the 18th-century evolutionists, recognized the role of isolation in species formation; he also saw the unity in nature and conceived the idea of the evolutionary tree.

A complete theory of evolution was not announced, however, until the publication in 1859 of Charles Darwin’s On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. In his book Darwin stated that all living creatures multiply so rapidly that if left unchecked they would soon overpopulate the world. According to Darwin, the checks on population size are maintained by competition for the means of life. Hence, if any member of a species differs in some way that makes it better fitted to survive, then it will have an advantage that its offspring would be likely to perpetuate. Darwin’s work reflects the influence of the British economist Thomas Robert Malthus, who in 1838 published an essay on population in which he warned that if humans multiply more rapidly than their food supply, competition for existence will result. Darwin was also influenced by the British geologist Charles Lyell, who realized from his studies of geological formations that the relative ages of deposits could be estimated by means of the proportion of living and extinct mollusks. But it was not until after his travels aboard the Beagle (1831–36), during which he observed a great richness and diversity of island fauna, that Darwin began to develop his theory of evolution. Alfred Russel Wallace had reached conclusions similar to those of Darwin following his studies of plants and animals in the Malay Archipelago. A short paper dealing with this subject sent by Wallace to Darwin finally resulted in the publication of Darwin’s own theories.

Conceptually, the theory was of the utmost significance, accounting as it did for the formation of new species. Following the subsequent discovery of the chromosomal basis of inheritance and the laws of heredity, it could be seen that natural selection does not involve the sharp alternatives of life or death but results from the differential survival of variants. Today the universal principle of natural selection, which is the central concept of Darwin’s theory, is firmly established.

A question posed by Aristotle was whether the embryo is preformed and therefore only enlarges during development or whether it differentiates from an amorphous beginning. Two conflicting schools of thought had been based on that question: the preformation school maintained that the egg contains a miniature individual that develops into the adult stage in the proper environment; the epigenesis school believed that the egg is initially undifferentiated and that development occurs as a series of steps. Prominent supporters of the preformation doctrine, which was widely held until the 18th century, included Malpighi, Swammerdam, and Leeuwenhoek. In the 19th century, as criticism of preformation mounted, the Prussian Estonian embryologist Karl Ernst von Baer provided the final evidence against the theory. His discovery of the mammalian egg and his recognition of the formation of the germ layers out of which the embryonic organs develop laid the foundations of modern embryology.

Despite the many early descriptions of spermatozoa, their essential role in fertilization was not proved until 1879, when the Swiss physician and zoologist Hermann Fol observed the penetration of a spermatozoon into an ovum. Prior to that discovery, during the period from 1823 to 1830, the existence of the sexual process in flowering plants had been demonstrated by the Italian astronomer and optician Giovanni Battista Amici and confirmed by others. The discovery of fertilization in plants was of great importance to the development of plant hybrids, which are produced by cross-pollination between different species; it was also of great significance to the studies of genetics and evolution.

The universal occurrence and remarkable similarity of the fertilization process, regardless of the organism in which it occurs, provoked many of the leading investigators of the time to search for the underlying mechanism. It was realized that there must be some way by which the number of chromosomes is reduced before fertilization; otherwise, the chromosome number would double every time a sperm fused with an egg. In 1883 the Belgian embryologist and cytologist Edouard van Beneden showed that the eggs and the sperm in the worm Ascaris contain half the number of chromosomes found in the body cells. To account for the halving of the chromosomes in the sex cells, a process known as meiosis, in 1887 the German biologist August Weismann suggested that there must be two different types of cell division, and by 1900 the details of meiosis had been elucidated.

The fundamental laws of heredity were discovered in 1865 by the Austrian botanist, teacher, and Augustinian prelate Gregor Mendel, though his work was ignored until its rediscovery in 1900. There were, however, a number of views on the subject that had been expressed long before Mendel. The Greek philosophers, for example, believed that the traits of individuals were acquired from contact with the environment and that such acquired characteristics could be inherited by offspring. Because Lamarck was the most famous proponent of the inheritance of acquired characteristics, the theory is called Lamarckism. This concept, which emphasized the use and disuse of organs as the significant factor in determining the characteristics of an individual, postulated that any alterations in the individual could be transmitted to the offspring through the gametes.

In 1885 Weismann suggested that hereditary characteristics were transmitted by what he called germ plasm—as distinguished from the somatoplasm (body cells)—which linked the generations by a continuous stream of dividing germ cells. In stating definitely seven years later that the material of heredity was in the chromosomes, Weismann anticipated the chromosomal basis of inheritance.

The English explorer, anthropologist, and eugenicist Francis Galton made a number of important contributions to genetics in the 19th century, one of which was a study of the hereditary nature of ability, from which he developed the concept that judicious breeding could improve the human race ( eugenics). Galton’s most-significant work was the demonstration that each generation of ancestors makes a proportionate contribution to the total makeup of the individual. Thus, he suggested, if a tall man marries a short woman, each should contribute half of the total heritage, and the resultant offspring should be intermediate between the two parents.

The fame of Gregor Mendel, the father of genetics, rests on experiments he did with garden peas, which possess sharply contrasting characteristics—for example, tall versus short; round seed versus wrinkled seed. When Mendel fertilized short plants with pollen from tall plants, he found the offspring (first filial generation) to be uniformly tall. But if he allowed the plants of that generation to self-pollinate (fertilize themselves), their offspring (the second filial generation) exhibited the characters of the grandparents in a rather consistent ratio of three tall to one short. Furthermore, if allowed to self-pollinate, the short plants always bred true—they never produced anything but short plants. From those results Mendel developed the concept of dominance, based on the supposition that each plant carried two trait units, one of which dominated the other. Nothing was known at that time about chromosomes or meiosis, yet Mendel deduced from his results that the trait units, later called genes, could be a kind of physical particle that was transmitted from one generation to another through the reproductive mechanism.

Mendel’s most-important concept was the idea that the paired genes present in the parent separate or segregate during the formation of the gametes. Moreover, in later experiments in which he studied the inheritance of two pairs of traits, Mendel showed that one pair of genes is independent of another. Thus, the principles of segregation and of independent assortment were established.

Mendel’s findings were ignored for 35 years, probably for two reasons. Because the distinguished Swiss botanist Karl Wilhelm von Nägeli failed to recognize the significance of the work after Mendel sent him the results, he did nothing to encourage Mendel. Nägeli’s great prestige and the lack of his endorsement indirectly weighed against widespread recognition of Mendel’s work. Moreover, when the work was published, little was known about the cell, and the processes of mitosis and meiosis were completely unknown. Mendel’s work was finally rediscovered in 1900, when three botanists independently recognized the worth of his studies from their own research and cited his publication in their work.

By 1901 it was understood how the hereditary units postulated by Mendel are distributed; it was also known that the somatic (body) cells have a double, or diploid, complement of chromosomes, while the reproductive cells have a single, or haploid, chromosome number. The experimental demonstration of the chromosomal basis for heredity had been firmly established by the German cytologist Theodor Boveri soon after the turn of the century and subsequently confirmed by others. To account for the large number of observed hereditary characters, Boveri suggested that each chromosome in a pair can exchange the hereditary factors it carries with those of the other chromosome. At first, the American geneticist Thomas Hunt Morgan dismissed that concept. Later, however, when he found that it agreed with his own laboratory findings, Morgan and his collaborators assigned the hereditary units (genes) specific positions, or loci, within the chromosomes. With the genes established as the carriers of hereditary traits, the English biologist William Bateson coined the term genetics for the experimental study of heredity and evolution.

By utilizing modern methods of investigation, such as X-ray diffraction and electron microscopy, to explore levels of cellular organization beyond that visible with a light microscope—the ultrastructure of the cell—new concepts of cellular function were produced. As a result, the study of the molecular organization of the cell had tremendous impact on biology during the 20th and 21st centuries. It also led directly to the convergence of many different scientific disciplines in order to acquire a better understanding of life processes.

Technologies such as DNA sequencing and the polymerase chain reaction also were developed, allowing biologists to peer into the genetic blueprints that give rise to organisms. First-generation sequencing technologies emerged in the 1970s and were followed several decades later by so-called next-generation sequencing technologies, which were superior in speed and cost-efficiency. Next-generation sequencing provided researchers with massive amounts of genetic data, typically gigabases in size (1 gigabase = 1,000,000,000 base pairs of DNA). Bioinformatics, which linked biological data with tools and techniques for data analysis, storage, and distribution, became an increasingly important part of biological studies, particularly those involving very large sets of genetic data.

This computerized image of anthrax shows the various structural relationships of seven units within the protein and demonstrates the interaction of a drug (shown in yellow) bound to the protein to block the so-called lethal factor unit. Bioinformatics plays an important role in enabling scientists to predict where a drug molecule will bind within a protein, given the individual structures of the molecules.

In the 1970s the development of recombinant DNA technology opened the way to genetic engineering, which enabled researchers to recombine nucleic acids and thereby modify organisms’ genetic codes, giving the organisms new abilities or eliminating undesirable traits. Those developments were followed by advances in cloning technologies, which led to the generation in 1996 of Dolly the sheep, the first clone of an adult mammal. Together, recombinant DNA technology and reproductive cloning (the method used to produce a living animal clone) facilitated great progress in the development of genetically modified organisms (GMOs). Such organisms became crucial components of biomedical research, where genetically modified (GM) mice and other animals were developed to model certain human diseases, thereby facilitating the investigation of new therapies and the factors that cause disease. Recombinant DNA technology played a crucial role in the generation of GM crops, including pest-resistant forms of cotton and herbicide-resistant forms of maize (corn) and soybeans.

In gene knockout a functional gene is replaced by an inactivated gene that is created using recombinant DNA technology. When a gene is “knocked out,” the resulting mutant phenotype (observable characteristics) often reveals the gene's biological function.

In the 1990s and early 2000s, researchers worldwide increasingly came together in consortiums and other collaborative groups to accomplish major feats in biology. The first major success of those efforts was the sequencing of the human genome, which was accomplished through the Human Genome Project (HGP). The HGP began in 1990, supported by the U.S. Department of Energy and the National Institutes of Health (NIH). NIH researchers later joined forces with Celera Genomics, a private-sector enterprise, and the project was completed in 2003. Other collaborative projects soon followed, including the International HapMap Project, an outgrowth of the HGP, and the 1000 Genomes Project, which built on data from the HapMap effort.

The 20th and 21st centuries also saw major advances in areas of biology dealing with ecosystems, the environment, and conservation. In the 20th century, scientists realized that humans are as dependent upon Earth’s natural resources as are other animals. However, humans were contributing to the progressive destruction of the environment, in part because of an increase in population pressure and certain technological advances. Lifesaving advances in medicine, for example, had allowed people to live longer and resulted in a dramatic drop in death rates (primarily in developed countries), contributing to an explosive increase in the human population. Chemical contaminants introduced into the environment by manufacturing processes, pesticides, automobile emissions, and other means seriously endangered all forms of life. Hence, biologists began to pay much greater attention to the relationships of living things to each other as well as to their biotic and abiotic environments.

The growing significance of climate change and its impact on ecosystems fueled advances in ecology, as well as the development of fields such as conservation biology and conservation genetics. As in almost every other area of biology, molecular biology came to fulfill an important role in those fields, with techniques such as whole genome sequencing being used to gather information on the genetic diversity of populations of endangered species and techniques such as cloning and genome editing raising the possibility of someday resurrecting extinct species (a process known as de-extinction). Information on the DNA sequences of a wide range of species also aided progress in scientists’ understanding of evolution and systematics (the study of evolutionary relationships and the diversification of life).

By the 21st century, there were many important categories in the biological sciences and hence numerous specialties within fields. Botany, zoology, and microbiology dealt with types of organisms and their relationships with each other. Such disciplines had long been subdivided into more-specialized categories—for example, ichthyology, the study of fishes, and algology, the study of algae. Disciplines such as embryology and physiology, which dealt with the development and function of an organism, were divided further according to the kind of organism studied—for example, invertebrate embryology and mammalian physiology. Many developments in physiology and embryology had resulted from studies in cell biology, biophysics, and biochemistry. Likewise, research in cell physiology and cytochemistry, along with ultrastructural studies, helped scientists correlate cell structure with function. Ecology, which focused on relationships between organisms and their environment, included both the physical features of the environment and other organisms that may compete for food and shelter. Emphasis on different environments and certain features of organisms resulted in the subdivision of the field into a range of specialties, such as freshwater ecology, marine ecology, and population ecology.

scale in ecological studies A forest patch nested within a landscape mosaic.

Many areas of study in the biological sciences cross the boundaries that traditionally separated the various branches of the sciences. In biophysics, for example, researchers apply the principles and methods of physics to investigate and find solutions to problems in biology. Evolutionary biologists and paleontologists are familiar with the principles of geology and may even work closely with geologists while attempting to determine the age of biological remains. Likewise, anthropologists and archaeologists apply knowledge of human culture and society to biological findings in order to more fully understand humankind. Astrobiology arose through the activities of the scientists and engineers concerned with the exploration of space. As a result, the field of biology has received contributions from and made contributions to many other disciplines, in the humanities as well as in the sciences.

Through the 20th and 21st centuries, as biology became increasingly interconnected with other areas of science, it also came to encompass a number of disciplines itself. In some of those disciplines, multiple levels of organization were recognized—for example, population biology (the study of populations of living things) and organismic biology (the study of the whole organism) and cell biology and molecular biology. In the latter part of the 20th century, molecular biology spawned still more disciplines, and the advent of genomics led to the emergence of sophisticated subdisciplines, such as developmental genomics and functional genomics. Genetics continued to expand, giving rise to new areas such as conservation genetics. Despite their diverse scope, however, in the 21st century many areas of the biological sciences continued to draw on common unifying principles and ideas, particularly those that were central to taxonomy, genetics, and evolution.

In the 20th and 21st centuries, biologists’ role in society as well as their moral and ethical responsibility in the discovery and development of new ideas led to a reassessment of individual social and scientific value systems. Scientists cannot afford to ignore the consequences of their discoveries; they are as concerned with the possible misuses of their findings as they are with the basic research in which they are involved. In the 20th century, the emerging social and political role of the biologist and all other scientists required a weighing of values that could not be done with the accuracy or objectivity of a laboratory balance. As members of society, it became necessary for biologists to redefine their social obligations and functions, particularly in the realm of making judgments about ethical problems, such as human control of the environment or the manipulation of genes to direct further evolutionary development.

Of particular consequence in the biological sciences was the development of genetic engineering. In cases of genetic deficiencies and disease, genetic engineering opened up the possibility of correcting gene defects to restore physiological function, potentially improving patients’ quality of life. Gene therapy, in which a normal gene would be introduced into an individual’s genome in order to repair a disease-causing mutation, was one means by which researchers could potentially achieve that goal. However, the possibilities for misuse of genetic engineering were vast. There was significant concern, for example, about genetically modified organisms, particularly modified crops, and their impacts on human and environmental health. The emergence of cloning technologies, including somatic cell nuclear transfer, also raised concerns. The Declaration on Human Cloning passed in 2005 by the United Nations called upon member states to prohibit the cloning of humans, though it left open the pursuit of therapeutic cloning.

Similarly, in 2015, researchers who had developed technologies for gene editing, which enabled scientists to customize an organism’s genetic makeup by altering specific bases in its DNA sequence, called for a moratorium on the application of the technologies in humans. The impacts of gene editing on human genetics were unknown, and there were no regulations in place to guide its use. Indeed, in the absence of strict regulation, a Chinese scientist moved forward with gene editing in humans, in late 2018 claiming the birth of the world’s first babies carrying edited genomes. The scientist claimed to have edited human embryos to disable a gene that normally facilitates the entry of HIV into cells; the embryos were then implanted into a woman and carried to term. Meanwhile, researchers in the United States attempted to use gene editing to alter genes in human sperm, which would enable the edited genes to be passed on to subsequent generations. In particular, the researchers sought to alter genes that increase the risk of certain types of cancer, with the aim of reducing cancer risk in offspring. The debate over gene editing renewed earlier discussions about the ethical and social impacts of genetic engineering in humans, especially its potential to be used to alter traits such as intelligence and appearance.

Other challenges confronting biologists included the search for ways to curb environmental pollution without interfering with efforts to improve the quality of life for humankind. Contributing to the problem of pollution was the problem of surplus human population. A rise in global human population had placed greater demands on the land, especially in the area of food production, and had necessitated increases in the operations of modern industry, the waste products of which contributed to the pollution of air, water, and soil. To find solutions to global warming, pollution, and other environmental problems, biologists worked with social scientists and other members of society in order to determine the requirements necessary for maintaining a healthy and productive planet. For although many of humankind’s present and future problems may seem to be essentially social, political, or economic in nature, they have biological ramifications that could affect the very existence of life itself.

The word biology is formed by combining the Greek βίος (bios), meaning "life", and so the suffix '-logy', meaning "science of", "knowledge of", "study of", "about of", based on the Greek verb λέγειν, 'legein' "to select", "to gather" (cf. the noun λόγος, 'logos' "word"). The term biology in its modern sense appears to have been introduced independently by Thomas Beddoes (in 1799), Karl Friedrich Burdach (in 1800), Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur, 1802) and Jean-Baptiste Lamarck (Hydrogéologie, 1802). The word itself appears in the title of Volume 3 of Michael Christoph Hanow's Philosophiae naturalis sive physicae dogmaticae: Geologia, biologia, phytologia generalis et dendrologia, published in 1766.

Before biology, there were several terms used for the study of animals and plants. Natural history referred to the descriptive aspects of biology, though it also included mineralogy and other non-biological fields; from the Middle Ages through the Renaissance, the unifying framework of natural history was the scala naturae or Great Chain of Being. Natural philosophy and natural theology encompassed the conceptual and metaphysical basis of plant and animal life, dealing with problems of why organisms exist and behave the way they do, though these subjects also included what is now geology, physics, chemistry, and astronomy. Physiology and (botanical) pharmacology were the province of medicine. Botany, zoology, and (in the case of fossils) geology replaced natural history and natural philosophy in the 18th and 19th centuries before biology was widely adopted. To this day, "botany" and "zoology" are widely used, although they have been joined by other sub-disciplines of biology.

The earliest humans must have had and passed on knowledge about plants and animals to increase their chances of survival. This may have included knowledge of human and animal anatomy and aspects of animal behavior (such as migration patterns). However, the first major turning point in biological knowledge came with the Neolithic Revolution about 10,000 years ago. Humans first domesticated plants for farming, then livestock animals to accompany the resulting sedentary societies.

The ancient cultures of Mesopotamia, Egypt, the Indian subcontinent, and China, among others, produced renowned surgeons and students of the natural sciences such as Susruta and Zhang Zhongjing, reflecting independent sophisticated systems of natural philosophy. However, the roots of modern biology are usually traced back to the secular tradition of ancient Greek philosophy.

The Mesopotamians seem to have had little interest in the natural world as such, preferring to study how the gods had ordered the universe. Animal physiology was studied for divination, including especially the anatomy of the liver, seen as an important organ in haruspicy. Animal behavior too was studied for divinatory purposes. Most information about the training and domestication of animals was probably transmitted orally, but one text dealing with the training of horses has survived.

The ancient Mesopotamians had no distinction between "rational science" and magic. When a person became ill, doctors prescribed both magical formulas to be recited and medicinal treatments. The earliest medical prescriptions appear in Sumerian during the Third Dynasty of Ur (c. 2112 – c. 2004 BCE). The most extensive Babylonian medical text, however, is the Diagnostic Handbook written by the ummânū, or chief scholar, Esagil-kin-apli of Borsippa, during the reign of the Babylonian king Adad-apla-iddina (1069 – 1046 BCE). In East Semitic cultures, the main medicinal authority was an exorcist-healer known as an āšipu. The profession was passed down from father to son and was held in high regard. Of less frequent recourse was the asu, a healer who treated physical symptoms using remedies composed of herbs, animal products, and minerals, as well as potions, enemas, and ointments or poultices. These physicians, who could be either male or female, also dressed wounds, set limbs, and performed simple surgeries. The ancient Mesopotamians also practiced prophylaxis and took measures to prevent the spread of disease.

In ancient China, biological topics can be found dispersed across several different disciplines, including the work of herbologists, physicians, alchemists, and philosophers. The Taoist tradition of Chinese alchemy, for example, can be considered part of the life sciences due to its emphasis on health (with the ultimate goal being the elixir of life). The system of classical Chinese medicine usually revolved around the theory of yin and yang, and the five phases. Taoist philosophers, such as Zhuangzi in the 4th century BCE, also expressed ideas related to evolution, such as denying the fixity of biological species and speculating that species had developed differing attributes in response to differing environments.

One of the oldest organised systems of medicine is known from the Indian subcontinent in the form of Ayurveda which originated around 1500 BCE from Atharvaveda (one of the four most ancient books of Indian knowledge, wisdom and culture).

The ancient Indian Ayurveda tradition independently developed the concept of three humours, resembling that of the four humours of ancient Greek medicine, though the Ayurvedic system included further complications, such as the body being composed of five elements and seven basic tissues. Ayurvedic writers also classified living things into four categories based on the method of birth (from the womb, eggs, heat & moisture, and seeds) and explained the conception of a fetus in detail. They also made considerable advances in the field of surgery, often without the use of human dissection or animal vivisection. One of the earliest Ayurvedic treatises was the Sushruta Samhita, attributed to Sushruta in the 6th century BCE. It was also an early materia medica, describing 700 medicinal plants, 64 preparations from mineral sources, and 57 preparations based on animal sources.

Over a dozen medical papyri have been preserved, most notably the Edwin Smith Papyrus (the oldest extant surgical handbook) and the Ebers Papyrus (a handbook of preparing and using materia medica for various diseases), both from the 16th century BCE.

Ancient Egypt is also known for developing embalming, which was used for mummification, in order to preserve human remains and forestall decomposition.

The pre-Socratic philosophers asked many questions about life but produced little systematic knowledge of specifically biological interest — though the attempts of the atomists to explain life in purely physical terms would recur periodically through the history of biology. However, the medical theories of Hippocrates and his followers, especially humorism, had a lasting impact.

The philosopher Aristotle was the most influential scholar of the living world from classical antiquity. Though his early work in natural philosophy was speculative, Aristotle's later biological writings were more empirical, focusing on biological causation and the diversity of life. He made countless observations of nature, especially the habits and attributes of plants and animals in the world around him, which he devoted considerable attention to categorizing. In all, Aristotle classified 540 animal species, and dissected at least 50. He believed that intellectual purposes, formal causes, guided all natural processes.

Aristotle, and nearly all Western scholars after him until the 18th century, believed that creatures were arranged in a graded scale of perfection rising from plants on up to humans: the scala naturae or Great Chain of Being. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany — the History of Plants — which survived as the most important contribution of antiquity to botany, even into the Middle Ages. Many of Theophrastus' names survive into modern times, such as carpos for fruit, and pericarpion for seed vessel. Dioscorides wrote a pioneering and encyclopaedic pharmacopoeia, De Materia Medica, incorporating descriptions of some 600 plants and their uses in medicine. Pliny the Elder, in his Natural History, assembled a similarly encyclopaedic account of things in nature, including accounts of many plants and animals.

A few scholars in the Hellenistic period under the Ptolemies—particularly Herophilus of Chalcedon and Erasistratus of Chios—amended Aristotle's physiological work, even performing dissections and vivisections. Claudius Galen became the most important authority on medicine and anatomy. Though a few ancient atomists such as Lucretius challenged the teleological Aristotelian viewpoint that all aspects of life are the result of design or purpose, teleology (and after the rise of Christianity, natural theology) would remain central to biological thought essentially until the 18th and 19th centuries. Ernst W. Mayr argued that "Nothing of any real consequence happened in biology after Lucretius and Galen until the Renaissance." The ideas of the Greek traditions of natural history and medicine survived, but they were generally taken unquestioningly in medieval Europe.

The decline of the Roman Empire led to the disappearance or destruction of much knowledge, though physicians still incorporated many aspects of the Greek tradition into training and practice. In Byzantium and the Islamic world, many of the Greek works were translated into Arabic and many of the works of Aristotle were preserved.

During the High Middle Ages, a few European scholars such as Hildegard of Bingen, Albertus Magnus and Frederick II wrote on natural history. The rise of European universities, though important for the development of physics and philosophy, had little impact on biological scholarship.

The European Renaissance brought expanded interest in both empirical natural history {?} and physiology. In 1543, Andreas Vesalius inaugurated the modern era of Western medicine with his seminal human anatomy treatise De humani corporis fabrica, which was based on dissection of corpses. Vesalius was the first in a series of anatomists who gradually replaced scholasticism with empiricism in physiology and medicine, relying on first-hand experience rather than authority and abstract reasoning. Via herbalism, medicine was also indirectly the source of renewed empiricism in the study of plants. Otto Brunfels, Hieronymus Bock and Leonhart Fuchs wrote extensively on wild plants, the beginning of a nature-based approach to the full range of plant life. Bestiaries—a genre that combines both the natural and figurative knowledge of animals—also became more sophisticated, especially with the work of William Turner, Pierre Belon, Guillaume Rondelet, Conrad Gessner, and Ulisse Aldrovandi.

Artists such as Albrecht Dürer and Leonardo da Vinci, often working with naturalists, were also interested in the bodies of animals and humans, studying physiology in detail and contributing to the growth of anatomical knowledge. The traditions of alchemy and natural magic, especially in the work of Paracelsus, also laid claim to knowledge of the living world. Alchemists subjected organic matter to chemical analysis and experimented liberally with both biological and mineral pharmacology. This was part of a larger transition in world views (the rise of the mechanical philosophy) that continued into the 17th century, as the traditional metaphor of nature as organism was replaced by the nature as machine metaphor.

Systematizing, naming and classifying dominated natural history throughout much of the 17th and 18th centuries. Carl Linnaeus published a basic taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species. While Linnaeus conceived of species as unchanging parts of a designed hierarchy, the other great naturalist of the 18th century, Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable — even suggesting the possibility of common descent. Though he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought; his work would influence the evolutionary theories of both Lamarck and Darwin.

The discovery and description of new species and the collection of specimens became a passion of scientific gentlemen and a lucrative enterprise for entrepreneurs; many naturalists traveled the globe in search of scientific knowledge and adventure.

Extending the work of Vesalius into experiments on still living bodies (of both humans and animals), William Harvey and other natural philosophers investigated the roles of blood, veins and arteries. Harvey's De motu cordis in 1628 was the beginning of the end for Galenic theory, and alongside Santorio Santorio's studies of metabolism, it served as an influential model of quantitative approaches to physiology.

In the early 17th century, the micro-world of biology was just beginning to open up. A few lensmakers and natural philosophers had been creating crude microscopes since the late 16th century, and Robert Hooke published the seminal Micrographia based on observations with his own compound microscope in 1665. But it was not until Antonie van Leeuwenhoek's dramatic improvements in lensmaking beginning in the 1670s — ultimately producing up to 200-fold magnification with a single lens — that scholars discovered spermatozoa, bacteria, infusoria and the sheer strangeness and diversity of microscopic life. Similar investigations by Jan Swammerdam led to new interest in entomology and built the basic techniques of microscopic dissection and staining.

As the microscopic world was expanding, the macroscopic world was shrinking. Botanists such as John Ray worked to incorporate the flood of newly discovered organisms shipped from across the globe into a coherent taxonomy, and a coherent theology (natural theology). Debate over another flood, the Noachian, catalyzed the development of paleontology; in 1669 Nicholas Steno published an essay on how the remains of living organisms could be trapped in layers of sediment and mineralized to produce fossils. Although Steno's ideas about fossilization were well known and much debated among natural philosophers, an organic origin for all fossils would not be accepted by all naturalists until the end of the 18th century due to philosophical and theological debate about issues such as the age of the earth and extinction.

Up through the 19th century, the scope of biology was largely divided between medicine, which investigated questions of form and function (i.e., physiology), and natural history, which was concerned with the diversity of life and interactions among different forms of life and between life and non-life. By 1900, much of these domains overlapped, while natural history (and its counterpart natural philosophy) had largely given way to more specialized scientific disciplines — cytology, bacteriology, morphology, embryology, geography, and geology.

Widespread travel by naturalists in the early-to-mid-19th century resulted in a wealth of new information about the diversity and distribution of living organisms. Of particular importance was the work of Alexander von Humboldt, which analyzed the relationship between organisms and their environment (i.e., the domain of natural history) using the quantitative approaches of natural philosophy (i.e., physics and chemistry). Humboldt's work laid the foundations of biogeography and inspired several generations of scientists.

The emerging discipline of geology also brought natural history and natural philosophy closer together; the establishment of the stratigraphic column linked the spatial distribution of organisms to their temporal distribution, a key precursor to concepts of evolution. Georges Cuvier and others made great strides in comparative anatomy and paleontology in the late 1790s and early 19th century. In a series of lectures and papers that made detailed comparisons between living mammals and fossil remains Cuvier was able to establish that the fossils were remains of species that had become extinct — rather than being remains of species still alive elsewhere in the world, as had been widely believed. Fossils discovered and described by Gideon Mantell, William Buckland, Mary Anning, and Richard Owen among others helped establish that there had been an 'age of reptiles' that had preceded even the prehistoric mammals. These discoveries captured the public imagination and focused attention on the history of life on earth. Most of these geologists held to catastrophism, but Charles Lyell's influential Principles of Geology (1830) popularised Hutton's uniformitarianism, a theory that explained the geological past and present on equal terms.

The most significant evolutionary theory before Darwin's was that of Jean-Baptiste Lamarck; based on the inheritance of acquired characteristics (an inheritance mechanism that was widely accepted until the 20th century), it described a chain of development stretching from the lowliest microbe to humans. The British naturalist Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell, Thomas Malthus's writings on population growth, and his own morphological expertise, created a more successful evolutionary theory based on natural selection; similar evidence led Alfred Russel Wallace to independently reach the same conclusions.

The 1859 publication of Darwin's theory in On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life is often considered the central event in the history of modern biology. Darwin's established credibility as a naturalist, the sober tone of the work, and most of all the sheer strength and volume of evidence presented, allowed Origin to succeed where previous evolutionary works such as the anonymous Vestiges of Creation had failed. Most scientists were convinced of evolution and common descent by the end of the 19th century. However, natural selection would not be accepted as the primary mechanism of evolution until well into the 20th century, as most contemporary theories of heredity seemed incompatible with the inheritance of random variation.

Wallace, following on earlier work by de Candolle, Humboldt and Darwin, made major contributions to zoogeography. Because of his interest in the transmutation hypothesis, he paid particular attention to the geographical distribution of closely allied species during his field work first in South America and then in the Malay archipelago. While in the archipelago he identified the Wallace line, which runs through the Spice Islands dividing the fauna of the archipelago between an Asian zone and a New Guinea/Australian zone. His key question, as to why the fauna of islands with such similar climates should be so different, could only be answered by considering their origin. In 1876 he wrote The Geographical Distribution of Animals, which was the standard reference work for over half a century, and a sequel, Island Life, in 1880 that focused on island biogeography. He extended the six-zone system developed by Philip Sclater for describing the geographical distribution of birds to animals of all kinds. His method of tabulating data on animal groups in geographic zones highlighted the discontinuities; and his appreciation of evolution allowed him to propose rational explanations, which had not been done before.

The scientific study of heredity grew rapidly in the wake of Darwin's Origin of Species with the work of Francis Galton and the biometricians. The origin of genetics is usually traced to the 1866 work of the monk Gregor Mendel, who would later be credited with the laws of inheritance. However, his work was not recognized as significant until 35 years afterward. In the meantime, a variety of theories of inheritance (based on pangenesis, orthogenesis, or other mechanisms) were debated and investigated vigorously. Embryology and ecology also became central biological fields, especially as linked to evolution and popularized in the work of Ernst Haeckel. Most of the 19th century work on heredity, however, was not in the realm of natural history, but that of experimental physiology.

Over the course of the 19th century, the scope of physiology expanded greatly, from a primarily medically oriented field to a wide-ranging investigation of the physical and chemical processes of life—including plants, animals, and even microorganisms in addition to man. Living things as machines became a dominant metaphor in biological (and social) thinking.

Advances in microscopy also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the cell. In 1838 and 1839, Schleiden and Schwann began promoting the ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, though they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory.

Cell theory led biologists to re-envision individual organisms as interdependent assemblages of individual cells. Scientists in the rising field of cytology, armed with increasingly powerful microscopes and new staining methods, soon found that even single cells were far more complex than the homogeneous fluid-filled chambers described by earlier microscopists. Robert Brown had described the nucleus in 1831, and by the end of the 19th century cytologists identified many of the key cell components: chromosomes, centrosomes mitochondria, chloroplasts, and other structures made visible through staining. Between 1874 and 1884 Walther Flemming described the discrete stages of mitosis, showing that they were not artifacts of staining but occurred in living cells, and moreover, that chromosomes doubled in number just before the cell divided and a daughter cell was produced. Much of the research on cell reproduction came together in August Weismann's theory of heredity: he identified the nucleus (in particular chromosomes) as the hereditary material, proposed the distinction between somatic cells and germ cells (arguing that chromosome number must be halved for germ cells, a precursor to the concept of meiosis), and adopted Hugo de Vries's theory of pangenes. Weismannism was extremely influential, especially in the new field of experimental embryology.

By the mid-1850s the miasma theory of disease was largely superseded by the germ theory of disease, creating extensive interest in microorganisms and their interactions with other forms of life. By the 1880s, bacteriology was becoming a coherent discipline, especially through the work of Robert Koch, who introduced methods for growing pure cultures on agar gels containing specific nutrients in Petri dishes. The long-held idea that living organisms could easily originate from nonliving matter (spontaneous generation) was attacked in a series of experiments carried out by Louis Pasteur, while debates over vitalism vs. mechanism (a perennial issue since the time of Aristotle and the Greek atomists) continued apace.

In chemistry, one central issue was the distinction between organic and inorganic substances, especially in the context of organic transformations such as fermentation and putrefaction. Since Aristotle these had been considered essentially biological (vital) processes. However, Friedrich Wöhler, Justus Liebig and other pioneers of the rising field of organic chemistry—building on the work of Lavoisier — showed that the organic world could often be analyzed by physical and chemical methods. In 1828 Wöhler showed that the organic substance urea could be created by chemical means that do not involve life, providing a powerful challenge to vitalism. Cell extracts ("ferments") that could effect chemical transformations were discovered, beginning with diastase in 1833. By the end of the 19th century the concept of enzymes was well established, though equations of chemical kinetics would not be applied to enzymatic reactions until the early 20th century.

Physiologists such as Claude Bernard explored (through vivisection and other experimental methods) the chemical and physical functions of living bodies to an unprecedented degree, laying the groundwork for endocrinology (a field that developed quickly after the discovery of the first hormone, secretin, in 1902), biomechanics, and the study of nutrition and digestion. The importance and diversity of experimental physiology methods, within both medicine and biology, grew dramatically over the second half of the 19th century. The control and manipulation of life processes became a central concern, and experiment was placed at the center of biological education.

At the beginning of the 20th century, biological research was largely a professional endeavour. Most work was still done in the natural history mode, which emphasized morphological and phylogenetic analysis over experiment-based causal explanations. However, anti-vitalist experimental physiologists and embryologists, especially in Europe, were increasingly influential. The tremendous success of experimental approaches to development, heredity, and metabolism in the 1900s and 1910s demonstrated the power of experimentation in biology. In the following decades, experimental work replaced natural history as the dominant mode of research.

In the early 20th century, naturalists were faced with increasing pressure to add rigor and preferably experimentation to their methods, as the newly prominent laboratory-based biological disciplines had done. Ecology had emerged as a combination of biogeography with the biogeochemical cycle concept pioneered by chemists; field biologists developed quantitative methods such as the quadrat and adapted laboratory instruments and cameras for the field to further set their work apart from traditional natural history. Zoologists and botanists did what they could to mitigate the unpredictability of the living world, performing laboratory experiments and studying semi-controlled natural environments such as gardens; new institutions like the Carnegie Station for Experimental Evolution and the Marine Biological Laboratory provided more controlled environments for studying organisms through their entire life cycles.

The ecological succession concept, pioneered in the 1900s and 1910s by Henry Chandler Cowles and Frederic Clements, was important in early plant ecology. Alfred Lotka's predator-prey equations, G. Evelyn Hutchinson's studies of the biogeography and biogeochemical structure of lakes and rivers (limnology) and Charles Elton's studies of animal food chains were pioneers among the succession of quantitative methods that colonized the developing ecological specialties. Ecology became an independent discipline in the 1940s and 1950s after Eugene P. Odum synthesized many of the concepts of ecosystem ecology, placing relationships between groups of organisms (especially material and energy relationships) at the center of the field.

In the 1960s, as evolutionary theorists explored the possibility of multiple units of selection, ecologists turned to evolutionary approaches. In population ecology, debate over group selection was brief but vigorous; by 1970, most biologists agreed that natural selection was rarely effective above the level of individual organisms. The evolution of ecosystems, however, became a lasting research focus. Ecology expanded rapidly with the rise of the environmental movement; the International Biological Program attempted to apply the methods of big science (which had been so successful in the physical sciences) to ecosystem ecology and pressing environmental issues, while smaller-scale independent efforts such as island biogeography and the Hubbard Brook Experimental Forest helped redefine the scope of an increasingly diverse discipline.

1900 marked the so-called rediscovery of Mendel: Hugo de Vries, Carl Correns, and Erich von Tschermak independently arrived at Mendel's laws (which were not actually present in Mendel's work). Soon after, cytologists (cell biologists) proposed that chromosomes were the hereditary material. Between 1910 and 1915, Thomas Hunt Morgan and the "Drosophilists" in his fly lab forged these two ideas — both controversial — into the "Mendelian-chromosome theory" of heredity. They quantified the phenomenon of genetic linkage and postulated that genes reside on chromosomes like beads on string; they hypothesized crossing over to explain linkage and constructed genetic maps of the fruit fly Drosophila melanogaster, which became a widely used model organism.

Hugo de Vries tried to link the new genetics with evolution; building on his work with heredity and hybridization, he proposed a theory of mutationism, which was widely accepted in the early 20th century. Lamarckism, or the theory of inheritance of acquired characteristics also had many adherents. Darwinism was seen as incompatible with the continuously variable traits studied by biometricians, which seemed only partially heritable. In the 1920s and 1930s — following the acceptance of the Mendelian-chromosome theory — the emergence of the discipline of population genetics, with the work of R.A. Fisher, J.B.S. Haldane and Sewall Wright, unified the idea of evolution by natural selection with Mendelian genetics, producing the modern synthesis. The inheritance of acquired characters was rejected, while mutationism gave way as genetic theories matured.

In the second half of the century the ideas of population genetics began to be applied in the new discipline of the genetics of behavior, sociobiology, and, especially in humans, evolutionary psychology. In the 1960s W.D. Hamilton and others developed game theory approaches to explain altruism from an evolutionary perspective through kin selection. The possible origin of higher organisms through endosymbiosis, and contrasting approaches to molecular evolution in the gene-centered view (which held selection as the predominant cause of evolution) and the neutral theory (which made genetic drift a key factor) spawned perennial debates over the proper balance of adaptationism and contingency in evolutionary theory.

In the 1970s Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibrium which holds that stasis is the most prominent feature of the fossil record, and that most evolutionary changes occur rapidly over relatively short periods of time. In 1980 Luis Alvarez and Walter Alvarez proposed the hypothesis that an impact event was responsible for the Cretaceous-Paleogene extinction event. Also in the early 1980s, statistical analysis of the fossil record of marine organisms published by Jack Sepkoski and David M. Raup led to a better appreciation of the importance of mass extinction events to the history of life on earth.

By the end of the 19th century all of the major pathways of drug metabolism had been discovered, along with the outlines of protein and fatty acid metabolism and urea synthesis. In the early decades of the 20th century, the minor components of foods in human nutrition, the vitamins, began to be isolated and synthesized. Improved laboratory techniques such as chromatography and electrophoresis led to rapid advances in physiological chemistry, which — as biochemistry — began to achieve independence from its medical origins. In the 1920s and 1930s, biochemists — led by Hans Krebs and Carl and Gerty Cori — began to work out many of the central metabolic pathways of life: the citric acid cycle, glycogenesis and glycolysis, and the synthesis of steroids and porphyrins. Between the 1930s and 1950s, Fritz Lipmann and others established the role of ATP as the universal carrier of energy in the cell, and mitochondria as the powerhouse of the cell. Such traditionally biochemical work continued to be very actively pursued throughout the 20th century and into the 21st.

Following the rise of classical genetics, many biologists—including a new wave of physical scientists in biology—pursued the question of the gene and its physical nature. Warren Weaver—head of the science division of the Rockefeller Foundation—issued grants to promote research that applied the methods of physics and chemistry to basic biological problems, coining the term molecular biology for this approach in 1938; many of the significant biological breakthroughs of the 1930s and 1940s were funded by the Rockefeller Foundation.

Like biochemistry, the overlapping disciplines of bacteriology and virology (later combined as microbiology), situated between science and medicine, developed rapidly in the early 20th century. Félix d'Herelle's isolation of bacteriophage during World War I initiated a long line of research focused on phage viruses and the bacteria they infect.

The development of standard, genetically uniform organisms that could produce repeatable experimental results was essential for the development of molecular genetics. After early work with Drosophila and maize, the adoption of simpler model systems like the bread mold Neurospora crassa made it possible to connect genetics to biochemistry, most importantly with Beadle and Tatum's one gene-one enzyme hypothesis in 1941. Genetics experiments on even simpler systems like tobacco mosaic virus and bacteriophage, aided by the new technologies of electron microscopy and ultracentrifugation, forced scientists to re-evaluate the literal meaning of life; virus heredity and reproducing nucleoprotein cell structures outside the nucleus ("plasmagenes") complicated the accepted Mendelian-chromosome theory.

Oswald Avery showed in 1943 that DNA was likely the genetic material of the chromosome, not its protein; the issue was settled decisively with the 1952 Hershey–Chase experiment—one of many contributions from the so-called phage group centered around physicist-turned-biologist Max Delbrück. In 1953 James Watson and Francis Crick, building on the work of Maurice Wilkins and Rosalind Franklin, suggested that the structure of DNA was a double helix. In their famous paper "Molecular structure of Nucleic Acids", Watson and Crick noted coyly, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." After the 1958 Meselson-Stahl experiment confirmed the semiconservative replication of DNA, it was clear to most biologists that nucleic acid sequence must somehow determine amino acid sequence in proteins; physicist George Gamow proposed that a fixed genetic code connected proteins and DNA. Between 1953 and 1961, there were few known biological sequences — either DNA or protein — but an abundance of proposed code systems, a situation made even more complicated by expanding knowledge of the intermediate role of RNA. To actually decipher the code, it took an extensive series of experiments in biochemistry and bacterial genetics, between 1961 and 1966 — most importantly the work of Nirenberg and Khorana.

In addition to the Division of Biology at Caltech, the Laboratory of Molecular Biology (and its precursors) at Cambridge, and a handful of other institutions, the Pasteur Institute became a major center for molecular biology research in the late 1950s. Scientists at Cambridge, led by Max Perutz and John Kendrew, focused on the rapidly developing field of structural biology, combining X-ray crystallography with Molecular modelling and the new computational possibilities of digital computing (benefiting both directly and indirectly from the military funding of science). A number of biochemists led by Frederick Sanger later joined the Cambridge lab, bringing together the study of macromolecular structure and function. At the Pasteur Institute, François Jacob and Jacques Monod followed the 1959 PaJaMo experiment with a series of publications regarding the lac operon that established the concept of gene regulation and identified what came to be known as messenger RNA. By the mid-1960s, the intellectual core of molecular biology — a model for the molecular basis of metabolism and reproduction — was largely complete.

The late 1950s to the early 1970s was a period of intense research and institutional expansion for molecular biology, which had only recently become a somewhat coherent discipline. In what organismic biologist E. O. Wilson called "The Molecular Wars", the methods and practitioners of molecular biology spread rapidly, often coming to dominate departments and even entire disciplines. Molecularization was particularly important in genetics, immunology, embryology, and neurobiology, while the idea that life is controlled by a "genetic program"—a metaphor Jacob and Monod introduced from the emerging fields of cybernetics and computer science—became an influential perspective throughout biology. Immunology in particular became linked with molecular biology, with innovation flowing both ways: the clonal selection theory developed by Niels Jerne and Frank Macfarlane Burnet in the mid-1950s helped shed light on the general mechanisms of protein synthesis.

Resistance to the growing influence of molecular biology was especially evident in evolutionary biology. Protein sequencing had great potential for the quantitative study of evolution (through the molecular clock hypothesis), but leading evolutionary biologists questioned the relevance of molecular biology for answering the big questions of evolutionary causation. Departments and disciplines fractured as organismic biologists asserted their importance and independence: Theodosius Dobzhansky made the famous statement that "nothing in biology makes sense except in the light of evolution" as a response to the molecular challenge. The issue became even more critical after 1968; Motoo Kimura's neutral theory of molecular evolution suggested that natural selection was not the ubiquitous cause of evolution, at least at the molecular level, and that molecular evolution might be a fundamentally different process from morphological evolution. (Resolving this "molecular/morphological paradox" has been a central focus of molecular evolution research since the

Biotechnology in the general sense has been an important part of biology since the late 19th century. With the industrialization of brewing and agriculture, chemists and biologists became aware of the great potential of human-controlled biological processes. In particular, fermentation proved a great boon to chemical industries. By the early 1970s, a wide range of biotechnologies were being developed, from drugs like penicillin and steroids to foods like Chlorella and single-cell protein to gasohol — as well as a wide range of hybrid high-yield crops and agricultural technologies, the basis for the Green Revolution.

Biotechnology in the modern sense of genetic engineering began in the 1970s, with the invention of recombinant DNA techniques. Restriction enzymes were discovered and characterized in the late 1960s, following on the heels of the isolation, then duplication, then synthesis of viral genes. Beginning with the lab of Paul Berg in 1972 (aided by EcoRI from Herbert Boyer's lab, building on work with ligase by Arthur Kornberg's lab), molecular biologists put these pieces together to produce the first transgenic organisms. Soon after, others began using plasmid vectors and adding genes for antibiotic resistance, greatly increasing the reach of the recombinant techniques.

Wary of the potential dangers (particularly the possibility of a prolific bacteria with a viral cancer-causing gene), the scientific community as well as a wide range of scientific outsiders reacted to these developments with both enthusiasm and fearful restraint. Prominent molecular biologists led by Berg suggested a temporary moratorium on recombinant DNA research until the dangers could be assessed and policies could be created. This moratorium was largely respected, until the participants in the 1975 Asilomar Conference on Recombinant DNA created policy recommendations and concluded that the technology could be used safely.

Following Asilomar, new genetic engineering techniques and applications developed rapidly. DNA sequencing methods improved greatly (pioneered by Frederick Sanger and Walter Gilbert), as did oligonucleotide synthesis and transfection techniques. Researchers learned to control the expression of transgenes, and were soon racing — in both academic and industrial contexts — to create organisms capable of expressing human genes for the production of human hormones. However, this was a more daunting task than molecular biologists had expected; developments between 1977 and 1980 showed that, due to the phenomena of split genes and splicing, higher organisms had a much more complex system of gene expression than the bacteria models of earlier studies. The first such race, for synthesizing human insulin, was won by Genentech. This marked the beginning of the biotech boom (and with it, the era of gene patents), with an unprecedented level of overlap between biology, industry, and law.

By the 1980s, protein sequencing had already transformed methods of scientific classification of organisms (especially cladistics) but biologists soon began to use RNA and DNA sequences as characters; this expanded the significance of molecular evolution within evolutionary biology, as the results of molecular systematics could be compared with traditional evolutionary trees based on morphology. Following the pioneering ideas of Lynn Margulis on endosymbiotic theory, which holds that some of the organelles of eukaryotic cells originated from free living prokaryotic organisms through symbiotic relationships, even the overall division of the tree of life was revised. Into the 1990s, the five domains (Plants, Animals, Fungi, Protists, and Monerans) became three (the Archaea, the Bacteria, and the Eukarya) based on Carl Woese's pioneering molecular systematics work with 16S rRNA sequencing.

The development and popularization of the polymerase chain reaction (PCR) in mid-1980s (by Kary Mullis and others at Cetus Corp.) marked another watershed in the history of modern biotechnology, greatly increasing the ease and speed of genetic analysis.Coupled with the use of expressed sequence tags, PCR led to the discovery of many more genes than could be found through traditional biochemical or genetic methods and opened the possibility of sequencing entire genomes.

The unity of much of the morphogenesis of organisms from fertilized egg to adult began to be unraveled after the discovery of the homeobox genes, first in fruit flies, then in other insects and animals, including humans. These developments led to advances in the field of evolutionary developmental biology towards understanding how the various body plans of the animal phyla have evolved and how they are related to one another.

The Human Genome Project — the largest, most costly single biological study ever undertaken — began in 1988 under the leadership of James D. Watson, after preliminary work with genetically simpler model organisms such as E. coli, S. cerevisiae and C. elegans. Shotgun sequencing and gene discovery methods pioneered by Craig Venter — and fueled by the financial promise of gene patents with Celera Genomics — led to a public–private sequencing competition that ended in compromise with the first draft of the human DNA sequence announced in 2000.

At the beginning of the 21st century, biological sciences converged with previously differentiated new and classic disciplines like Physics into research fields like Biophysics. Advances were made in analytical chemistry and physics instrumentation including improved sensors, optics, tracers, instrumentation, signal processing, networks, robots, satellites, and compute power for data collection, storage, analysis, modeling, visualization, and simulations. These technology advances allowed theoretical and experimental research including internet publication of molecular biochemistry, biological systems, and ecosystems science. This enabled worldwide access to better measurements, theoretical models, complex simulations, theory predictive model experimentation, analysis, worldwide internet observational data reporting, open peer-review, collaboration, and internet publication. New fields of biological sciences research emerged including Bioinformatics, Neuroscience, Theoretical biology, Computational genomics, Astrobiology and Synthetic Biology.

• c. 520 BC – Alcmaeon of Croton distinguished veins from arteries and discovered the optic nerve.
• c. 450 BC – Sushruta wrote the Sushruta Samhita, redacted versions of which, by the third century AD, describe over 120 surgical instruments and 300 surgical procedures, classify human surgery into eight categories, and introduce cosmetic surgery.
• c. 450 BC – Xenophanes examined fossils and a speculated on the evolution of life.
• c. 380 BC – Diocles wrote the oldest known anatomy book and was the first to use the term anatomy.
• c. 350 BC – Aristotle attempted a comprehensive classification of animals. His written works include Historion Animalium, a general biology of animals, De Partibus Animalium, a comparative anatomy and physiology of animals, and De Generatione Animalium, on developmental biology.
• c. 300 BC – Theophrastos (or Theophrastus) began the systematic study of botany.
• c. 300 BC – Herophilos dissected the human body.
• c. 50–70 AD – Historia Naturalis by Pliny the Elder (Gaius Plinius Secundus) was published in 37 volumes.
• 130–200 – Claudius Galen wrote numerous treatises on human anatomy.
• c. 1010 – Avicenna (Abu Ali al Hussein ibn Abdallah ibn Sina) published The Canon of Medicine.
• 1543 – Andreas Vesalius publishes the anatomy treatise De humani corporis fabrica.

• ?? – Jan Baptist van Helmont performed his famous tree plant experiment in which he shows that the substance of a plant derives from water, a forerunner of the discovery of photosynthesis.
• 1628 – William Harvey published An Anatomical Exercise on the Motion of the Heart and Blood in Animals
• 1651 – William Harvey concluded that all animals, including mammals, develop from eggs, and spontaneous generation of any animal from mud or excrement was an impossibility.
• 1665 – Robert Hooke saw cells in cork using a microscope.
• In 1661, 1664 and 1665, the blood cells were discerned by Marcello Malpighi. In 1678, the red blood corpuscles was described by Jan Swammerdam of Amsterdam, a Dutch naturalist and physician. The first complete account of the red cells was made by Anthony van Leeuwenhoek of Delft in the last quarter of the 17th century.
• 1668 – Francesco Redi disproved spontaneous generation by showing that fly maggots only appear on pieces of meat in jars if the jars are open to the air. Jars covered with cheesecloth contained no flies.
• 1672 – Marcello Malpighi published the first description of chick development, including the formation of muscle somites, circulation, and nervous system.
• 1676 – Anton van Leeuwenhoek observed protozoa and calls them animalcules.
• 1677 – Anton van Leeuwenhoek observed spermatozoa.
• 1683 – Anton van Leeuwenhoek observed bacteria. Leeuwenhoek's discoveries renew the question of spontaneous generation in microorganisms.

• 1767 – Kaspar Friedrich Wolff argued that the tissues of a developing chick form from nothing and are not simply elaborations of already-present structures in the egg.
• 1768 – Lazzaro Spallanzani again disproved spontaneous generation by showing that no organisms grow in a rich broth if it is first heated (to kill any organisms) and allowed to cool in a stoppered flask. He also showed that fertilization in mammals requires an egg and semen.
• 1771 – Joseph Priestley demonstrated that plants produce a gas that animals and flames consume. Those two gases are carbon dioxide and oxygen.
• 1798 – Thomas Malthus discussed human population growth and food production in An Essay on the Principle of Population.

• 1801 – Jean-Baptiste Lamarck began the detailed study of invertebrate taxonomy.
• 1802 – The term biology in its modern sense was propounded independently by Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur) and Lamarck (Hydrogéologie). The word was coined in 1800 by Karl Friedrich Burdach.
• 1809 – Lamarck proposed a modern theory of evolution based on the inheritance of acquired characteristics.
• 1817 – Pierre-Joseph Pelletier and Joseph Bienaimé Caventou isolated chlorophyll.
• 1820 – Christian Friedrich Nasse formulated Nasse's law: hemophilia occurs only in males and is passed on by unaffected females.
• 1824 – J. L Prevost and J. B. Dumas showed that the sperm in semen were not parasites, as previously thought, but, instead, the agents of fertilization.
• 1826 – Karl von Baer showed that the eggs of mammals are in the ovaries, ending a 200-year search for the mammalian egg.
• 1828 – Friedrich Woehler synthesized urea; first synthesis of an organic compound from inorganic starting materials.
• 1836 – Theodor Schwann discovered pepsin in extracts from the stomach lining; first isolation of an animal enzyme.
• 1837 – Theodor Schwann showeds that heating air will prevent it from causing putrefaction.
• 1838 – Matthias Schleiden proposed that all plants are composed of cells.
• 1839 – Theodor Schwann proposed that all animal tissues are composed of cells. Schwann and Schleinden argued that cells are the elementary particles of life.
• 1843 – Martin Barry reported the fusion of a sperm and an egg for rabbits in a 1-page paper in the Philosophical Transactions of the Royal Society of London.
• 1856 – Louis Pasteur stated that microorganisms produce fermentation.
• 1858 – Charles R. Darwin and Alfred Wallace independently proposed a theory of biological evolution ("descent through modification") by means of natural selection. Only in later editions of his works did Darwin used the term "evolution."
• 1858 – Rudolf Virchow proposed that cells can only arise from pre-existing cells; "Omnis cellula e celulla," all cell from cells. The Cell Theory states that all organisms are composed of cells (Schleiden and Schwann), and cells can only come from other cells (Virchow).
• 1864 – Louis Pasteur disproved the spontaneous generation of cellular life.
• 1865 – Gregor Mendel demonstrated in pea plants that inheritance follows definite rules. The Principle of Segregation states that each organism has two genes per trait, which segregate when the organism makes eggs or sperm. The Principle of Independent Assortment states that each gene in a pair is distributed independently during the formation of eggs or sperm. Mendel's trailblazing foundation for the science of genetics went unnoticed, to his lasting disappointment.
• 1865 – Friedrich August Kekulé von Stradonitz realized that benzene is composed of carbon and hydrogen atoms in a hexagonal ring.
• 1869 – Friedrich Miescher discovered nucleic acids in the nuclei of cells.
• 1874 – Jacobus van 't Hoff and Joseph-Achille Le Bel advanced a three-dimensional stereochemical representation of organic molecules and propose a tetrahedral carbon atom.
• 1876 – Oskar Hertwig and Hermann Fol independently described (in sea urchin eggs) the entry of sperm into the egg and the subsequent fusion of the egg and sperm nuclei to form a single new nucleus.
• 1884 – Emil Fischer began his detailed analysis of the compositions and structures of sugars.
• 1892 – Hans Driesch separated the individual cells of a 2-cell sea urchin embryo and shows that each cell develops into a complete individual, thus disproving the theory of preformation and showing that each cell is "totipotent," containing all the hereditary information necessary to form an individual.
• 1898 – Martinus Beijerinck used filtering experiments to show that tobacco mosaic disease is caused by something smaller than a bacterium, which he names a virus.

• 1900 – Hugo de Vries, Carl Correns and Erich von Tschermak independently rediscovered Mendel's paper on heredity.
• 1902 – Walter Sutton and Theodor Boveri, independently proposed that the chromosomes carry the hereditary information.
• 1905 – William Bateson coined the term "genetics" to describe the study of biological inheritance.
• 1906 – Mikhail Tsvet discovered the chromatography technique for organic compound separation.
• 1907 – Ivan Pavlov demonstrated conditioned responses with salivating dogs.
• 1907 – Hermann Emil Fischer artificially synthesized peptide amino acid chains and thereby shows that amino acids in proteins are connected by amino group-acid group bonds.
• 1909 – Wilhelm Johannsen coined the word "gene."
• 1911 – Thomas Hunt Morgan proposed that genes are arranged in a line on the chromosomes.
• 1922 – Aleksandr Oparin proposed that the Earth's early atmosphere contained methane, ammonia, hydrogen, and water vapor, and that these were the raw materials for the origin of life.
• 1926 – James B. Sumner showed that the urease enzyme is a protein.
• 1928 – Otto Diels and Kurt Alder discovered the Diels-Alder cycloaddition reaction for forming ring molecules.
• 1928 – Alexander Fleming discovered the first antibiotic, penicillin
• 1929 – Phoebus Levene discovered the sugar deoxyribose in nucleic acids.
• 1929 – Edward Doisy and Adolf Butenandt independently discovered estrone.
• 1930 – John Howard Northrop showed that the pepsin enzyme is a protein.
• 1931 – Adolf Butenandt discovered androsterone.
• 1932 – Hans Adolf Krebs discovered the urea cycle.
• 1933 – Tadeus Reichstein artificially synthesized vitamin C; first vitamin synthesis.
• 1935 – Rudolf Schoenheimer used deuterium as a tracer to examine the fat storage system of rats.
• 1935 – Wendell Stanley crystallized the tobacco mosaic virus.
• 1935 – Konrad Lorenz described the imprinting behavior of young birds.
• 1937 – Dorothy Crowfoot Hodgkin discovered the three-dimensional structure of cholesterol.
• 1937 – Hans Adolf Krebs discovered the tricarboxylic acid cycle.
• 1937 – In Genetics and the Origin of Species, Theodosius Dobzhansky applies the chromosome theory and population genetics to natural populations in the first mature work of neo-Darwinism, also called the modern synthesis, a term coined by Julian Huxley.
• 1938 – Marjorie Courtenay-Latimer discovered a living coelacanth off the coast of southern Africa.
• 1940 – Donald Griffin and Robert Galambos announced their discovery of echolocation by bats.
• 1942 – Max Delbrück and Salvador Luria demonstrated that bacterial resistance to virus infection is caused by random mutation and not adaptive change.
• 1944 – Oswald Avery shows that DNA carried the hereditary information in pneumococcus bacteria.
• 1944 – Robert Burns Woodward and William von Eggers Doering synthesized quinine.
• 1945 – Dorothy Crowfoot Hodgkin discovered the three-dimensional structure of penicillin.
• 1948 – Erwin Chargaff showed that in DNA the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units.

• 1951 – The research group of Robert Robinson with John Cornforth (Oxford University) publishes their synthesis of cholesterol, while Robert Woodward (Harvard University) publishes his synthesis of cortisone.
• 1951 – Fred Sanger, Hans Tuppy, and Ted Thompson completed their chromatographic analysis of the insulin amino acid sequence.
• 1952 – American developmental biologists Robert Briggs and Thomas King cloned the first vertebrate by transplanting nuclei from leopard frogs embryos into enucleated eggs. More differentiated cells were the less able they are to direct development in the enucleated egg.
• 1952 – Alfred Hershey and Martha Chase showed that DNA is the genetic material in bacteriophage viruses.
• 1952 – Rosalind Franklin concluded that DNA is a double helix with a diameter of 2 nm and the sugar-phosphate backbones on the outside of the helix, based on x ray diffraction studies. She suspected the two sugar-phosphate backbones have a peculiar relationship to each other.
• 1953 – After examining Franklin's unpublished data, James D. Watson and Francis Crick published a double-helix structure for DNA, with one sugar-phosphate backbone running in the opposite direction to the other. They further suggested a mechanism by which the molecule can replicate itself and serve to transmit genetic information. Their paper, combined with the Hershey-Chase experiment and Chargaff's data on nucleotides, finally persuaded biologists that DNA is the genetic material, not protein.
• 1953 – Stanley Miller showed that amino acids can be formed when simulated lightning is passed through vessels containing water, methane, ammonia, and hydrogen
• 1954 – Dorothy Crowfoot Hodgkin discovered the three-dimensional structure of vitamin B12.
• 1955 – Marianne Grunberg-Manago and Severo Ochoa discovered the first nucleic-acid-synthesizing enzyme (polynucleotide phosphorylase), which links nucleotides together into polynucleotides.
• 1955 – Arthur Kornberg discovered DNA polymerase enzymes.
• 1958 – John Gurdon used nuclear transplantation to clone an African Clawed Frog; first cloning of a vertebrate using a nucleus from a fully differentiated adult cell.
• 1958 – Matthew Stanley Meselson and Franklin W. Stahl proved that DNA replication is semiconservative in the Meselson-Stahl experiment
• 1959 – Max Perutz comes up with a model for the structure of oxygenated hemoglobin.
• 1959 – Severo Ochoa and Arthur Kornberg received the Nobel Prize for their work.
• 1960 – John Kendrew described the structure of myoglobin, the oxygen-carrying protein in muscle.
• 1960 – Four separate researchers (S. Weiss, J. Hurwitz, Audrey Stevens and J. Bonner) discovered bacterial RNA polymerase, which polymerizes nucleotides under the direction of DNA.
• 1960 – Robert Woodward synthesized chlorophyll.
• 1961 – J. Heinrich Matthaei cracked the first codon of the genetic code (the codon for the amino acid phenylalanine) using Grunberg-Manago's 1955 enzyme system for making polynucleotides.
• 1961 – Joan Oró found that concentrated solutions of ammonium cyanide in water can produce the nucleotide adenine, a discovery that opened the way for theories on the origin of life.
• 1962 – Max Perutz and John Kendrew shared the Nobel prize for their work on the structure of hemoglobin and myoglobin.
• 1966 – Genetic code fully cracked through trial-and-error experimental work.(Tobin, Allan, and Dusheck, Jennie, (1998), Asking About Life,Saunders College Publishing, pg 267, ISBN 003072046X)
• 1966 – Kimishige Ishizaka discovered a new type of immunoglobulin, IgE, that develops allergy and explains the mechanisms of allergy at molecular and cellular levels.
• 1966 – Lynn Margulis proposed the endosymbiotic theory, that the eukaryotic cell is a symbiotic union of primitive prokaryotic cells. Richard Dawkins called the theory "one of the great achievements of twentieth-century evolutionary biology."
• 1968 – Fred Sanger used radioactive phosphorus as a tracer to chromatographically decipher a 120 base long RNA sequence.
• 1969 – Dorothy Crowfoot Hodgkin deciphered the three-dimensional structure of insulin.
• 1970 – Hamilton Smith and Daniel Nathans discovered DNA restriction enzymes.
• 1970 – Howard Temin and David Baltimore independently discovered reverse transcriptase enzymes.
• 1972 – Albert Eschenmoser and Robert Woodward synthesized vitamin B12.
• 1972 – Stephen Jay Gould and Niles Eldredge proposed an idea they call "punctuated equilibrium", which states that the fossil record is an accurate depiction of the pace of evolution, with long periods of "stasis" (little change) punctuated by brief periods of rapid change and species formation (within a lineage).
• 1972 – Seymour Jonathan Singer and Garth L. Nicholson developed the fluid mosaic model, which deals with the make-up of the membrane of all cells.
• 1974 – Manfred Eigen and Manfred Sumper showed that mixtures of nucleotide monomers and RNA replicase will give rise to RNA molecules which replicate, mutate, and evolve.
• 1974 – Leslie Orgel showed that RNA can replicate without RNA-replicase and that zinc aids this replication.
• 1977 – John Corliss and ten coauthors discovered chemosynthetically based animal communities located around submarine hydrothermal vents on the Galapagos Rift.
• 1977 – Walter Gilbert and Allan Maxam present a rapid DNA sequencing technique which uses cloning, base destroying chemicals, and gel electrophoresis.
• 1977 – Frederick Sanger and Alan Coulson presented a rapid gene sequencing technique which uses dideoxynucleotides and gel electrophoresis.
• 1978 – Frederick Sanger presented the 5,386 base sequence for the virus PhiX174; first sequencing of an entire genome.
• 1982 – Stanley B. Prusiner proposed the existence of infectious proteins, or prions. His idea is widely derided in the scientific community, but he wins a Nobel Prize in 1997.
• 1983 – Kary Mullis invented "PCR" ( polymerase chain reaction), an automated method for rapidly copying sequences of DNA.
• 1984 – Alec Jeffreys devised a genetic fingerprinting method.
• 1985 – Harry Kroto, J.R. Heath, S.C. O'Brien, R.F. Curl, and Richard Smalley discovered the unusual stability of the buckminsterfullerene molecule and deduce its structure.
• 1986 – Alexander Klibanov demonstrated that enzymes can function in non-aqueous environments.
• 1986 – Rita Levi-Montalcini and Stanley Cohen received the Nobel Prize in Physiology or Medicine for their discovery of Nerve growth factor (NGF).

• 1990 – French Anderson et al. performed the first approved gene therapy on a human patient
• 1990 – Napoli, Lemieux and Jorgensen discovered RNA interference (1990) during experiments aimed at the color of petunias.
• 1990 – Wolfgang Krätschmer, Lowell Lamb, Konstantinos Fostiropoulos, and Donald Huffman discovered that Buckminsterfullerene can be separated from soot because it is soluble in benzene.
• 1995 – Publication of the first complete genome of a free-living organism.
• 1996 – Dolly the sheep was first clone of an adult mammal.
• 1998 – Mello and Fire publish their work on RNAi in c.elegans, for which they shared the 2006 Nobel Prize in Physiology or Medicine.
• 1999 – Researchers at the Institute for Human Gene Therapy at the University of Pennsylvania accidentally kill Jesse Gelsinger during a clinical trial of a gene therapy technique, leading the FDA to halt further gene therapy trials at the Institute.
• 2001 – Publication of the first drafts of the complete human genome (see Craig Venter).
• 2002 – First virus produced 'from scratch', an artificial polio virus that paralyzes and kills mice.
• 2007 – Commercialization of Illumina Next generation Sequencing tools. This has become the most popular high-throughput sequencing system.
• 2012 – Use of CRISPR-Cas9 as a DNA-editing biotechnology tool.