Biofelsefe — Yeryüzünün Tarihi
NFA 2020 / Aziz Yardımlı


Biofelsefe — Yeryüzünün Tarihi


Artist's conception of a protoplanetary disk.

Artist's impression of the Hadean Eon.

📂 Hadean (W)

Hadean (W)

The Hadean is a geologic eon of the Earth pre-dating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the International Commission on Stratigraphy (ICS), 4 billion years ago. As of 2016, the ICS describes its status as "informal". Geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term, the "Priscoan period", from priscus, the Latin word for 'ancient'. Other, older texts refer to the eon as the Pre-Archean.
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"Hadean" (from Hades, the Greek god of the underworld, and the underworld itself) describes the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies.

Since few geological traces of this eon remain on Earth, there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean, so these are sometimes used in an informal sense to refer to the same periods of time on Earth.

The Lunar divisions are:

In 2010, an alternative scale was proposed that includes the addition of the Chaotian and Prenephelean Eons preceding the Hadean, and divides the Hadean into three eras with two periods each. The Paleohadean era consists of the Hephaestean (4.5–4.4 Ga) and the Jacobian periods (4.4–4.3 Ga). The Mesohadean is divided into the Canadian (4.3–4.2 Ga) and the Procrustean periods (4.2–4.1 Ga). The Neohadean is divided into the Acastan (4.1–4.0 Ga) and the Promethean periods (4.0–3.9 Ga). As of February 2017, this has not been adopted by the IUGS.

Hadean rocks

Further information: Oldest dated rocks

In the last decades of the 20th century geologists identified a few Hadean rocks from western Greenland, northwestern Canada, and Western Australia. In 2015, traces of carbon minerals interpreted as "remains of biotic life" were found in 4.1-billion-year-old rocks in Western Australia.

The oldest dated zircon crystals, enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the Narryer Gneiss Terrane of Western Australia, date to 4.404 ± 0.008 Ga. This zircon is a slight outlier, with the oldest consistently-dated zircon falling closer to 4.35 Ga — around 200 million years after the hypothesized time of the Earth's formation.

In many other areas, xenocryst (or relict) Hadean zircons enclosed in older rocks indicate that younger rocks have formed on older terranes and have incorporated some of the older material. One example occurs in the Guiana shield from the Iwokrama Formation of southern Guyana where zircon cores have been dated at 4.22 Ga.

Atmosphere and oceans

A sizable quantity of water would have been in the material that formed the Earth. Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. Hydrogen and helium are expected to continually escape (even to the present day) due to atmospheric escape.

Part of the ancient planet is theorized to have been disrupted by the impact that created the Moon, which should have caused melting of one or two large regions of the Earth. Earth's present composition suggests that there was not complete remelting as it is difficult to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapor. Liquid water oceans existed despite the surface temperature of 230 °C (446 °F) because at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO2 atmosphere, water is still liquid. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.

Studies of zircons have found that liquid water must have existed as long ago as 4.4 billion years ago, very soon after the formation of the Earth. This requires the presence of an atmosphere. The cool early Earth theory covers a range from about 4.4 to about 4.1 billion years.

A September 2008 study of zircons found that Australian Hadean rock holds minerals pointing to the existence of plate tectonics as early as 4 billion years ago (approximately 600 million years after Earth's formation). If this is true, the time when Earth finished its transition from having a hot, molten surface and atmosphere full of carbon dioxide, to being very much like it is today, can be roughly dated to about 4.0 billion years ago. The actions of plate tectonics and the oceans trapped vast amounts of carbon dioxide, thereby reducing the greenhouse effect and leading to a much cooler surface temperature and the formation of solid rock, and possibly even life.

Hadean Eon
4000 Ma and earlier.

Date Event
4600 Ma The planet Earth forms from the accretion disc revolving around the young Sun, with organic compounds (complex organic molecules) necessary for life having perhaps formed in the protoplanetary disk of cosmic dust grains surrounding it before the formation of the Earth itself.
4500 Ma According to the giant impact hypothesis, the Moon originated when the planet Earth and the hypothesized planet Theia collided, sending a very large number of moonlets into orbit around the young Earth which eventually coalesced to form the Moon. The gravitational pull of the new Moon stabilised the Earth's fluctuating axis of rotation and set up the conditions in which abiogenesis could occur.
4400 Ma First appearance of liquid water on Earth.
4374 Ma The age of the oldest discovered zircon crystals.
4280 Ma Earliest possible appearance of life on Earth.



Artist's impression of the Archean Eon.

📂 Archean (W)

Archean (W)

The Archean Eon (also spelled Archaean or Archæan) is one of the four geologic eons of Earth's history, occurring 4,000 to 2,500 million years ago (4 to 2.5 Gya). During the Archean, the Earth's crust had cooled enough to allow the formation of continents and the beginning of life on Earth.
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Etymology and changes in classification

The word 'Archean' comes from the ancient Greek word Αρχή (Arkhē), meaning ‘beginning, origin.’ It was first used in 1872, when it meant "of the earliest geological age." Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540 million years ago (Mya) until 2,500 Mya.

Instead of being based on stratigraphy, the beginning and end of the Archean Eon are defined chronometrically. The eon's lower boundary or starting point of 4 Gya (4 billion years ago) is officially recognized by the International Commission on Stratigraphy.[


When the Archean began, the Earth’s heat flow was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, from the formation of the metallic core, and from the decay of radioactive elements.

Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in Greenland, Siberia, the Canadian Shield, Montana and Wyoming (exposed parts of the Wyoming Craton), the Baltic Shield, the Rhodope Massif, Scotland, India, Brazil, western Australia, and southern Africa. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids. Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks. The metamorphosed igneous rocks were derived from volcanic island arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, being both types of metamorphosed rock, represent sutures between the protocontinents.

The Earth's continents started to form in the Archean, although details about their formation are still being debated, due to lack of extensive geological evidence. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma. Although the first continents formed during this eon, rock of this age makes up only 7% of the present world's cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5–40% of the present area of continents formed during the Archean.

By the end of the Archean around 2500 Ma (2.5 Gya), plate tectonic activity may have been similar to that of the modern Earth. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed.


The Archean atmosphere is thought to have nearly lacked free oxygen. Astronomers think that the Sun had about 70–75 percent of the present luminosity, yet temperatures on Earth appear to have been near modern levels after only 500 Ma of Earth's formation (the faint young Sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.

Early life

The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon.

The earliest evidence for life on Earth are graphite of biogenic origin found in 3.7-billion-year-old metasedimentary rocks discovered in Western Greenland.

The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia. Stromatolites are found throughout the Archean and become common late in the Archean. Cyanobacteria were instrumental in creating free oxygen in the atmosphere.

Further evidence for early life is found in 3.47 billion-year-old baryte, in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%, which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34.

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation.

Earth was very hostile to life before 4.2–4.3 Ga and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before the Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon.

Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called Prokaryota. In addition to the domain Bacteria, microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during the Archean without leaving any. Fossil steranes, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.

Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago.

Archean Eon
4000 Ma – 2500 Ma

Date Event
4000 Ma Formation of a greenstone belt of the Acasta Gneiss of the Slave craton in Northwest Territories, Canada, the oldest rock belt in the world.
4100–3800 Ma Late Heavy Bombardment (LHB): extended barrage of impact events upon the inner planets by meteoroids. Thermal flux from widespread hydrothermal activity during the LHB may have been conducive to abiogenesis and life's early diversification. "Remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. This is when life most likely arose.
3900–2500 Ma Cells resembling prokaryotes appear.[24] These first organisms are chemoautotrophs: they use carbon dioxide as a carbon source and oxidize inorganic materials to extract energy. Later, prokaryotes evolve glycolysis, a set of chemical reactions that free the energy of organic molecules such as glucose and store it in the chemical bonds of ATP. Glycolysis (and ATP) continue to be used in almost all organisms, unchanged, to this day.
3800 Ma Formation of a greenstone belt of the Isua complex of the western Greenland region, whose rocks show an isotope frequency suggestive of the presence of life. The earliest evidences for life on Earth are 3.8 billion-year-old biogenic hematite in a banded iron formation of the Nuvvuagittuq Greenstone Belt in Canada, graphite in 3.7 billion-year-old metasedimentary rocks discovered in western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.
3500 Ma Lifetime of the last universal common ancestor (LUCA); the split between bacteria and archaea occurs.

Bacteria develop primitive forms of photosynthesis which at first did not produce oxygen.[34] These organisms generated Adenosine triphosphate (ATP) by exploiting a proton gradient, a mechanism still used in virtually all organisms.

3200 Ma Diversification and expansion of acritarchs.
3000 Ma Photosynthesizing cyanobacteria evolved; they used water as a reducing agent, thereby producing oxygen as a waste product. The oxygen initially oxidizes dissolved iron in the oceans, creating iron ore. The oxygen concentration in the atmosphere slowly rose, acting as a poison for many bacteria and eventually triggering the Great Oxygenation Event. The Moon, still very close to Earth, caused tides 1,000 feet (305 m) high. The Earth was continually wracked by hurricane-force winds. These extreme mixing influences are thought to have stimulated evolutionary processes.
2800 Ma Oldest evidence for microbial life on land in the form of organic matter-rich paleosols, ephemeral ponds and alluvial sequences, some of them bearing microfossils.



Diorama of Ediacaran sealife displayed at the Smithsonian Institution.

📂 Proterozoic (W)

Proterozoic (W)

The Proterozoic is a geological eon spanning the time from the appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two forms of ultimately Greek origin: protero- meaning “former, earlier,” and -zoic, a suffix related to zoe “life.” The Proterozoic Eon extended from 2500 mya to 541 mya (million years ago), and is the most recent part of the Precambrian "supereon." The Proterozoic is the longest eon of the Earth's geologic time scale and it is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.

The well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic; several glaciations, which produced the hypothesized Snowball Earth during the Cryogenian Period in the late Neoproterozoic Era; and the Ediacaran Period (635 to 541 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms and provides us with the first obvious fossil evidence of life on earth.

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The Proterozoic record

The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of those rocks are less metamorphosed than there are Archean ones, and many are unaltered. Studies of these rocks have shown that the eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity (orogeny).

There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during the Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations.

The accumulation of oxygen

One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted. Until roughly 2.3 billion years ago, oxygen was probably only 1% to 2% of its current level. The Banded iron formations, which provide most of the world's iron ore, are one mark of that mineral sink process. Their accumulation ceased after 1.9 billion years ago, after the iron in the oceans had all been oxidized.

Red beds, which are colored by hematite, indicate an increase in atmospheric oxygen 2 billion years ago. Such massive iron oxide formations are not found in older rocks. The oxygen buildup was probably due to two factors: exhaustion of the chemical sinks, and an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere.

Subduction processes

The Proterozoic Eon was a very tectonically active period in the Earth's history. The late Archean Eon to Early Proterozoic Eon corresponds to a period of increasing crustal recycling, suggesting subduction. Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6 Ga. The occurrence of eclogite, (a type of metamorphic rock created by high pressure, > 1 GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor the formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon. As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes.

The long-term tectonic stability of those cratons is why we find continental crust ranging up to a few billion years in age. It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic. Studies by Condie (2000) and Rino et al. (2004) suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period.

Tectonic history (supercontinents)

Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed the deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles (Wilson cycle).

In the late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia. An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after the breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma). The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny.

Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are a few plausible models that explain tectonics of the early Earth prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia).



The first advanced single-celled, eukaryotes and multi-cellular life, preserved as the Francevillian Group Fossils, roughly coincides with the start of the accumulation of free oxygen. This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria. It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved.

The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1200 million years ago.

The earliest fossils possessing features typical of fungi date to the Paleoproterozoic era, some 2,400 million years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis.

Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian Period when the first fossils of animals, including trilobites and archeocyathids, as well as the animal-like Caveasphaera, appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 541 Ma.


Proterozoic Eon
2500 Ma – 542 Ma. Contains the Palaeoproterozoic, Mesoproterozoic and Neoproterozoic eras.

Date Event
2500 Ma Great Oxidation Event led by cyanobacteria's oxygenic photosynthesis. Commencement of plate tectonics with old marine crust dense enough to subduct.
By 1850 Ma Eukaryotic cells appear. Eukaryotes contain membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis. (See Symbiogenesis and Endosymbiont). Bacterial viruses (bacteriophage) emerge before, or soon after, the divergence of the prokaryotic and eukaryotic lineages. The appearance of red beds show that an oxidising atmosphere had been produced. Incentives now favoured the spread of eukaryotic life.
1400 Ma Great increase in stromatolite diversity.
1300 Ma Earliest land fungi
By 1200 Ma Meiosis and sexual reproduction are present in single-celled eukaryotes, and possibly in the common ancestor of all eukaryotes. Sex may even have arisen earlier in the RNA world. Sexual reproduction first appears in the fossil records; it may have increased the rate of evolution.
1000 Ma The first non-marine eukaryotes move onto land. They were photosynthetic and multicellular, indicating that plants evolved much earlier than originally thought.
750 Ma First protozoa (ex: Melanocyrillium); beginning of animal evolution
850–630 Ma A global glaciation may have occurred. Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution. It is believed that this was due to evolution of the first land plants, which increased the amount of oxygen and lowered the amount of carbon dioxide in the atmosphere.
600 Ma The accumulation of atmospheric oxygen allows the formation of an ozone layer. Prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonisation of the land.
580–542 Ma The Ediacara biota represent the first large, complex aquatic multicellular organisms — although their affinities remain a subject of debate.
580–500 Ma Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion.
550 Ma First fossil evidence for Ctenophora (comb jellies), Porifera (sponges), Anthozoa (corals and sea anemones). Appearance of Ikaria wariootia (an early Bilaterian).



Diorama of Ediacaran sealife displayed at the Smithsonian Institution.

📂 Phanerozoic (W)

Phanerozoic (W)

The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present, and began with the Cambrian Period when animals first developed hard shells preserved in the fossil record. The time before the Phanerozoic, called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.

The time span of the Phanerozoic starts with the sudden appearance of fossilized evidence of a number of animal phyla; the evolution of those phyla into diverse forms; the emergence and development of complex plants; the evolution of fish; the emergence of insects and tetrapods; and the development of modern fauna. Plant life on land appeared in the early Phanerozoic eon. During this time span, tectonic forces caused the continents to move and eventually collect into a single landmass known as Pangaea (the most recent supercontinent), which then separated into the current continental landmasses.

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Etymology of the term

Its name was derived from the Ancient Greek words φανερός (phanerós) and ζωή (zōḗ), meaning visible life, since it was once believed that life began in the Cambrian, the first period of this eon. The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick (1876–1953).


Proterozoic-Phanerozoic boundary

The Proterozoic-Phanerozoic boundary is at 541 million years ago. In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan) fossils but several hundred groups (taxa) of metazoa of the earlier Proterozoic era have been identified since the systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and reef-building animals (archaeocyatha) such as corals and others appear; at the first appearance of a complex feeding burrow called Treptichnus pedum; or at the first appearance of a group of small, generally disarticulated, armored forms termed 'the small shelly fauna'. The three different dividing points are within a few million years of each other.

In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature.

📥 Phanerozoic (W)









The Ediacaran Period: Glimpses of the Earth’s Earliest Animals


📹 The Ediacaran Period: Glimpses of the Earth’s Earliest Animals


Cosmic Calendar (W)

Earth’s history with time-spans of the eons to scale (W)


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