Timeline of evolutionary history of life

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Not to be confused with History of evolutionary thought.
This article is about the evolution of all life on Earth. For human evolution specifically, see timeline of human evolution. For more detailed and comprehensive coverage, see Evolutionary history of life.
Visual representation of the history of life on Earth as a spiral
Life Graphical Timeline
view • discuss • edit
-4500 —
-4000 —
-3500 —
-3000 —
-2500 —
-2000 —
-1500 —
-1000 —
-500 —
0 —
Formation
of Earth
Atmospheric oxygen
Modern-looking
humans
Axis scale: millions of years ago.
Dates prior to 1000 million years ago are speculative.

This timeline of evolution of life represents the current scientific theory outlining the major events during the development of life on planet Earth. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, and individual organisms and molecules such as DNA and proteins. The similarities between all present day organisms indicate the presence of a common ancestor from which all known species, living and extinct, have diverged through the process of evolution.

The dates given in this article are estimates based on scientific evidence.

Basic timeline[edit]

In its 4.6 billion years circling the sun, the Earth has harbored an increasing diversity of life forms:

Periodic extinctions have temporarily reduced diversity, eliminating:

Dates are approximate.

Detailed timeline[edit]

In this timeline, Ma (for megaannum) means "million years ago", ka (for kiloannum) means "thousand years ago", and ya means "years ago".

Hadean Eon[edit]

Main article: Hadean

4000 Ma and earlier.

Date Event
4600 Ma The planet Earth forms from the accretion disc revolving around the young Sun; complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.[1]
4500 Ma According to the giant impact hypothesis the Moon is formed when the planet Earth and the planet Theia collide, sending a very large number of moonlets into orbit around the young Earth which eventually coalesce to form the Moon.[2] The gravitational pull of the new Moon stabilises the Earth's fluctuating axis of rotation and sets up the conditions in which life formed.[3]

Archean Eon[edit]

Main article: Archean

4000 Ma – 2500 Ma

Date Event
4000 Ma Formation of Greenstone belt of the Acasta Gneiss of the Great Slave Region, in Canada, the oldest rock belt in the world.[4]
4100–3800 Ma Late Heavy Bombardment: 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 life's emergence and early diversification.[5]
3900–2500 Ma Cells resembling prokaryotes appear.[6] 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.[7][8]
3800 Ma Formation of Greenstone belt of the Isua complex of the western Greenland Region, whose rocks show an isotope frequency suggestive of the presence of life.[4] The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[9] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[10][11]
3500 Ma Lifetime of the last universal ancestor;[12][13] the split between bacteria and archaea occurs.[14]

Bacteria develop primitive forms of photosynthesis which at first do not produce oxygen.[15] These organisms generate ATP by exploiting a proton gradient, a mechanism still used in virtually all organisms.

3000 Ma Photosynthesizing cyanobacteria evolve; they use water as a reducing agent, thereby producing oxygen as a waste product.[16] The oxygen initially oxidizes dissolved iron in the oceans, creating iron ore. The oxygen concentration in the atmosphere slowly rises, acting as a poison for many bacteria. The Moon is still very close to Earth and causes tides 1,000 feet (305 m) high. The Earth is continually wracked by hurricane-force winds. These extreme mixing influences are thought to stimulate evolutionary processes. (See Oxygen catastrophe). Life on land likely developed at this time [17]

Proterozoic Eon[edit]

Main article: Proterozoic

2500 Ma – 542 Ma

Date Event
2500 Ma Great Oxidation Event led by Cyanobacteria's oxygenic photosynthesis.[16] Commencement of plate tectonics with old marine crust dense enough to subduct.[4]
2000 Ma Diversification and expansion of acritarchs.[18]
By 1850 Ma Eukaryotic cells appear. Eukaryotes contain membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis. (See Endosymbiosis). Bacterial viruses (bacteriophage) emerge before, or soon after, the divergence of the prokaryotic and eukaryotic lineages.[19] The appearance of red beds show that an oxidising atmosphere had been produced. Incentives now favoured the spread of eukaryotic life.[20][21][22]
1400 Ma Great increase in stromatolite diversity.
By 1200 Ma Meiosis and sexual reproduction are present in single-celled eukaryotes, and possibly in the common ancestor of all eukaryotes.[23] Sex may even have arisen earlier in the RNA world.[24] Sexual reproduction first appears in the fossil records; it may have increased the rate of evolution.[25]
1200 Ma Simple multicellular organisms evolve, mostly consisting of cell colonies of limited complexity. First multicellular red algae evolve.
1100 Ma Earliest dinoflagellates
1000 Ma First vaucherian algae (ex: Palaeovaucheria)
750 Ma First protozoa (ex: Melanocyrillium)
850–630 Ma A global glaciation may have occurred.[26][27] Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution.[28][29][30]
600 Ma The accumulation of atmospheric oxygen allows the formation of an ozone layer.[31] Prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonisation of the land.[17]
580–542 Ma The Ediacaran biota represent the first large, complex multicellular organisms — although their affinities remain a subject of debate.[32]
580–500 Ma Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion.[33][34]
560 Ma Earliest fungi
550 Ma First fossil evidence for ctenophora (comb jellies), porifera (sponges), and anthozoa (corals & anemones)

Phanerozoic Eon[edit]

Main article: Phanerozoic

542 Ma – present

The Phanerozoic Eon, literally the "period of well-displayed life", marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, which are divided by major mass extinctions.

Paleozoic Era[edit]

Main article: Paleozoic

542 Ma – 251.0 Ma

Date Event
535 Ma Major diversification of living things in the oceans: chordates, arthropods (e.g. trilobites, crustaceans), echinoderms, mollusks, brachiopods, foraminifers and radiolarians, etc.
530 Ma The first known footprints on land date to 530 Ma, indicating that early animal explorations may have predated the development of terrestrial plants.[35]
525 Ma Earliest graptolites.
510 Ma First cephalopods (Nautiloids) and chitons.
505 Ma Fossilization of the Burgess Shale.
485 Ma First vertebrates with true bones (jawless fishes).
450 Ma First complete conodonts and echinoids appear.
440 Ma First agnathan fishes: Heterostraci, Galeaspida, and Pituriaspida.
434 Ma The first primitive plants move onto land,[36] having evolved from green algae living along the edges of lakes.[37] They are accompanied by fungi[citation needed], which may have aided the colonization of land through symbiosis.
420 Ma Earliest ray-finned fishes, trigonotarbid arachnids, and land scorpions.
410 Ma First signs of teeth in fish. Earliest nautiid nautiloids, lycophytes, and trimerophytes.
395 Ma First lichens, stoneworts. Earliest harvestman, mites, hexapods (springtails) and ammonoids. The first known tetrapod tracks on land.
363 Ma By the start of the Carboniferous Period, the Earth begins to be recognisable. Insects roamed the land and would soon take to the skies; sharks swam the oceans as top predators,[38] and vegetation covered the land, with seed-bearing plants and forests soon to flourish.

Four-limbed tetrapods gradually gain adaptations which will help them occupy a terrestrial life-habit.

360 Ma First crabs and ferns. Land flora dominated by seed ferns.
350 Ma First large sharks, ratfishes, and hagfish.
340 Ma Diversification of amphibians.
330 Ma First amniote vertebrates (Paleothyris).
320 Ma Synapsids (pre-cursors to mammals) separate from sauropsids (reptiles) in late Carboniferous.[39]
305 Ma Earliest diapsid reptiles (e.g. Petrolacosaurus).
280 Ma Earliest beetles, seed plants and conifers diversify while lepidodendrids and sphenopsids decrease. Terrestrial temnospondyl amphibians and pelycosaurs (e.g. Dimetrodon) diversify in species.
275 Ma Therapsids separate from synapsids.
251.4 Ma The Permian–Triassic extinction event eliminates over 90-95% of marine species. Terrestrial organisms were not as seriously affected as the marine biota. This "clearing of the slate" may have led to an ensuing diversification, but life on land took 30M years to completely recover.[40]

Mesozoic Era[edit]

Main article: Mesozoic
Date Event
From 251.4 Ma The Mesozoic Marine Revolution begins: increasingly well adapted and diverse predators pressurize sessile marine groups; the "balance of power" in the oceans shifts dramatically as some groups of prey adapt more rapidly and effectively than others.
245 Ma Earliest ichthyosaurs.
240 Ma Increase in diversity of gomphodont cynodonts and rhynchosaurs.
225 Ma Earliest dinosaurs (prosauropods), first cardiid bivalves, diversity in cycads, bennettitaleans, and conifers. First teleost fishes. First mammals (Adelobasileus).
220 Ma Gymnosperm forests dominate the land; herbivores grow to huge sizes to accommodate the large guts necessary to digest the nutrient-poor plants.[citation needed], first flies and turtles (Odontochelys). First Coelophysoid dinosaurs
200 Ma The first accepted evidence for viruses that infect eukaryotic cells (at least, the group Geminiviridae) exists.[41] Viruses are still poorly understood and may have arisen before "life" itself, or may be a more recent phenomenon.

Major extinctions in terrestrial vertebrates and large amphibians. Earliest examples of Ankylosaurian dinosaurs

195 Ma First pterosaurs with specialized feeding (Dorygnathus). First sauropod dinosaurs. Diversification in small, ornithischian dinosaurs: heterodontosaurids, fabrosaurids, and scelidosaurids.
190 Ma Pliosaurs appear in the fossil record. First lepidopteran insects (Archaeolepis), hermit crabs, modern starfish, irregular echinoids, corbulid bivalves, and tubulipore bryozoans. Extensive development of sponge reefs.
176 Ma First members of the Stegosauria group of dinosaurs
170 Ma Earliest salamanders, newts, cryptoclidid & elasmosaurid plesiosaurs, and cladotherian mammals. Sauropod dinosaurs diversify.
165 Ma First rays and glycymeridid bivalves.
163 Ma Pterodactyloid pterosaurs first appear.[42]
161 Ma Ceratopsian dinosaurs appear in the fossil record (Yinlong)
155 Ma First blood-sucking insects (ceratopogonids), rudist bivalves, and cheilostome bryozoans. Archaeopteryx, a possible ancestor to the birds, appears in the fossil record, along with triconodontid and symmetrodont mammals. Diversity in stegosaurian and theropod dinosaurs.
130 Ma The rise of the Angiosperms: These flowering plants boast structures that attract insects and other animals to spread pollen. This innovation causes a major burst of animal evolution through co-evolution. First freshwater pelomedusid turtles.
120 Ma Oldest fossils of heterokonts, including both marine diatoms and silicoflagellates.
115 Ma First monotreme mammals.
110 Ma First hesperornithes, toothed diving birds. Earliest limopsid, verticordiid, and thyasirid bivalves.
106 Ma Spinosaurus, the largest theropod dinosaur, appears in the fossil record.
100 Ma Earliest bees.
90 Ma Extinction of ichthyosaurs. Earliest snakes and nuculanid bivalves. Large diversification in angiosperms: magnoliids, rosids, hamamelidids, monocots, and ginger. Earliest examples of ticks. Probable origins of placental mammals (earliest undisputed fossil evidence is 66 Ma).
80 Ma First ants.
70 Ma Multituberculate mammals increase in diversity. First yoldiid bivalves.
68 Ma Tyrannosaurus, the largest terrestrial predator of North America appears in the fossil record. First species of Triceratops.

Cenozoic Era[edit]

Main article: Cenozoic

66 Ma – present

Date Event
66 Ma The Cretaceous–Paleogene extinction event eradicates about half of all animal species, including mosasaurs, pterosaurs, plesiosaurs, ammonites, belemnites, rudist and inoceramid bivalves, most planktic foraminifers, and all of the dinosaurs excluding their descendants, the birds.[43]
From 66 Ma Rapid dominance of conifers and ginkgos in high latitudes, along with mammals becoming the dominant species. First psammobiid bivalves. Rapid diversification in ants.
63 Ma Evolution of the creodonts, an important group of carnivorous mammals.
60 Ma Diversification of large, flightless birds. Earliest true primates, along with the first semelid bivalves, edentates, carnivorous and lipotyphlan mammals, and owls. The ancestors of the carnivorous mammals (miacids) were alive.
56 Ma Gastornis, a large, flightless bird appears in the fossil record, becoming an apex predator at the time.
55 Ma Modern bird groups diversify (first song birds, parrots, loons, swifts, woodpeckers), first whale (Himalayacetus), earliest rodents, lagomorphs, armadillos, appearance of sirenians, proboscideans, perissodactyl and artiodactyl mammals in the fossil record. Angiosperms diversify. The ancestor (according to theory) of the species in Carcharodon, the early mako shark Isurus hastalis, is alive.
52 Ma First bats appear (Onychonycteris).
50 Ma Peak diversity of dinoflagellates and nanofossils, increase in diversity of anomalodesmatan and heteroconch bivalves, brontotheres, tapirs, rhinoceroses, and camels appear in the fossil record, diversification of primates.
40 Ma Modern-type butterflies and moths appear. Extinction of Gastornis. Basilosaurus, one of the first of the giant whales, appeared in the fossil record.
37 Ma First Nimravid carnivores ("False Saber-toothed Cats") — these species are unrelated to modern-type felines
35 Ma Grasses evolve from among the angiosperms; grasslands begin to expand. Slight increase in diversity of cold-tolerant ostracods and foraminifers, along with major extinctions of gastropods, reptiles, and amphibians. Many modern mammal groups begin to appear: first glyptodonts, ground sloths, dogs, peccaries, and the first eagles and hawks. Diversity in toothed and baleen whales.
33 Ma Evolution of the thylacinid marsupials (Badjcinus).
30 Ma First balanids and eucalypts, extinction of embrithopod and brontothere mammals, earliest pigs and cats.
28 Ma Paraceratherium appears in the fossil record, the largest terrestrial mammal that ever lived.
25 Ma Pelagornis sandersi appears in the fossil record, the largest bird that ever lived.
25 Ma First deer.
20 Ma First giraffes, hyenas, bears and giant anteaters, increase in bird diversity.
15 Ma Mammut appears in the fossil record, first bovids and kangaroos, diversity in Australian megafauna.
10 Ma Grasslands and savannas are established, diversity in insects, especially ants and termites, horses increase in body size and develop high-crowned teeth, major diversification in grassland mammals and snakes.
6.5 Ma First hominin (Sahelanthropus).
6 Ma Australopithecines diversify (Orrorin, Ardipithecus)
5 Ma First tree sloths and hippopotami, diversification of grazing herbivores like zebras and elephants, large carnivorous mammals like lions and dogs, burrowing rodents, kangaroos, birds, and small carnivores, vultures increase in size, decrease in the number of perissodactyl mammals. Extinction of Nimravid carnivores
4.8 Ma Mammoths appear in the fossil record.
4 Ma Evolution of Australopithecus, Stupendemys appears in the fossil record as the largest freshwater turtle, first modern elephants, giraffes, zebras, lions, rhinos and gazelles appear in the fossil record.
3 Ma The Great American Interchange, where various land and freshwater faunas migrated between North and South America. Armadillos, opossums, hummingbirds, and vampire bats traveled to North America while horses, tapirs, saber-toothed cats, and deer entered South America. The first short-faced bears (Arctodus) appear.
2.7 Ma Evolution of Paranthropus
2.5 Ma The earliest species of Smilodon evolve
2 Ma First members of the genus Homo appear in the fossil record. Diversification of conifers in high latitudes. The eventual ancestor of cattle, Bos primigenus evolves in India
1.7 Ma Extinction of australopithecines.
1.2 Ma Evolution of Homo antecessor. The last members of Paranthropus die out.
600 ka Evolution of Homo heidelbergensis
350 ka Evolution of Neanderthals
300 ka Gigantopithecus, a giant relative of the orangutan dies out from Asia
200 ka Anatomically modern humans appear in Africa.[44][45][46] Around 50,000 years before present they start colonising the other continents, replacing the Neanderthals in Europe and other hominins in Asia.
40 ka The last of the giant monitor lizards (Megalania) die out
30 ka Extinction of Neanderthals, first domestic dogs.
15 ka The last Woolly rhinoceros (Coelodonta) are believed to have gone extinct
11 ka The giant short-faced bears (Arctodus) vanish from North America, with the last Giant Ground Sloths dying out. All Equidae become extinct in North America
10 ka The Holocene Epoch starts 10,000[47] years ago after the Late Glacial Maximum. The last mainland species of Woolly mammoth (Mammuthus primigenus) die out, as does the last Smilodon species
Historical extinctions[edit]
Date Event
6000 ya Small populations of American Mastodon die off in places like Utah and Michigan
4500 ya The last members of a dwarf race of Woolly Mammoths vanish from Wrangel Island near Alaska
c. 600 ya (c. 1400) The moa and its predator, Haast's Eagle, die out in New Zealand
387 ya (1627) The last recorded wild Aurochs die out
326 ya (1688) The dodo goes extinct
246 ya (1768) The Steller's sea cow goes extinct
131 ya (1883) The quagga, a subspecies of zebra, goes extinct
100 ya (1914) Martha, last known Passenger Pigeon, dies
78 ya (1936) The Thylacine goes extinct in a Tasmanian zoo, the last member of the family Thylacinidae
62 ya (1952) The Caribbean monk seal goes extinct[48]
6 ya (2008) The Baiji, the Yangtze river dolphin, becomes functionally extinct

See also[edit]

Further reading[edit]

References[edit]

  1. ^ Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Retrieved 30 March 2012. 
  2. ^ Planetary Science Institute page on the Giant Impact Hypothesis. Hartmann and Davis belonged to the PSI. This page also contains several paintings of the impact by Hartmann himself.
  3. ^ "Because the Moon helps stabilize the tilt of the Earth's rotation, it prevents the Earth from wobbling between climatic extremes. Without the Moon, seasonal shifts would likely outpace even the most adaptable forms of life." [1] Astrobiology Magazine. (URL accessed on August 7, 2010)
  4. ^ a b c Bjornerud, Marcia, (2005), "Reading the Rocks: the autobiography of the Earth" (Basic Books
  5. ^ Abramov, Oleg; Mojzsis, Stephen J. last2=Mojzsis (2009). "Microbial habitability of the Hadean Earth during the late heavy bombardment". Nature 459 (7245): 419–422. Bibcode:2009Natur.459..419A. doi:10.1038/nature08015. PMID 19458721. Retrieved May 26, 2013. 
  6. ^ Carl Woese, J Peter Gogarten, "When did eukaryotic cells (cells with nuclei and other internal organelles) first evolve? What do we know about how they evolved from earlier life-forms?" Scientific American, October 21, 1999.
  7. ^ Romano, AH, Conway, T. (1996). "Evolution of carbohydrate metabolic pathways". Res Microbiol 147 (6–7): 448–55. doi:10.1016/0923-2508(96)83998-2. PMID 9084754. 
  8. ^ Knowles JR (1980). "Enzyme-catalyzed phosphoryl transfer reactions". Annu. Rev. Biochem. 49 (1): 877–919. doi:10.1146/annurev.bi.49.070180.004305. PMID 6250450. 
  9. ^ Yoko Ohtomo, Takeshi Kakegawa, Akizumi Ishida, Toshiro Nagase, Minik T. Rosing (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. doi:10.1038/ngeo2025. Retrieved 9 Dec 2013. 
  10. ^ Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013. 
  11. ^ Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology (journal). doi:10.1089/ast.2013.1030. Retrieved 15 November 2013. 
  12. ^ Doolittle, W. Ford (February, 2000). Uprooting the tree of life. Scientific American 282 (6): 90–95.
  13. ^ Nicolas Glansdorff, Ying Xu & Bernard Labedan: The Last Universal Common Ancestor : emergence, constitution and genetic legacy of an elusive forerunner. Biology Direct 2008, 3:29.
  14. ^ Hahn, Jürgen; Pat Haug (1986). "Traces of Archaebacteria in ancient sediments". System Applied Microbiology 7 (Archaebacteria '85 Proceedings): 178–83. doi:10.1016/S0723-2020(86)80002-9. 
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  18. ^ Javaux, E.; Marshall, C.; Bekker, A. (2010-02-07). "Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits". Nature 463 (7283): 934–938. Bibcode:2010Natur.463..934J. doi:10.1038/nature08793. PMID 20139963.  edit
  19. ^ Bernstein H, Bernstein C (May 1989). "Bacteriophage T4 genetic homologies with bacteria and eucaryotes". J. Bacteriol. 171 (5): 2265–70. PMC 209897. PMID 2651395. 
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  21. ^ Knoll, Andrew H.; Javaux, E.J, Hewitt, D. and Cohen, P. (2006). "Eukaryotic organisms in Proterozoic oceans". Philosophical Transactions of the Royal Society B 361 (1470): 1023–38. doi:10.1098/rstb.2006.1843. PMC 1578724. PMID 16754612. 
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  23. ^ Bernstein H, Bernstein C, Michod RE (2012). DNA repair as the primary adaptive function of sex in bacteria and eukaryotes. Chapter 1: pp.1-49 in: DNA Repair: New Research, Sakura Kimura and Sora Shimizu editors. Nova Sci. Publ., Hauppauge, N.Y. ISBN 978-1-62100-808-8 https://www.novapublishers.com/catalog/product_info.php?products_id=31918
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  26. ^ Hoffman, P.F.; Kaufman, A.J.; Halverson, G.P.; Schrag, D.P. (1998-08-28). "A Neoproterozoic Snowball Earth". Science 281 (5381): 1342–6. Bibcode:1998Sci...281.1342H. doi:10.1126/science.281.5381.1342. PMID 9721097. Retrieved 2007-05-04.  Full online article (pdf 260 Kb)
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  28. ^ http://researchpages.net/media/resources/2007/06/21/richtimhywelfinal.pdf
  29. ^ Corsetti, F.A.; Awramik, S.M.; Pierce, D. (2003-04-15). "A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA". Proceedings of the National Academy of Sciences 100 (8): 4399–4404. Bibcode:2003PNAS..100.4399C. doi:10.1073/pnas.0730560100. PMC 153566. PMID 12682298. Retrieved 2007-06-28. 
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  33. ^ The Cambrian Period
  34. ^ The Cambrian Explosion – Timing
  35. ^ "The oldest fossils of footprints ever found on land hint that animals may have beaten plants out of the primordial seas. Lobster-sized, centipede-like or slug like animals such as Protichnites and Climactichnites made the prints wading out of the ocean and scuttling over sand dunes about 530 million years ago. Previous fossils indicated that animals didn't take this step until 40 million years later." Oldest fossil footprints on land
  36. ^ "The oldest fossils reveal evolution of non-vascular plants by the middle to late Ordovician Period (~450–440 Ma) on the basis of fossil spores" Transition of plants to land
  37. ^ "The land plants evolved from the algae, more specifically green algae, as suggested by certain common biochemical traits" The first land plants
  38. ^ "The ancestry of sharks dates back more than 200 million years before the earliest known dinosaur. Introduction to shark evolution, geologic time and age determination
  39. ^ "Amniota - Palaeos". 
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  41. ^ "Viruses of nearly all the major classes of organisms—animals, plants, fungi and bacteria/archaea—probably evolved with their hosts in the seas, given that most of the evolution of life on this planet has occurred there. This means that viruses also probably emerged from the waters with their different hosts, during the successive waves of colonisation of the terrestrial environment." Origins of Viruses (URL accessed on January 9, 2005)
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  48. ^ "It's official: Caribbean monk seal is extinct", MSNBC.com 6 June 2008

External links[edit]