Evolutionary history of life


Evolutionary history of life

Although evidence of early life is scarce and often difficult to interpret, it appears that life appeared on Earth relatively soon (on the geologic time scale) after the planet had cooled enough for liquid water to be present. The dominant theory within the scientific community is that the first living organisms originated on Earth, evolving from non-living self-replicating molecules, and gained other characteristics of living organisms (see abiogenesis). An alternative theory suggests that the first simple organisms came from outer space, perhaps on a meteorite (see panspermia).

Overview

For the majority of its history, life has been restricted to the sea, and has been dominated by microbial mats. These layered colonies consisted of bacteria and other microbes with a diverse range of behaviours, from decomposition to photosynthesis. The evolution of oxygenic photosynthesis produced free oxygen, which initially reacted with the Earth's crust. When this weathering process had stabilised, free oxygen began to accumulate. This allowed organisms to grow larger and more complex, which in turn paved the way for multicellularity and sex.

Sexual reproduction allows for more rapid evolution, but there were no organisms large enough to handle until shortly before the Cambrian period, Ma|Cambrian. The Cambrian period saw an explosion of animal diversity, and most recognisable groups of organisms—as well as some bizarre extinct types—appeared in this period. Although the seas flourished with life, the land was still barren—a number of biological challenges had to be overcome before land could inhabited. Plants, animals and fungi all seem to have become established on the land around the same time, around the Silurian period (Ma|Silurian); they diversified and expanded over the coming millions of years, and the appearance of forests of land plants changed Earth's environment sufficiently to trigger one of the five largest extinction events. Other such events dramatically changed the dominant organisms on both land and sea; the largest—the Permian-Triassic extinction event —wiped out the vast majority of animal taxa Ma|Triassic.

Following the P–T extinction event, vertebrates such as the dinosaurs rose to become the dominant large land animals. The flora was revolutionised with the advent of the flowering plants in the Cretaceous period (Ma|Cretaceous); co-evolution with insects may have hastened their diversification and given rise to social insects, which, while a small part of the insect "family tree", account for 50% of the total mass of all insects. The extinction of the dinosaurs Ma|Tertiary, and a subsequent warm spell, allowed mammals to radiate.

Earliest history of Earth

Graphical timeline

help=off
link-to=Eras_graphical_timeline
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title=History of Earth and its life
title-colour={colour|period color|Evolutionary history of life}

from=-period start|Hadean
to=0

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period1=Hadean
period1-text==
"background-color:white">Hadean
period2=Archean
period2-text=="background-color:white">Archean
period3 = Proterozoic
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-zoic

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period4 = Phanerozoic
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-zoic

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period1-right=0.30
period2-right=0.30
period3-right=0.30
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period5 = Eoarchean
period5-text = Eo
period6 = Paleoarchean
period6-text = Paleo
period7 = Mesoarchean
period7-text = Meso
period8 = Neoarchean
period8-text = Neo

period5-left=0.305
period6-left=0.305
period7-left=0.305
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period5-right=0.55
period6-right=0.55
period7-right=0.55
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period9 = Paleoproterozoic
period9-text = Paleo
period10 = Mesoproterozoic
period10-text = Meso
period11 = Neoproterozoic
period11-text = Neo

period12 = Paleozoic
period12-text = Paleo
period13 = Mesozoic
period13-text = Meso
period14 = Cenozoic
period14-text = Ceno

period9-left=0.305
period10-left=0.305
period11-left=0.305
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period14-left=0.305
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period15-right=-0.01
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period15-border-colour=#c33

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bar1-left = 0.30
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bar2-colour=black
bar2-left=0.551
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bar2-from=-period start|eoarchean

note1=Solar system formed
note1-at=-4550
note1-nudge-left=5.5
note1-nudge-down=0.7
note2= Impact formed Moon
note2-at=-4510
note2-nudge-left=5.5
note2-nudge-down=0.3
note3=? Cool surface, oceans, atmosphere
note3-at=-4450
note3-nudge-left=5.5
note3-nudge-up=0.0
note4=Late Heavy Bombardment
note4-at=-3900
note4-nudge-left=5.5
note4-nudge-down=0.3
bar4-colour=gray
bar4-left=0.555
bar4-right = 0.60
bar4-from=-4000
bar4-to=-3800
note5=? Earliest evidence of life
note5-at=-3800
note5-nudge-left=5.5
note5-nudge-down=0.0
note6= Oxygenation of atmosphere
note6-at=-2400
note6-nudge-left=5.5
note6-nudge-down=0.3

caption=Scale:
Millions of years

The oldest meteorite fragments found on Earth are about ma|4540| million years old, and this has convinced scientists that the whole Solar system, including Earth, formed around then. About 40 million years later a planetoid struck the Earth, throwing into orbit the material that formed the Moon. [cite journal |title = Origin of the Earth-Moon System |last = E. M. Galimov and A. M. Krivtsov | journal = J. Earth Syst. Sci. | volume = 114 | pages = 593–600 | month = December | year = 2005 |issue = 6 |doi = 10.1007/BF02715942 [http://www.ias.ac.in/jessci/dec2005/ilc-3.pdf] ]

Until recently the oldest rocks found on Earth were about ma|3800| million years old,
*cite book
first=G.B. | last=Dalrymple | year=1991 | title=The Age of the Earth
publisher=Stanford University Press | location=California | id=ISBN 0-8047-1569-6

*cite web
last=Newman | first=William L. | date=2007-07-09 | url=http://pubs.usgs.gov/gip/geotime/age.html
title=Age of the Earth | publisher=Publications Services, USGS | accessdate=2008-08-29

*cite journal
last=Dalrymple | first=G. Brent
title=The age of the Earth in the twentieth century: a problem (mostly) solved
journal=Geological Society, London, Special Publications | year=2001 | volume=190 | pages=205–221
url=http://sp.lyellcollection.org/cgi/content/abstract/190/1/205
accessdate=2007-09-20
doi = 10.1144/GSL.SP.2001.190.01.14
] and this led scientists to believe for decades that Earth's surface was molten until then. Hence they named this part of Earth's history the Hadean eon, whose name means "hellish". [cite journal
author=Cohen, B.A., Swindle, T.D., and Kring, D.A.
title=Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages
journal=Science | month=December | year=2000 | volume=290 | issue=5497 | pages=1754–1756
doi=10.1126/science.290.5497.1754
url=http://www.sciencemag.org/cgi/content/abstract/290/5497/1754 | accessdate=2008-08-31
pmid=11099411
] However analysis of zircons formed ma|4400|4000 indicates that Earth's crust solidified about 100 million years after the planet's formation and that Earth quickly acquired oceans and an atmosphere, which may have been capable of supporting life. [
*cite web | title = Early Earth Likely Had Continents And Was Habitable | date = 2005-11-17 | url = http://www.colorado.edu/news/releases/2005/438.html
*cite journal
last = Cavosie | first = A. J. | coauthors = J. W. Valley, S. A., Wilde, and E.I.M.F. | date = July 15, 2005
title = Magmatic δ18O in 4400-3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean
journal = Earth and Planetary Science Letters | volume = 235 | issue = 3-4 | pages = 663–681
doi = 10.1016/j.epsl.2005.04.028
url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4GDKB05-3&_coverDate=07%2F15%2F2005&_alid=382434001&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5801&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=be47e49c535d059be188b66c6e596dd5
]

Evidence from the Moon indicates that from ma|4000|3800 it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar system, and Earth, having stronger gravity, should have experienced an even heavier bombardment.cite web
last = Britt | first = Robert Roy | date = 2002-07-24
url = http://www.space.com/scienceastronomy/planetearth/earth_bombarded_020724.html
title = Evidence for Ancient Bombardment of Earth | publisher = Space.com | accessdate = 2006-04-15
] [cite journal
author=Cohen, B.A., Swindle, T.D., and Kring, D.A.
title=Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages
journal=Science | month=December | year=2000 | volume=290 | issue=5497 | pages=1754–1756
doi=10.1126/science.290.5497.1754
url=http://www.sciencemag.org/cgi/content/abstract/290/5497/1754 | accessdate=2008-08-31
pmid=11099411
] While there is no direct evidence of conditions on Earth ma|4000|3800, there is no reason to think that the Earth was not also affected by this late heavy bombardment. [ cite journal
author=Valley, J.W., Peck, W.H., King, E.M., and Wilde, S.A. | date=April 2002
title = A cool early Earth
journal=Geology | volume=30 | issue=4 | pages=351–354 | doi=10.1130/0091-7613(2002)030<0351:ACEE>2.0.CO;2
url=http://www.geology.wisc.edu/zircon/Valley2002Cool_Early_Earth.pdf | accessdate=2008-09-13
] This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although volcanic outgassing on Earth would have contributed at least half. [cite journal
author=Dauphas, N., Robert, F., and Marty, B. | date=December 2000
title=The Late Asteroidal and Cometary Bombardment of Earth as Recorded in Water Deuterium to Protium Ratio
journal=Icarus | issue=2 | pages=508–512 | doi=10.1006/icar.2000.6489
volume=148
]

Earliest evidence for life on Earth

The earliest organisms were minute and relatively featureless, so their fossils look like small rods, which are very difficult to tell apart from structures which form through physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to ma|3000. Other finds in rocks dated to about Ma|3500 have been interpreted as bacteria, [
*cite journal
author=Schopf, J.W. | month=April | year=1993
title=Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life
journal=Science| volume=260| issue=5108 | pages=640–646 | doi=10.1126/science.260.5108.640
url= [http://www.sciencemag.org/cgi/content/abstract/260/5108/640 | accessdate=2008-08-30
pmid=11539831

*cite journal |author=Altermann, W.; J. Kazmierczak |title=Archean microfossils: a reappraisal of early life on Earth |journal=Res Microbiol |volume=154 |issue=9 |pages=611&ndash;7 |year=2003 |pmid=14596897 |doi=10.1016/j.resmic.2003.08.006
] and geochemical evidence seemed to show the presence of life ma|3800. [ cite journal
author=Mojzsis, S.J., Arrhenius, G., McKeegan, K.D.,Harrison, T.M., Nutman, A.P., and Friend, C.R.L. | month=November | year=1996
title=Evidence for life on Earth before 3,800 million years ago
journal=Nature | volume=384 | pages=55–59 | doi=10.1038/384055a0
url=http://www.nature.com/nature/journal/v384/n6604/abs/384055a0.html | accessdate=2008-08-30
] However these analyses were closely scrutinised, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.cite journal
author=Grotzinger, J.P., and Rothman, D.H. | year=1996
title=An abiotic model for stomatolite morphogenesis
journal=Nature | volume=383 |pages=423–425 | doi=10. 1038/383423a0
] [
*cite journal
author=Fedo, C.M., and Whitehouse, M.J. | month=May | year=2002
title=Metasomatic Origin of Quartz-Pyroxene Rock, Akilia, Greenland, and Implications for Earth's Earliest Life
journal=Science | volume=296 | issue=5572 | pages=1448–1452 | doi=10.1126/science.1070336
url=http://www.sciencemag.org/cgi/content/abstract/296/5572/1448 | accessdate=2008-08-30
pmid=12029129

*cite journal
author=Lepland, A., van Zuilen, M.A., Arrhenius, G., Whitehouse M.J., and Fedo, C.M.
month=January | year=2005
title=Questioning the evidence for Earth's earliest life — Akilia revisited
journal=Geology | volume=33 | issue=1 | pages=77–79 | doi=10.1130/G20890.1
url=http://geology.geoscienceworld.org/cgi/content/abstract/33/1/77 | accessdate=2008-08-30
] While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Currently, the oldest unchallenged evidence for life is geochemical signatures from rocks deposited ma|3400,cite journal
author=Brasier, M., McLoughlin, N., Green, O., and Wacey, D. | month=June | year=2006
title=A fresh look at the fossil evidence for early Archaean cellular life
journal=Philosophical Transactions of the Royal Society: Biology | volume=361 | issue=1470
pages=887–902 | doi=10.1098/rstb.2006.1835
url=http://physwww.mcmaster.ca/~higgsp/3D03/BrasierArchaeanFossils.pdf | accessdate=2008-08-30
format=PDF
] [cite journal |author=Schopf, J. |title=Fossil evidence of Archaean life |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=869&ndash;85 |year=2006 |pmid=16754604 |doi=10.1098/rstb.2006.1834] although there has been little time for these recent reports (2006) to be examined by critics.

Origins of life on Earth

Biochemists reason that all living organisms on Earth must share a single Last Universal Common Ancestor, because it would be unbelievable that two or more separate lineages could have independently developed the many complex biochemical mechanisms shared by all living organisms. [cite journal
author=Mason, S.F. | title=Origins of biomolecular handedness
journal=Nature | volume=311 | issue=5981 | pages=19&ndash;23 |year=1984 | pmid=6472461
doi=10.1038/311019a0
] [cite journal
author= Orgel, L.E. | month=October | year=1994 | title=The origin of life on the earth
journal=Scientific American | volume=271 | issue=4 | pages=76–83
url=http://courses.washington.edu/biol354/The%20Origin%20of%20Life%20on%20Earth.pdf
accessdate=2008-08-30
format=PDF
Also available as a [http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html web page]
] However the earliest organisms for which fossil evidence is available are bacteria, which are far too complex to have arisen directly from non-living materials. The lack of fossil or geochemical evidence for earlier types of organism has left plenty of scope for hypotheses, which fall into two main groups: that life arose spontaneously on Earth, and that it was "seeded" from elsewhere in the universe.

Life "seeded" from elsewhere

The idea that life Earth was "seeded" from elsewhere in the universe dates back at least to the fifth century BC. [ cite book
author=O'Leary, M.R. | year=2008
title=Anaxagoras and the Origin of Panspermia Theory | publisher=iUniverse, Inc. | isbn=0595495966
] In the twentieth century it was proposed by the physical chemist Svante Arrhenius,cite journal
author=Arrhenius, S. | title=The Propagation of Life in Space | year=1903
journal=Die Umschau volume=7
Reprinted in cite book
editor=Goldsmith, D., | title=The Quest for Extraterrestrial Life
publisher=University Science Books | isbn=0198557043
] by the astronomers Fred Hoyle and Chandra Wickramasinghe,Citation
last = Crick | first = F. H. | last2 = Orgel | first2 = L. E.| title = Directed Panspermia
journal = Icarus | volume = 19 | pages = 341–348 | year = 1973| doi = 10.1016/0019-1035(73)90110-3
] There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar system via fragments knocked into space by a large meteor impact, in which case the only credible source is Mars;cite journal
last = Warmflash | first = D. | coauthors = B. Weiss | year = 2005
month = November | title = Did Life Come From Another World? | journal = Scientific American
pages = 64&ndash;71 | url = http://www.sciam.com/article.cfm?articleID=00073A97-5745-1359-94FF83414B7F0000&pageNumber=1&catID=2
accessdate = 2008-09-02
] by alien visitors, possibly as a result of accidental contamination by micro-organisms that they brought with them; and from outside the Solar system but by natural means. Experiments suggest that some micro-organisms can survive the shock of being catapulted into space and some can survive exposure to radiation for several days, but there is no proof that they can survive in space for much longer periods. Scientists are divided over the likelihood of life arising independently on Mars, [cite web
title=Claim of Martian Life Called 'Bogus' | month=August | year=2007
url=http://www.space.com/news/070823_mars_life.html | accessdate = 2008-09-02
] or on other planets in our galaxy.

Independent emergence on Earth

Research on how life might have emerged unaided from non-living chemicals focuses on three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. [cite journal |author=Peretó, J. |title=Controversies on the origin of life |url=http://www.im.microbios.org/0801/0801023.pdf |format=PDF |journal=Int. Microbiol. |volume=8 |issue=1 |pages=23&ndash;31 |year=2005 |pmid=15906258 |accessdate=2007-10-07] Research on abiogenesis still has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other. [cite journal
author=Szathmáry, E. | month=February | year=2005 | title=Life: In search of the simplest cell
journal=Nature | volume=433 | pages=469–470 | doi=10.1038/433469a
url=http://www.nature.com/nature/journal/v433/n7025/full/433469a.html | accessdate=2008-09-01
] [cite journal |author=Luisi, P.L.; F. Ferri, P. Stano |title=Approaches to semi-synthetic minimal cells: a review |journal=Naturwissenschaften |volume=93 |issue=1 |pages=1&ndash;13 |year=2006 |pmid=16292523 |doi=10.1007/s00114-005-0056-z]

Replication first: RNA world

Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance and self-replication. This system is far too complex to have emerged directly from non-living materials.cite book
author=Cowen, R. | title=History of Life | edition=3rd | publisher=Blackwell Science
isbn=0632044446 | page=p. 6
] The discovery that some RNA molecules can catalyze both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. [cite journal |author=Joyce, G.F. |title=The antiquity of RNA-based evolution |journal=Nature |volume=418 |issue=6894 |pages=214&ndash;21 |year=2002 |pmid=12110897 |doi=10.1038/418214a] These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.cite journal
author=Hoenigsberg, H.
title=Evolution without speciation but with selection: LUCA, the Last Universal Common Ancestor in Gilbert’s RNA world
journal=Genetic and Molecular Research | volume=2 | issue=4 | pages=366–375 | pmid=15011140
url=http://www.funpecrp.com.br/gmr/year2003/vol4-2/gmr0070_full_text.htm | accessdate=2008-08-30
(also available as [http://www.funpecrp.com.br/gmr/year2003/vol4-2//pdf/gmr0070.pdf PDF] ] RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have. [cite journal |author=Trevors, J.T.; D.L. Abel |title=Chance and necessity do not explain the origin of life |journal=Cell Biol. Int. |volume=28 |issue=11 |pages=729&ndash;39 |year=2004 |pmid=15563395 |doi=10.1016/j.cellbi.2004.06.006] [cite journal |author=Forterre, P.; N. Benachenhou-Lahfa, F. Confalonieri, M. Duguet, et al. |title=The nature of the last universal ancestor and the root of the tree of life, still open questions |journal=BioSystems |volume=28 |issue=1–3 |pages=15&ndash;32 |year=1992 |pmid=1337989 |doi=10.1016/0303-2647(92)90004-I] Ribozymes remain as the main components of ribosomes, modern cells' "protein factories". [cite journal
author=Cech, T.R. | title=The ribosome is a ribozyme
journal=Science | month=August | year=2000 |volume=289 | issue=5481 | pages=878–9 | pmid=10960319
doi=10.1126/science.289.5481.878
url=http://www.sciencemag.org/cgi/content/short/289/5481/878 | accessdate=2008-09-01
]

Although short self-replicating RNA molecules have been artificially produced in laboratories, [cite journal
last=Johnston | first=W. K. | authorlink= | coauthors="et al." | year=2001 | month=
title=RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension
journal=Science | volume=292 | issue=5520 | pages=1319–1325 | doi=10.1126/science.1060786
pmid=11358999
] doubts have been raised about where natural non-biological synthesis of RNA is possible.
*cite journal |last=Levy |first=Matthew |authorlink= |coauthors=Miller, Stanley L. |year=1998 |month= |title=The stability of the RNA bases: Implications for the origin of life |journal=PNAS |volume=95 |issue= |pages=7933–7938 |id= |url= |quote=|doi=10.1073/pnas.95.14.7933|pmid=9653118
*cite journal |last=Larralde |first=R. |authorlink= |coauthors=Robertson, M. P.; Miller, S. L. |year=1995 |month= |title=Rates of Decomposition of Ribose and Other Sugars: Implications for Chemical Evolution |journal=PNAS |volume=92 |issue=18 |pages=8158–8160 |doi=10.1073/pnas.92.18.8158|pmid=7667262
*cite journal |last=Lindahl |first=Tomas |authorlink= |coauthors= |year=1993 |month= |title=Instability and decay of the primary structure of DNA |journal=Nature |volume=362 |issue=6422 |pages=709–715 |doi=10.1038/362709a0 ] The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA. [cite journal
last=Orgel | first=Leslie| month=Nov| year=2000 |title=A Simpler Nucleic Acid
journal=Science | volume=290 | issue=5495 | pages=1306–7 | doi=10.1126/science.290.5495.1306
pmid=11185405
] [cite journal
author=Nelson, K.E. |coauthors=Levy, M.; Miller, S.L.
title=Peptide nucleic acids rather than RNA may have been the first genetic molecule
year=2000 |month=Apr |journal=Proceedings of the National Academy of Sciences
volume=97 | issue=8 | pages=3868–71 |pmid=10760258 | doi= 10.1073/pnas.97.8.3868
]

In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about convert|100|°C|°F and ocean-bottom pressures near hydrothermal vents. In this hypothesis lipid membranes would be the last major cell components to appear and until then the proto-cells would be confined to the pores. [ cite journal
author=Martin, W. and Russell, M.J. | date=2003
title=On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells
journal=Philosophical Transactions of the Royal Society: Biological | volume=358 | pages=59–85
pmid=12594918
url=http://www.ncbi.nlm.nih.gov/pubmed/12594918 | accessdate=2008-09-05
doi=10.1098/rstb.2002.1183
]

Metabolism first: Iron-sulfur world

A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including carbon monoxide hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts. Most of the steps required temperatures of about convert|100|°C|°F and moderate pressures, although one stage required convert|250|°C|°F and a pressure equivalent to that found under convert|7|km|mi of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents. [cite journal
author=Günter Wächtershäuser | year = 2000 | month = August | title = Life as We Don't Know It
journal = Science | volume = 289 | pages = 1307–1308
doi = 10.1126/science.289.5483.1307 | pmid = 10979855
]

Membranes first: Lipid world

Annotated image | caption=Cross-section through a liposome.|

annotations =
It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step. [cite journal |author=Trevors, J.T.; R. Psenner |title=From self-assembly of life to present-day bacteria: a possible role for nanocells |journal=FEMS Microbiol. Rev. |volume=25 |issue=5 |pages=573&ndash;82 |year=2001 |pmid=11742692 |doi=10.1111/j.1574-6976.2001.tb00592.x] Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles", and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such RNA might then have formed more easily within the liposomes than they would have outside. [cite journal
author=Segré, D., Ben-Eli, D., Deamer, D., and Lancet, D.
title=The Lipid World | date=February-April 2001
journal=Origins of Life and Evolution of Biospheres 2001 | volume=31 | issue=1-2 | pages=119–45
pmid=11296516 | url=http://ool.weizmann.ac.il/Segre_Lipid_World.pdf | accessdate=2008-09-01
format=PDF
doi=10.1023/A:1006746807104
]

The clay theory

RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern; they are subject to an analog of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules. [citation
author=Cairns-Smith, A.G. | contribution=An approach to a blueprint for a primitive organism
editor=Waddington, C,H. | title=Towards a Theoretical Biology | volume=1 | pages=57–66 | date=1968
publisher=Edinburgh University Press
] Although this idea has not become the scientific consensus, it still has active supporters. [cite journal
author=Ferris, J.P. | title=Prebiotic Synthesis on Minerals: Bridging the Prebiotic and RNA Worlds
journal=Biological Bulletin | volume=196| issue=3 | series=Evolution: A Molecular Point of View
month=June | year=1999
url=http://www.jstor.org/pss/1542957 | accessdate=2008-09-01
doi=10.2307/1542957
pages=311
unused_data=|pages311-314
]

Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes. [cite journal
author=Hanczyc, M.M., Fujikawa, S.M. and Jack W. Szostak, J.W.
title=Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Division
journal=Science | month=October | year=2003 | volume=302 | issue=5645| pages=618–622
doi=10.1126/science.1089904
url=http://www.sciencemag.org/cgi/content/abstract/302/5645/618 | accessdate=2008-09-01
pmid=14576428
]

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids. [cite journal
author=Hartman, H. | month=October | year=1998 | title=Photosynthesis and the Origin of Life
journal=Origins of Life and Evolution of Biospheres | volume=28 | issue=4–6 | pages=512–521
url=http://www.springerlink.com/content/t1n325268n01217k/ | accessdate=2008-09-01
]

Environmental and evolutionary impact of microbial mats

These multi-layered colonies of bacteria and other organisms are generally only a few millimeters thick, but still contain a wide range of chemical environments. citation | contribution=Biofilm, Biodictyon, Biomat Microbialites, Oolites, Stromatolites, Geophysiology, Global Mechanism, Parahistology | author=Krumbein, W.E., Brehm, U., Gerdes, G., Gorbushina, A.A., Levit, G. and Palinska, K.A. | pages=1–28 | title=Fossil and Recent Biofilms: A Natural History of Life on Earth | editor=Krumbein, W.E., Paterson, D.M., and Zavarzin, G.A. | date=2003 | publisher=Kluwer Academic| isbn=1402015976 | url=http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf | accessdate=2008-07-09 ] In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there. To some extent each mat forms its own food chain, as the by-products of each group of micro-organisms generally serve as "food" for adjacent groups. [ cite journal | title=Community structure of a microbial mat: the phylogenetic dimension | author=Risatti, J,B., Capman, W.C., and Stahl, D.A. | journal=Proceedings of the National Academy of Sciences | date=October 11, 1994 | volume=91 | issue=21
pages=10173–10177 | url=http://www.pnas.org/content/91/21/10173.full.pdf | accessdate=2008-07-09 |format=PDF | doi=10.1073/pnas.91.21.10173 | pmid=7937858
]

Stromatolites are stubby pillars built as microbes in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water. Although earlier reports of fossilized stromatolites from about ma|3500 were criticized on the grounds that the structures in the rocks could have been produced by non-biological processes, in 2006 another find of stromatolites was reported from the same part of Australia, in rocks also dated to ma|3500. [cite journal
author=Allwood, A.C., Walter, M.R., Kamber, B.S., Marshall1, C.P., and Burch, I.W.
title= Stromatolite reef from the Early Archaean era of Australia
journal=Nature | volume=441 | pages=714–718 | date=June 2006) | doi=10.1038/nature04764
url=http://www.nature.com/nature/journal/v441/n7094/abs/nature04764.html | accessdate=2008-08-31
]

It is estimated that the appearance of oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The reducing agent used by oxygenic photosynthesis is water, which is much more plentiful than the geologically-produced reducing agents required by the earlier non-oxygenic photosynthesis. cite journal
title=Molecular evidence for the evolution of photosynthesis | author=Blankenship, R.E. | journal=Trends in Plant Science | volume=6 | issue=1 | date=1 January 2001 | pages=4–6 | doi=10.1038/35085554 | url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TD1-424KK4J-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=6f38f9f1d29b24fc90d0145837338b9e | accessdate=2008-07-14
] From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes. cite journal | title=The role of microbial mats in the production of reduced gases on the early Earth | author=Hoehler, T.M., Bebout, B.M., and Des Marais, D.J. | journal=Nature | volume=412 | pages=324–327 | date=19 July 2001 | doi=10.1038/35085554 | url=http://www.nature.com/nature/journal/v412/n6844/full/412324a0.html | accessdate=2008-07-14 ] Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms. cite journal | title=Toxic oxygen: The radical life-giver | author=Abele, D. | journal=Nature | volume=420 | issue=27 | date=7 November 2002 | doi=10.1038/420027a | url=http://www.nature.com/nature/journal/v420/n6911/full/420027a.html | accessdate=2008-07-14 | pages=27 ] [ cite web | url=http://trc.ucdavis.edu/biosci10v/bis10v/week3/06aerobicrespirintro.html | title=Introduction to Aerobic Respiration | publisher=University of California, Davis | accessdate=2008-07-14 ]

Oxygen became a significant component of Earth's atmosphere about ma|2400. [ cite journal
author=Goldblatt, C., Lenton, T.M., and Watson, A.J.
title=The Great Oxidation at ~2.4 Ga as a bistability in atmospheric oxygen due to UV shielding by ozone
journal=Geophysical Research Abstracts | volume=8 | issue=00770 | year=2006
url=http://www.cosis.net/abstracts/EGU06/00770/EGU06-J-00770.pdf | accessdate=2008-09-01
format=PDF
] Although eucaryotes may have been present much earlier,cite journal
author=Glansdorff, N., Xu, Y., and Labedan, B. | year=2008
title=The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner
journal=Biology Direct | volume=3 | issue=29 | doi=10.1186/1745-6150-3-29
pages=29
] cite journal
author=Brocks, J.J., Logan, G.A., Buick, R., and Summons, R.E. | year=1999
title=Archaean molecular fossils and the rise of eukaryotes
journal=Science | volume=285 | pages=1033–1036
url=http://www.sciencemag.org/cgi/content/abstract/285/5430/1033 | accessdate=2008-09-02
doi=10.1126/science.285.5430.1033
pmid=10446042
] the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eucaryotic cells, from which all multicellular organisms are built. cite journal
title=A molecular timescale of eukaryote evolution and the rise of complex multicellular life
author=Hedges, S.B., Blair, J.E, Venturi, M.L., and Shoe, J.L
journal=BMC Evolutionary Biology| volume=4 | issue=2 | doi=10.1186/1471-2148-4-2 | month=January | year=2004
url=http://www.biomedcentral.com/1471-2148/4/2/abstract/ | accessdate=2008-07-14
pages=2
] The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created selection pressure for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via endosymbiosis, where one organism lives inside another and both of them benefit from their association.

Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient of the mat organisms and were well-adapted to strike out on their own both as floating mats and as the first of the phytoplankton, providing the basis of most marine food chains. cite journal | title=Archaean metabolic evolution of microbial mats | author=Nisbet, E.G., and Fowler, C.M.R. | journal=Proceedings of the Royal Society: Biology | date=December 7 1999 | volume=266 | issue=1436 | pages=p. 2375 | doi=10.1098/rspb.1999.0934 | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1690475 | accessdate=2008-07-16 - abstract with link to free full content (PDF)]

Diversification of eucaryotes

clade
label1=Eucaryotes
1=clade
label1=Bikonta
1=clade
label1=
1=Apusozoa
label2=
2=Archaeplastida (Land plants, green algae, red algae, and glaucophytes)
label3=
3=Chromalveolata
label4=
4=Rhizaria
label5=
5=Excavata


label2=Unikonta
2=clade
1=Amoebozoa
label2=Opisthokonta
2=clade
label1=
1=Metazoa (Animals)
label2=
2=Choanozoa
label3=
3=Eumycota (Fungi)

One possible family tree of eucaryotes [cite journal
author = Burki, F., Shalchian-Tabrizi, K., Minge, M., Skjæveland, Å., Nikolaev, S.I., "et al."
year = 2007 | title = Phylogenomics Reshuffles the Eukaryotic Supergroups
journal = PLoS ONE | volume = 2 | issue = 8: e790 | doi = 10.1371/journal.pone.0000790
pages = e790
] [cite journal
url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1713255
title = Evaluating Support for the Current Classification of Eukaryotic Diversity
author = Parfrey, L.W., Barbero, E., Lasser, E., Dunthorn, M., Bhattacharya, D., Patterson, D.J., and Katz, L.A.
doi = 10.1371/journal.pgen.0020220 | journal = PLoS Genetics |month=December | year=2006| volume = 2
issue = 12 | pages = e220 | pmid = 17194223
]

Eucaryotes may have been present long before the oxygenation of the atmosphere, but most modern eucaryotes require oxygen, which their mitochondria use to fuel the production of ATP, the internal energy supply of all known cells. In the 1970s it was proposed and, after much debate, widely accepted that eucaryotes emerged as a result of a sequence of endosymbioses between "procaryotes". For example: a predatory micro-organism invaded a large procaryote, probably an archaean, but the attack was neutralized, and the attacker took up residence and evolved into the first of the mitochondria; one of these chimeras later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of plants; and so on. After each endosymbiosis began, the partners would have eliminated unproductive duplication of genetic functions by re-arranging their genomes, a process which sometimes involved transfer of genes between them. [cite book
author= Margulis, L. | year=1981 | title=Symbiosis in cell evolution
publisher=W.H. Freeman | location=San Francisco | isbn=0716712563
] [ cite journal
author=Vellai, T. and Vida, G. | year=1999
title=The origin of eukaryotes; the difference between eukaryotic and prokaryotic cells
journal=Proceedings of the Royal Society: Biology | volume=266 | pages=1571–1577
doi=10.1098/rspb.1999.0817
] [ cite journal
author=Selosse, M-A., Abert, B., and Godelle, B.
title=Reducing the genome size of organelles favours gene transfer to the nucleus
journal=Trends in ecology & evolution | volume=16 | issue=3 | pages=135–141
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VJ1-429XTFM-H&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=8370ca16bcde45bfa1c050068a2d6e19
accessdate=2008-09-02 | doi=10.1016/S0169-5347(00)02084-X
year=2001
] Another hypothesis proposes that mitochondria were originally sulfur- or hydrogen-metabolising endosymbionts, and became oxygen-consumers later. [cite journal
title=Supertrees disentangle the chimerical origin of eukaryotic genomes
author=Pisani, D., Cotton, J.A., McInerney, J.O.
journal= Mol Biol Evol.| year=2007| volume=24 | issue=8| pages=1752–60| pmid=17504772
doi=10.1093/molbev/msm095
] On the other hand mitochondria might have been part of eucaryotes' original equipment. [ cite journal
author=Gray, M.W., Burger, G., and Lang, B.F. | year=1999
title=Mitochondrial evolution | journal=Science | volume=283 | issue=5407 | pages=1476–1481
doi=10.1126/science.283.5407.1476
url=http://www.sciencemag.org/cgi/content/abstract/283/5407/1476 | accessdate=2008-09-02
pmid=10066161
]

The presence of steranes in Australian shales indicates that eukaryotes were present ma|2700. Fossils of the alga "Grypania" have been reported in ma|1850|million-year-old rocks (originally dated to ma|2100 but later revised), and indicates that eucaryotes with organelles had already evolved. [cite journal
author=Han, T.M. and Runnegar, B. | month=July | year=1992
title=Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation, Michigan |journa=Science | vloume=257 | issue=5067 | pages=232–235 | dio=10.1126/science.1631544
url=http://www.sciencemag.org/cgi/content/abstract/257/5067/232 | accessdate=2008-09-02
doi=10.1126/science.1631544
url=http://www.sciencemag.org/cgi/content/abstract/257/5067/232 | accessdate=2008-09-02
pmid=1631544
] A diverse collection of fossil algae were found in rocks dated between ma|1500| and ma|1400. [cite journal
author=Javaux, E.J, Knoll, A.H., and Walter, M.R. | month=September | year=2004
title=TEM evidence for eukaryotic diversity in mid-Proterozoic oceans
journal=Geobiology| volume=2 | issue=3| pages=121–132 | doi=10.1111/j.1472-4677.2004.00027.x
url=http://www3.interscience.wiley.com/journal/118814335/abstract | accessdate=2008-09-02
] The earliest known fossils of fungi date from ma|1430.cite journal
author = Butterfield, N.J. | year = 2005 | title = Probable Proterozoic fungi
journal = Paleobiology | volume = 31 | issue = 1 | pages = 165–182
doi =10.1666/0094-8373(2005)031<0165:PPF>2.0.CO;2
url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/31/1/165 | accessdate=2008-09-02
]

Multicellular organisms and sexual reproduction

Multicellularity

The simplest definitions of "multicellular", for example "having multiple cells", could include colonial cyanobacteria like Nostoc. Even a professional biologist's definition such as "having the same genome but different types of cell" would still include some genera of the green alga Volvox, which have cells that specialize in reproduction.cite journal
author=Fedonkin, M.A. | month=March | year=2003
title=The origin of the Metazoa in the light of the Proterozoic fossil record
journal=Paleontological Research | volume=7 | issue=1 | pages=9–41
url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf | accessdate=2008-09-02
format=PDF
doi=10.2517/prpsj.7.9
] For the sake of brevity this article focusses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the evolution of complexity could be regarded as "rather anthropocentric". cite journal
author=Bonner, J.T. |month=January | year=1999 | title=The Origins of Multicellularity
journal=Integrative Biology | volume=1 | issue=1 | pages=27–36
url=http://doi.wiley.com/10.1002/(SICI)1520-6602(1998)1:1%3C27::AID-INBI4%3E3.0.CO;2-6
accessdate=2008-09-03
doi=10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6
accessdate=2008-09-03
]

The initial advantages of multicellularity may have included: increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis; and even the opportunity for a group of cells to behave "intelligently" by sharing information.cite journal
author=Nakagaki, T., Yamada H. and Tóth, Á. | title=Intelligence: Maze-solving by an amoeboid organism
journal=Nature | volume=407 | month=September | year=2000 | doi=10.1038/35035159
url=http://www.nature.com/nature/journal/v407/n6803/abs/407470a0.html | accessdate=2008-09-03 | pages=470
] These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.

Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity.

The available evidence indicates that eucaryotes evolved much earlier but remained inconspicuous until a rapid diversification around ma|1000. The only respect in which eucaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eucaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function.

How sex evolved

The following hypotheses attempt to explain how and why sex evolved:
*It may have enabled organisms to repair genetic damage.cite journal
author=Bernstein, H., Byerly, H., Hopf, F., and Michod, R. | title = Origin of sex | year = 1984
journal = Journal of Theoretical Biology | volume=110 | issue=3 | pages = 323–351
pmid = 6209512 | doi = 10.1016/S0022-5193(84)80178-2
] The most primitive form of sex may have been one organism repairing damaged DNA by replicating an undamaged strand from a similar organism.cite book | author = Judson, O. | title = Dr. Tatiana's sex advice to all creation | publisher = Metropolitan Books | location = New York | year = 2002 | id = ISBN 0-8050-6331-5 | pages =233&ndash;4]
*Sexual reproduction may have originated from selfish parasitic genetic elements propagating themselves by transfer to new hosts.cite journal | author = Hickey, D. | title = Selfish DNA: a sexually-transmitted nuclear parasite | journal = Genetics | volume = 101 | issue = 3&ndash;4 | pages = 519&ndash;31 | year = 1982 | pmid = 6293914]
*It may have evolved from cannibalism, where some of the victim's DNA was incorporated into the cannibal organism.
*Sexual reproduction may evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids. [cite journal | title=Extreme Microbes | first=S. | last=DasSarma | contribution=Extreme Microbes | work=American Scientist | year=2007 | volume=95 | pages= page 224–231 ]
*Or it may have evolved as a form of vaccination in which infected hosts exchanged weakened symbiotic copies of parasitic DNA as protection against more virulent versions. The meiosis stage of sexual reproduction may then have evolved as a way of removing the symbiotes.cite journal | author = Sterrer, W. | title = On the origin of sex as vaccination | journal = Journal of Theoretical Biology | volume = 216 | pages = 387&ndash;96 | year = 2002 | doi = 10.1006/jtbi.2002.3008
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WMD-46DM0JD-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=3f5b76ba0b7fff999ee670479f06e5af
accessdate=2008-08-02
]

Bacteria also exchange DNA by bacterial conjugation, the benefits of which include resistance to antibiotics and other toxins, and the ability to utilize new metabolites.citation
author = Holmes R.K., and Jobling, M.G. | contribution=Genetics: Exchange of Genetic Information
title=Baron's Medical Microbiology | editor=Baron, S. | edition=4th | year=1996
publisher = University of Texas Medical Branch | location=Galveston | isbn = 0-9631172-1-1
url = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?highlight=conjugation&rid=mmed.section.468
accessdate=2008-09-02
] However conjugation is not a means of reproduction and is not limited to members of the same species, and there are cases where bacteria transfer DNA to plants and animals. [cite journal
author=Christie, P.J. | month=April | year=2001
title=Type IV secretion: intercellular transfer of macromolecules by systems ancestrally related to conjugation machines
journal=Molecular Microbiology | volume=40 | issue=22 | pages=294–305
url=http://lib.bioinfo.pl/meid:10183 | accessdate=2008-09-02
doi=10.1046/j.1365-2958.2001.02302.x
] Nevertheless it may be an example of the "selfish genetic element" hypothesis, as it transfers DNA by means of such a "selfish gene", the F-plasmid.

Fossil evidence for multicellularity and sexual reproduction

The earliest known fossil organism that is clearly multicellular, "Qingshania", Name given as in Butterfield's paper "Bangiomorpha pubescens" ..." (2000). A fossil fish, also from China, has also been named "Qingshania". The name of one of these will have to change.] , dated to ma|1700, appears to consist of virtually identical cells. A red alga called "Bangiomorpha", dated at ma|1200, is the earliest known organism which has differentiated, specialized cells, and is also the oldest known sexually-reproducing organism.cite journal
author=Butterfield, N.J. | month=September | year=2000
title="Bangiomorpha pubescens" n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes
journal=Paleobiology | volume=26 | issue=3 | pages=386–404
doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2
url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/386 | accessdate=2008-09-02
] The ma|1430|million-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells. The "string of beads" organism "Horodyskia", found in rocks dated from ma|1500| to ma|900, may have been an early metazoan; however it has also been interpreted as a colonial foraminiferan. [cite journal
author=Dong, L., Xiao, S., Shen, B., and Zhou, C. | month=January | year=2008
title=Silicified "Horodyskia" and "Palaeopascichnus" from upper Ediacaran cherts in South China: tentative phylogenetic interpretation and implications for evolutionary stasis
journal=Journal of the Geological Society | volume=165 | pages=367–378 | doi=10.1144/0016-76492007-074
url=http://findarticles.com/p/articles/mi_qa3721/is_200801/ai_n24394476/pg_1?tag=artBody;col1
accessdate=2008-09-02
]

Emergence of animals

clade
label1=
1=clade
label1=
1=clade
label1=
1=clade
label1=
1=clade
label1=Bilaterians
1=clade
label1=
1=clade
1=Deuterostomes (chordates, hemichordates, echinoderms
label2=Protostomes
2=clade
label1=
1=Ecdysozoa (anthropods, nematodes, tardigrades
label2=
2=Lophotrochozoa (molluscs, annelids, brachiopods

label2=
2=Acoelomorpha

2=Cnidaria (jellyfish, sea anemones, hydras)
3=Ctenophora (comb jellies)

2=Placozoa
3=Porifera (sponges): Calcarea

2=Porifera: Hexactinellida & Demospongiae

2=Choanoflagellata
3=Mesomycetozoea
A family tree of the animals. [cite journal
author=Gaidos, E., Dubuc, T., Dunford, M., McAndrew, P., Padilla-gamiño, J., Studer, B., Weersing, K., and Stanley, S.
title=The Precambrian emergence of animal life: a geobiological perspective
journal=Geobiology | year=2007 | doi=10.1111/j.1472-4669.2007.00125.x
url=http://www.soest.hawaii.edu/GG/FACULTY/GAIDOS/geobiology2007.pdf | accessdate=2008-09-03
format=PDF
volume=5
pages=351
]
Animals are multicellular eucaryotes,Myxozoa were thought to be an exception, but are now thought to be heavily modified members of the Cnidaria: cite journal
author=Jímenez-Guri, E., Philippe, H., Okamura, B., and Holland, P.W.H.
month=Jul |year=2007 |title="Buddenbrockia" is a cnidarian worm
journal=Science |volume=317 |issue=116 |pages=116–118 |doi=10.1126/science.1142024
pmid=17615357 | url=http://www.sciencemag.org/cgi/content/abstract/317/5834/116 | accessdate=2008-09-03
] and are distinguished from plants, algae, and fungi by lacking cell walls. [>cite web
url=http://micro.magnet.fsu.edu/cells/animalcell.html | author=Davidson, M.W.
title=Animal Cell Structure |accessdate=2008-09-03 | language=English | publisher=Florida State University
] All animals are motile, [cite web
url=http://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm
author=Saupe, S.G| title=Concepts of Biology | publisher=College of St. Benedict / St. John's University
accessdate=2008-09-03 |language=English
] if only at certain life stages. All animals except sponges have bodies differentiated into separate tissues, including muscles, which move parts of the animal by contracting, and nerve tissue, which transmits and processes signals.

The earliest widely-accepted animal fossils are rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and hydras), possibly from around ma|580, although fossils from the Doushantuo Formation can only be dated approximately. Their presence implies that the cnidarian and bilaterian lineages had already diverged. [cite journal
author=Chen, J.-Y., Oliveri, P., Gao, F., Dornbos, S.Q., Li, C-W., Bottjer, D.J. and Davidson, E.H.
title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China
journal=Developmental Biology | volume=248 | issue=1 | month=August | year=2002
doi=10.1006/dbio.2002.0714
url=http://www.uwm.edu/~sdornbos/PDF's/Chen%20et%20al.%202002.pdf | accessdate=2008-09-03
format=PDF
pages=182–196
]

The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,cite journal | author = Grazhdankin | year = 2004 | doi = 10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 | issn = 0094–8373 | volume = 30 | pages = 203 | title = Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution | journal = Paleobiology ] were the first animals more than a very few centimeters long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate kingdom, Vendozoa.cite journal
author = Seilacher, A. | year = 1992
title = Vendobionta and Psammocorallia: lost constructions of Precambrian evolution
journal = Journal of the Geological Society, London | volume = 149 | issue = 4 | pages = 607–613
doi = 10.1144/gsjgs.149.4.0607 | issn = 0016–7649
url = http://jgs.lyellcollection.org/cgi/content/abstract/149/4/607 | accessdate = 2007-06-21
format = abstract
] Others, however, been interpreted as early molluscs ("Kimberella"),cite journal
author = Martin, M.W.
coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
year = 2000 | date = 2000-05-05
title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
journal = Science | volume = 288 | issue = 5467 | pages = 841 | doi = 10.1126/science.288.5467.841
pmid = 10797002
url = http://www.scienceonline.org/cgi/content/abstract/288/5467/841 | accessdate=2008-07-03
format = abstract
] cite journal
author =Fedonkin, M.A. | coauthors =Waggoner, B.
title =The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
journal=Nature | volume =388 | pages =868–871 | year =1997 | doi = 10.1038/42242
url =http://www.nature.com/nature/journal/v388/n6645/abs/388868a0.html | accessdate=2008-07-03
format =abstract
] echinoderms ("Arkarua"); [cite journal
author=Mooi, R. and Bruno, D.
title=Evolution within a bizarre phylum: Homologies of the first echinoderms
journal=American Zoologist | volume=38 | pages=965–974 | year=1999
url=http://icb.oxfordjournals.org/cgi/reprint/38/6/965.pdf | accessdate=2007-11-24
format=PDF
] and arthropods ("Spriggina", [cite journal
author =McMenamin, M.A.S | title ="Spriggina" is a trilobitoid ecdysozoan
journal =Abstracts with Programs | publisher=Geological Society of America | volume =35 | issue=6
pages =105 | year =2003 | url=http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm
accessdate=2007-11-24 | format =abstract
] "Parvancorina" [ cite journal
title= A "Parvancorina"-like arthropod from the Cambrian of South China
author=Lin, J-P., Gon, S.M., Gehling, J.G., Babcock, L.E., Zhao, Y-L., Zhang, X-L,, Hu, S-X., Yuan, J-L., Yu, M-Y., and Peng, J.
journal=Historical Biology: A Journal of Paleobiology | volume=18 | issue=1 | month=March | pages=33–45
doi= 10.1080/08912960500508689 | year= 2006
] ). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans.cite journal
author = Butterfield, N.J. | year = 2006
title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
journal = Bioessays | volume = 28 | issue = 12 | pages = 1161–6 | doi = 10.1002/bies.20507
accessdate =
] However there seems little doubt that "Kimberella" was at least a triploblastic bilaterian animal, in other words significantly more complex than cnidarians.

The small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Mid Cambrian periods. The earliest, "Cloudina", shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates", "Halkieria" and "Microdictyon", were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.citation | contribution=Early skeletal fossils
author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M.
title=Neoproterozoic - Cambrian Biological Revolutions
year = 2004 | journal = Palentological Society Papers | volume = 10 | pages = 67–78
url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf
accessdate=2008-07-18
]

In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of Pre-Cambrian animal fossils. A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as "Opabinia", which did not fit into any known phylum. At the time these were interpreted at evidence that the modern phyla had evolved very rapidly in the "Cambrian explosion" and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.cite book
title=Wonderful Life: The Burgess Shale and the Nature of History
author=Gould, S.J. | authorlink=Steven Jay Gould | year=1989 | publisher=W.W. Norton & Company
] Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groupscite journal
author = Budd, G.E. | year = 2003 | title = The Cambrian Fossil Record and the Origin of the Phyla
journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 157–165
doi = 10.1093/icb/43.1.157
url = http://intl-icb.oxfordjournals.org/cgi/content/abstract/43/1/157 | accessdate=2008-07-15
format = Free full text
] – for example that "Opabinia" was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have have been closely related to the modern tardigrades.cite journal
author=Budd, G.E. | year=1996
title=The morphology of "Opabinia regalis" and the reconstruction of the arthropod stem-group
journal=Lethaia | volume=29 | issue=1 | pages=1–14 | doi=10.1111/j.1502-3931.1996.tb01831.x
] Nevertheless there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.cite journal
author=Marshall, C.R. | title=Explaining the Cambrian “Explosion” of Animals
journal=Annu. Rev. Earth Planet. Sci. | year=2006 | volume=34 | pages=355–384
doi=10.1146/annurev.earth.33.031504.103001
url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.33.031504.103001?journalCode=earth
accessdate=2007-11-06
]

The animals at the heart of the Cambrian explosion debate are protostomes, one of the two main groups of complex animals. One deuterostome group, the echinoderms, many of which have hard calcite "shells", are fairly common from the Early Cambrian small shelly fauna onwards. Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the Chengjiang fauna, a lagerstätte in China. [ cite journal
author=Conway Morris, S. | title=Once we were worms | date=August 2nd, 2003
journal=New Scientist | volume=179 | issue=2406
url=http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm | accessdate=2008-09-05 | pages=p. 34
] The Chengjiang fossils "Haikouichthys" and "Myllokunmingia" appear to be true vertebrates, [ cite journal
author=Shu, D-G., Luo, H-L., Conway Morris, S., Zhang, X-L., Hu, S-X., Chen, L., J. Han, J., Zhu, M., Li, Y., and Chen, L-Z.
title=Lower Cambrian vertebrates from south China
journal=Nature | volume=402 |pages=42–46 | date=November 1999 | doi=10.1038/46965
url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf | accessdate=2008-09-05
] and "Haikouichthys" had distinct vertebrae, which may have been slightly mineralized. [ cite journal
author=Shu, D.-G., Conway Morris, S., Han, J., Zhang, Z.-F., Yasui, K., Janvier, P., Chen, L., Zhang, X.-L., Liu, J.-N., Li, Y., and Liu, H.-Q.
title=Head and backbone of the Early Cambrian vertebrate "Haikouichthys"
journal=Nature | volume=421 | pages=526–529 | date=January 2003 | doi=10.1038/nature01264
url=http://www.nature.com/nature/journal/v421/n6922/abs/nature01264.html | accessdate=2008-09-05
] Vertebrates with jaws, such as the Acanthodians, first appeared in the Late Ordovician. [citation
author=Sansom I.J., Smith, M.M., and Smith, M.P. | date=2001
contribution=The Ordovician radiation of vertebrates | pages=156–171
editor=Ahlberg, P.E. |title=Major Events in Early Vertebrate Evolution
publisher=Taylor and Francis | isbn=0-415-23370-4
]

Colonization of land

Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size have to resist gravity; respiration and gas exchange systems have to change; reproductive systems cannnot depend on water to carry eggs and sperm towards each each other.cite book
author=Cowen, R. | title=History of Life | edition=3rd | publisher=Blackwell Science
isbn=0632044446 | pages=120-122
]

Plants and the Late Devonian wood crisis

In aquatic algae, almost all cells are capable of photosynthesies and are nearly independent. Life on land required plants to become internally more complex and specialized: photosynthesis was most efficient at the top; roots were required in order to extract water from the ground; the parts in between became supports and transport systems for water and nutrients.

Spores of land plants, possibly rather like liverworts, have been found in Mid Ordovician rocks dated to about ma|476. In Mid Silurian rocks there are fossils of actual plants including clubmosses such as "Baragwanathia"; most were under convert|10|cm|in high, and some appear closely related to vascular plants, the group that includes trees.cite journal
author=Kenrick, P., and Crane, P.R. | title=The origin and early evolution of plants on land
journal=Nature | volume=389 | month=September | year=1997
url=http://botit.botany.wisc.edu/courses/botany_940/06EvidEvol/papers/KendrickCrane1997.pdf
accessdate=2008-09-05
format=PDF
doi=10.1038/37918
pages=33
]

By the Late Devonian, trees such as "Archaeopteris" were so abundant that they changed river systems from mostly braided to mostly meandering, because their roots bound the soil firmly. [citation
contribution="Afforestation – the First Forests" | author=Scheckler, S.E. | pages=67–70
title=Palaeobiology II: A Synthesis | editor=Briggs, D.E.G., and Crowther, P.R. | date=2001
publisher=Blackwell | isbn=0632051493
url=http://books.google.co.uk/books?id=AHsrhGOTRM4C&pg=PA69&lpg=PA69&dq=devonian+meandering+plants+trees&source=web&ots=ImrDW61pBt&sig=RsDgJXv-NNu6Rxk_yFpb5espyLY&hl=en&sa=X&oi=book_result&resnum=3&ct=result | accessdate=2008-09-05
] In fact they caused a "Late Devonian wood crisis", [The phrase "Late Devonian wood crisis" is used at cite web
url=http://www.palaeos.com/Vertebrates/Units/150Tetrapoda/150.150.html | title=Palaeos – Tetrapoda: "Acanthostega" | accessdate=2008-09-05
] because:
*They removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus causing an ice age in the Carboniferous period.cite journal
author=Algeo, T.J., and Scheckler, S.E.
title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events
journal=Philosophical Transactions of the Royal Society: Biology | year=1998 | volume=353 | pages=113–130
url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1692181&blobtype=pdf | accessdate=2008-09-05
doi=10.1098/rstb.1998.0195
] In later ecosystems the carbon dioxide "locked up" in wood is returned to the atmosphere by decomposition of dead wood. However the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian. [cite journal
author=Taylor T.N., and Osborn J.M. | year=1996
title=The importance of fungi in shaping the paleoecosystem
journal=Review of Paleobotany and Palynology | volume=90 |pages=249–262
doi=10.1016/0034-6667(95)00086-0
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6W-454YDFK-7&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=9d5d008d99d044684e947ad74b05514d | accessdate=2008-09-05
]
*The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused algal blooms whose high consumption of oxygen caused anoxic events in deeper waters, increasing the extinction rate among deep-water animals.

Land invertebrates

Animals had to change their feeding and excretory systems, and most land animals developed internal fertilization of their eggs. The difference in refractive index between water and air required changes in their eyes. On the other hand in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in air encouraged the development of hearing.

Some trace fossils from the Cambrian-Ordovician boundary about ma|490 are interpreted as the tracks of large amphibious arthropods on coastal sand dunes, and may have been made by euthycarcinoids, [cite journal
author=MacNaughton, R.B., Cole, J.M., Dalrymple, R.W., Braddy, S.J., Briggs, D.E.G., and Lukie, T.D.
title=First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada
journal=Geology | month=May | year=2002 | volume=30| issue=5 | pages=391–394
doi=10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2
url=http://geology.geoscienceworld.org/cgi/content/abstract/30/5/391 | accessdate=2008-09-05
] which are thought to be evolutionary "aunts" of myriapods. [cite journal
author=Vaccari, N.E., Edgecombe, G.D., Escudero, C. | year=2004
title=Cambrian origins and affinities of an enigmatic fossil group of arthropods
journal=Nature | volume=430 | pages =554–557
doi=10.1038/nature02705
] Other trace fossils from the Late Ordovician a little over ma|445 probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and alluvial plains shortly before the Silurian-Devonian boundary, about ma|415, including signs that some arthropods ate plants. [cite journal
author=Buatois, L.A., Mangano, M.G., Genise, JF., and Taylor, T.N. | month=June | year=1998
title=The ichnologic record of the continental invertebrate invasion; evolutionary trends in environmental expansion, ecospace utilization, and behavioral complexity
journal=Palaios | volume=13 | issue=3 | pages=217–240
url=http://palaios.sepmonline.org/cgi/content/abstract/13/3/217 | accessdate=2008-09-05
doi=10.2307/3515447
] Arthropods were well pre-adapted to colonise land, because their existing jointed exoskeletons provided protection against dessication, support against gravity and a means of locomotion that was not dependent on water.cite book
author=Cowen, R. | title=History of Life | edition=3rd | publisher=Blackwell Science
isbn=0632044446 | page=p. 126
]

The fossil record of other major invertebrate groups on land is poor: none at all for non-parasitic flatworms, nematodes or nemerteans; some parasitic nematodes have been fossilized in amber; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of gastropods on land date from the Late Carboniferous, and this group may have had to wait until leaf litter became abundant enough to provide the moist conditions they need.citation
contribution="Terrestrialization of Animals" | author=Selden, P.A. | pages=71–74
title=Palaeobiology II: A Synthesis | editor=Briggs, D.E.G., and Crowther, P.R. | date=2001
publisher=Blackwell | isbn=0632051493
url=http://books.google.co.uk/books?id=AHsrhGOTRM4C&pg=PA71&lpg=PA71&dq=%22Terrestrialization+of+Animals%22+selden&source=web&ots=ImrDW71qDp&sig=JptdVx34SMIjKamHXDnEpKSE78s&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA74,M1 | accessdate=2008-09-05
]

Land vertebrates

clade
label1="Fish"
1=clade
1=Osteolepiformes ("fish")
2=clade
1=Panderichthyidae
2=clade
1= Obruchevichthidae
2=clade
1="Acanthostega"
2=clade
1="Ichthyostega"
2=clade
1="Tulerpeton"
2=clade
1=Early amphibians
2=clade
1=Anthracosauria
2=Amniotes
Family tree of tetrapods

Tetrapods, vertebrates with four limbs, evolved from the most closely-related fish in a relatively short timespan in the Late Devonian, from ma|370| to ma|360. [cite journal
author=Gordon, M.S, Graham, J.B., and Wang, T. | title=Revisiting the Vertebrate Invasion of the Land
journal=Physiological and Biochemical Zoology | volume=77 | issue=5
date= September/October 2004 | doi=10.1086/425182 | pages=697–699
] From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However in 1987 nearly-complete fossils of "Acanthostega" from about ma|363 showed that this Late Devonian transitional animal had legs and both lungs and gills, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that "Acanthostega", which was about convert|1|m|ft long, was a wholly aquatic predator that hunted in shallow water . Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the shoulder girdle and it has a distinct neck.cite newspaper
author= Clack, J.A. | title=Getting a Leg Up on Land
newspaper=Scientific American | date=November, 2005 |
url=http://www.sciam.com/article.cfm?id=getting-a-leg-up-on-land | accessdate=2008-09-06
]

The Devonian proliferation of land plants may help to explain why air-breathing would have been advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.

Later discoveries revealed earlier transitional forms between "Acanthostega" and completely fish-like animals. [cite journal
author=Daeschler, E.B., Shubin, N.H., and Jenkins, F.A.
title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan | date=April 2006
journal=Nature | volume=440 | pages=757–763 | doi=10.1038/nature04639
url=http://www.com.univ-mrs.fr/~boudouresque/Publications_DOM_2006_2007/Daeschler_et_al_2006_Nature.pdf
accessdate=2008-09-06
] Unfortunately there is then a gap of about 30 million years between the fossils of ancestral tetrapods and Mid Carboniferous fossils of vertebrates that look well-adapted for life on land. Some of these look like early relatives of modern amphibians, most of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the amniotes, whose water-proof skins and eggs enable them to live and breed far from water.cite journal
author=Ahlberg, P.E., and Milner, A.R. | date=April 1994
title=The Origin and Early Diversification of Tetrapods | journal= Nature | volume=368 | pages=507–514
doi=10.1038/368507a0
url=http://www.nature.com/nature/journal/v368/n6471/abs/368507a0.html | accessdate=2008-09-06
]

Dinosaurs, birds and mammals

clade
label1=Amniotes
1=clade
label1=Synapsids
1=clade
1=Early synapsids (extinct)
label2=Pelycosaurs
2=clade
1=Extinct pelycosaurs
label2=Therapsids
2=clade
1=Extinct therapsids
label2=Mammaliformes
2=clade
label1=
1=Extinct mammaliformes
label2=
2=Mammals

label2=Sauropsids
2=clade
1=clade
1=Anapsids; whether turtles belong here is debated [cite journal
author=Debraga, M., and Rieppel, O. | date=July 1997
title=Reptile phylogeny and the interrelationships of turtles
journal=Zoological Journal of the Linnean Society | volume=120 | issue=3 | pages=281–354
doi=10.1111/j.1096-3642.1997.tb01280.x
url=http://www3.interscience.wiley.com/journal/119830935/abstract | acccessdate=2008-09-07
]
label2=
2=clade
1=Captorhinidae and Protorothyrididae
label2=Diapsids
2=clade
1=Araeoscelidia (extinct)
label2=
2=clade
label1=
1=Squamata (lizards and snakes)
label2=Archosaurs
2=clade
1=Extinct archosaurs
2=Crocodilians
label3=
3=clade
1=Pterosaurs (extinct)
label2=Dinosaurs
2=clade
label11=Saurischians
1=clade
label1=Theropods
1=clade
label1=
1=Extinct
theropods
label2=
2=Birds

2=Sauropods
(extinct)

label2=
2=Ornithischians (extinct)
Possible family tree of dinosaurs, birds and mammals
Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period, between ma|330| and ma|314. The earliest fossils of the two surviving amniote groups, synapsids and sauropsids, date from around ma|313.cite journal
author= Benton M.J., and Donoghue, P.C.J.
title=Paleontological Evidence to Date the Tree of Life | date=2007
journal=Molecular Biology and Evolution | volume=24 | issue=1 | pages=26–53 | doi=10.1093/molbev/msl150
url=http://mbe.oxfordjournals.org/cgi/content/full/24/1/26 | accessdate=2008-09-07
pmid=17047029
] cite journal
author= Benton M.J.
title=Phylogeny of the Major Tetrapod Groups: Morphological Data and Divergence Dates
journal=Journal of Molecular Evolution | volume=30 | issue=5 | date= May, 1990 | pages=409–424
doi=10.1007/BF02101113
url=http://www.springerlink.com/content/k152294003652458/ | accessdate=2008-09-07
] The synapsid pelycosaurs and their descendants the therapsids are the most common land vertebrates in the best-known Permian fossil beds, between ma|229| and ma|251. However at the time these were all in temperate zones at middle latitudes, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians. [cite journal
author=Sidor, C.A., O'Keefe, F.R., Damiani, R., Steyer, J.S., Smith, R.M.H., Larsson, H.C.E., Sereno, P.C., Ide, O, and Maga, A.
title=Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea | date=April 2005
journal=Nature | volume=434 | pages=886–889 | doi=10.1038/nature03393
url=http://www.nature.com/nature/journal/v434/n7035/full/nature03393.html | accessdate=2008-09-08
]

The Permian-Triassic extinction wiped out almost all land vertebrates, [cite journal
author=Smith, R., and Botha, J. | date=September-October 2005
title=The recovery of terrestrial vertebrate diversity in the South African Karoo Basin after the end-Permian extinction
journal=Comptes Rendus Palevol | volume=4 | issues=6-7 | pages=623–636 | doi=10.1016/j.crpv.2005.07.005
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6X1G-4GYH7VN-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=add24b0622f2aff0b41e7c42a3160fa7
accessdate=2008-09-08
] as well as the great majority of other life. [cite book
author=Benton M.J. | date=2005
title=When Life Nearly Died: The Greatest Mass Extinction of All Time
publisher=Thames & Hudson | isbn=978-0500285732
] During the slow recovery from this catastrophe, a previously obscure group became the most abundant and diverse terrestrial vertebrates: a few fossils of archosauriformes ("shaped like archosaurs") have been found in Late Permian rocks, [citation
author=Gauthier, J., Cannatella, D.C., de Queiroz, K., Kluge, A.G., and Rowe, T. | date=1989
chapter=Tetrapod Phylogeny | title=The Hierarchy of Life | publisher=Elsevier Science | page=p. 345
editor=B. Fernholm, B., Bremer K., and Jörnvall, H.
url=http://si-pddr.si.edu/dspace/bitstream/10088/4689/1/VZ_1989GauthieretalHierLife.pdf
accessdate=2008-09-08
] but by the Mid Triassic archosaurs were the dominant land vertebrates. Dinosaurs evolved from one group of archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods, between ma|199| and ma|65. [citation
author=Benton, M.J. | date=March 1983
title=Dinosaur Success in the Triassic: a Noncompetitive Ecological Model
journal=Quarterly Review of Biology | volume=58 | issue=1
url=http://palaeo.gly.bris.ac.uk/Benton/reprints/1983success.pdf | accessdate=2008-09-08
]

In the Late Jurassic birds evolved from one group of small, predatory theropod dinosaurs. The first birds inherited teeth and long, bony tails from their dinosaur ancestors,cite book |last=Padian |first=Kevin. |year=2004 |chapter=Basal Avialae |editor=Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.) |title=The Dinosauria |edition=Second Edition |location=Berkeley |publisher=University of California Press |pages=210-231 |isbn=0-520-24209-2] but some developed horny, toothless beaks by the very Late Jurassic [cite journal
author=Hou, L., Zhou, Z., Martin, L. D., and Feduccia, A. | date=October 2002
title=A beaked bird from the Jurassic of China
journal=Nature | volume=377 | pages=616–618 | doi=10.1038/377616a0
url=http://www.nature.com/nature/journal/v377/n6550/abs/377616a0.html | accessdate=2008-09-08
] and short pygostyle tails by the Early Cretaceous. [cite journal
author= Clarke, J.A., Zhou, Z., and Zhang, F. | date=2006
title=Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of "Yixianornis grabaui"
journal=Journal of Anatomy | volume=208 | issue=3 | pages=287–308 | doi=10.1111/j.1469-7580.2006.00534.x
url=http://www3.interscience.wiley.com/journal/118559634/abstract?CRETRY=1&SRETRY=0
accessdate=2008-09-08
]

While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the mammaliform successors of the therapsids could only survive as small, mainly nocturnal insectivores. This apparent set-back may actually have promoted the evolution of mammals, for example nocturnal life may have have accelerated the development of endothermy ("warm-bloodedness") and hair or fur. cite journal | author=Ruben, J.A., and Jones, T.D. | title=Selective Factors Associated with the Origin of Fur and Feathers | journal=American Zoologist | date=2000 | volume=40 | issue=4 | pages=585–596 |doi=10.1093/icb/40.4.585 | url=http://icb.oxfordjournals.org/cgi/content/full/40/4/585] By ma|195 in the Early Jurassic there were animals that were very nearly mammals. [cite journal
author=Luo, Z-X., Crompton, A.W., and Sun , A-L. | date=May 2001
title=A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics
journal=Science | volume=292 | issue=5521 | pages=1535–1540 | doi=10.1126/science.1058476
url=http://www.sciencemag.org/cgi/content/full/292/5521/1535 | accessdate=2008-09-08
pmid=11375489
] Unfortunately there is a gap in the fossil record throughout the Mid Jurassic.cite journal | last=Cifelli | first=R.L. | title=Early mammalian radiations | journal=Journal of Paleontology | date=November 2001 | url=http://findarticles.com/p/articles/mi_qa3790/is_200111/ai_n8958762/pg_6] However fossil teeth discovered in Madagascar indicate that true mammals existed at least ma|167. [cite journal
author=Flynn, J.J., Parrish, J.M. Rakotosamimanana, B., Simpson, W.F., and Wyss, A.R.
title=A Middle Jurassic mammal from Madagascar | date=September 1999
journal=Nature | volume=401 | pages=57–60 | doi=10.1038/43420
url=http://www.nature.com/nature/journal/v401/n6748/abs/401057a0.html | accessdate=2008-09-08
] After dominating land vertebrate niches for about 150 million years, the dinosaurs perished ma|65 in the Cretaceous–Tertiary extinction along with many other groups of organisms.cite journal |author=MacLeod N, Rawson PF, Forey PL, Banner FT, Boudagher-Fadel MK, Bown PR, Burnett JA, Chambers, P, Culver S, Evans SE, Jeffery C, Kaminski MA, Lord AR, Milner AC, Milner AR, Morris N, Owen E, Rosen BR, Smith AB, Taylor PD, Urquhart E, Young JR |title=The Cretaceous–Tertiary biotic transition |year=1997 |journal=Journal of the Geological Society |volume=154 |issue=2 |pages=265–292 |url=http://findarticles.com/p/articles/mi_qa3721/is_199703/ai_n8738406/print |doi=10.1144/gsjgs.154.2.0265] Mammals throughout the time of the dinosaurs had been restricted to a narrow range of taxa, sizes and shapes, but increased rapidly in size and diversity after the extinction, [cite journal
author=Alroy J. | date=March 1999
title=The fossil record of North American mammals: evidence for a Paleocene evolutionary radiation
journal=Systematic biology | volume=48 | issue=1 | pages=107–18 | pmid=12078635
doi=10.1080/106351599260472
] [cite journal
author=Archibald J.D., and Deutschman D.H. | date= June 2001
title=Quantitative Analysis of the Timing of the Origin and Diversification of Extant Placental Orders
journal= Journal of Mammalian Evolution | volume=8 | issue=2 | pages=107-124
url=http://www.ingentaconnect.com/content/klu/jomm/2001/00000008/00000002/00342277
accessdate=2008-09-24
] with bats taking to the air within 13 million years, [cite journal
author=Simmons, N.B., Seymour,K.L., Habersetzer, J.,and Gunnell, G.F. | date=February 2008
title=Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation
journal=Nature | volume=451 | pages=818–821 | doi=10.1038/nature06549
] and whales to the sea within 15 million years. [cite journal|author = J. G. M. Thewissen, S. I. Madar, and S. T. Hussain|title = "Ambulocetus natans", an Eocene cetacean (Mammalia) from Pakistan|journal = Courier Forschungsinstitut Senckenberg|volume = 191|pages = 1–86|year = 1996 ]

Flowering plants

The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, between ma|130| | and ma|90,citation
author=Crane, P.R., Friis, E.M., and Pedersen, K.R.
chapter=The Origin and Early Diversification of Angiosperms | pages=233–250
title=Shaking the Tree: Readings from Nature in the History of Life | editor=Gee, H.
publisher=University of Chicago Press | date=2000 | isbn=0226284964
url=http://books.google.co.uk/books?hl=en&lr=&id=ZJe_Dmdbm-QC&oi=fnd&pg=PA233&dq=evolution+flowering+plant+angiosperm&ots=abVpqx_cP8&sig=z1HvmrRLdJP9oPkN0bffbAyriEI#PPA233,M1
accessdate=2008-09-09
] cite journal
author=Crepet, W.L. | date=November 2000
title=Progress in understanding angiosperm history, success, and relationships: Darwin’s abominably "perplexing phenomenon"
journal=Proceedings of the National Academy of Sciences | volume=97 | issue=24 | pages= 12939–12941
url=http://www.pnas.org/content/97/24/12939.full.pdf+html | accessdate=2008-09-09
doi=10.1073/pnas.97.24.12939
pmid=11087846
] and that their rise was associated with that of pollinating insects. Among modern flowering plants Magnolias are thought to be close to the common ancestor of the group. However paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants.

ocial insects

The social insects are remarkable because the great majority of individuals in each colony are sterile. This appears contrary to basic concepts of evolution such as natural selection and the selfish gene. In fact there are very few eusocial insect species: only 15 out of approximately 2,600 living families of insects contain eusocial species, and it seems that eusociality has evolved independently only 12 times among arthropods, although some have lineages diversified into several familes. Nevertheless social insects have been spectacularly successful; for example although ants and termites account for only about 2% of known insect species, they form over 50% of the total mass of insects. Their ability to control a territory appears to be the foundation of their success.cite journal
author=Wilson, E.O., and Hölldobler, B. | title=Eusociality: Origin and consequences
journal=Proceedigs of the National Acedemy of Sciences | date=September 2005 | volume=102 | issue=38
pages=13367–13371 | doi=10.1073/pnas.0505858102
url=http://www.pnas.org/content/102/38/13367.full.pdf+html | accessdate=2008-09-07
pmid=16157878
]

The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual haplodiploid method of sex determination, which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed. [cite journal
author = Hughes, W.O.H., Oldroyd, B.P., Beekman, M., and Ratnieks, F.L.W.
title = Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality
journal = Science | volume = 320 | issue = 5880 | pages = 1213–1216
publisher = American Association for the Advancement of Science | date = 2008-05-30
url = http://www.sciencemag.org/cgi/content/abstract/320/5880/1213 | format = html
accessdate = 2008-08-04
doi = 10.1126/science.1156108
pmid = 18511689
] However Wilson and Hölldobler argue that this explanation is faulty: for example, it is based on kin selection, but there is no evidence of nepotism in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages though co-operative foraging. In support of this explanation they cite the appearance of eusociality in bathyergid mole rats, which are not haplodiploid. [cite journal
author=Lovegrove, B.G.
title=The evolution of eusociality in molerats (Bathyergidae): a question of risks, numbers, and costs
journal=Behavioral Ecology and Sociobiology | volume=28 | issue=1 | date=January, 1991 | pages=37–45
doi=10.1007/BF00172137
url=http://www.springerlink.com/content/k4n52v522l816125/ | accessdate=2008-09-07
]

The earliest fossils of insects have been found in Early Devonian rocks from about ma|400, which preserve only a few varieties of flightless insect. The Mazon Creek lagerstätten from the Late Carboniferous, about ma|300, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Mid Cenozoic. [citation
author=Labandeira, C., and Eble, G.J.
contribution=The Fossil Record of Insect Diversity and Disparity
title=Gondwana Alive: Biodiversity and the Evolving Biosphere
editor=Anderson, J., Thackeray, F., van Wyk, B., and de Wit, M.
publisher=Witwatersrand University Press | date=2000
url=http://www.santafe.edu/research/publications/workingpapers/00-08-044.pdf | accessdate=2008-09-07
]

Humans

Modern humans evolved from a lineage of upright-walking apes that has been traced back over ma|6 to "Sahelanthropus". [cite journal
author=Brunet M., Guy, F., Pilbeam, D., Mackaye, H.T., "et al"
title=A new hominid from the Upper Miocene of Chad, Central Africa
journal=Nature | volume=418 | pages=145–151 | date=July 2002 | doi=10.1038/nature00879
url=http://www.nature.com/nature/journal/v418/n6894/abs/nature00879.html | accessdate=2008-09-09
] The first known stone tools were made about ma|2.5, apparently by "Australopithecus garhi", and were found near animal bones that bear scratches made by these tools. [cite journal
author=de Heinzelin, J., Clark, J.D., White, T., "et al"
title=Environment and Behavior of 2.5-Million-Year-Old Bouri Hominids
journal=Science | volume=284 | issue=5414 | pages=625–629 | date=April 1999
doi=10.1126/science.284.5414.625
url=http://www.sciencemag.org/cgi/content/full/sci;284/5414/625 | accessdate=2008-09-09
pmid=10213682
] The earliest hominines had chimp-sized brains, but there has been a four-fold increase in the last 3 million years; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence. [cite journal
author=De Miguel, C., and M. Henneberg, M.
title=Variation in hominid brain size: How much is due to method?
journal=HOMO - Journal of Comparative Human Biology | issue=1 | date=2001
doi=10.1078/0018-442X-00019
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7GW4-4DPCHXC-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=aef79dbca1f189c885cfe9f36636b131
accessdate=2008-09-09
volume=52
pages=3–58
] There is a long-running debate about whether modern humans evolved all over the world from existing advanced hominines or are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species. [cite book|last=Leakey|first=Richard|title=The Origin of Humankind|publisher=Basic Books|location=New York, NY|date=1994|series=Science Masters Series|pages=pages 87-89|isbn=0465053130] There is also debate about whether anatomically-modern humans had an intellectual, cultural and technological "Great Leap Forward" under 100,000 years ago and, if so, whether this was due to neurological changes that are not visible in fossils. [cite journal
last=Mellars
first=Paul
authorlink=Paul Mellars
url=http://www.pnas.org/cgi/reprint/0510792103v1
title=Why did modern human populations disperse from Africa ca. 60,000 years ago?
year=2006
doi=10.1073/pnas.0510792103
journal=Proceedings of the National Academy of Sciences
volume=103
pages=9381
pmid=16772383
]

Mass extinctions

Life on earth has suffered occasional mass extinctions at least since ma|542. Although they are disasters at the time, mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one. [cite book | last=Benton | first =M.J. | title=Vertebrate Palaeontology | publisher=Blackwell | year=2004 | chapter=6. Reptiles Of The Triassic | url=http://www.blackwellpublishing.com/book.asp?ref=0632056371] [cite journal| last=Van Valkenburgh | first= B.| year=1999| url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.27.1.463 | title=Major patterns in the history of xarnivorous mammals | journal=Annual Review of Earth and Planetary Sciences| volume= 26| pages= 463–493 | doi= 10.1146/annurev.earth.27.1.463]

The fossil record appears to show that the gaps between mass extinctions are becoming longer and the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways:cite web
date=2001-01-06| first=Norman| last= MacLeod | title=Extinction!
url=http://www.firstscience.com/home/articles/earth/extinction-page-2-1_1258.html | accessdate=2008-09-11
]
*The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted. [cite journal| last=Martin| first= R.E.| year=1995| title=Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans| journal=Global and Planetary Change| volume =11| issue= 1| doi=10.1016/0921-8181(94)00011-2| pages=1] [cite journal| last=Martin| first= R.E.| year=1996| title=Secular increase in nutrient levels through the Phanerozoic: Implications for productivity, biomass, and diversity of the marine biosphere| journal=Palaios| volume= 11| pages=209–219| doi=10.2307/3515230]
*Reasonably complete fossils are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera which were often defined solely to accommodate these finds – the story of "Anomalocaris" is an example of this. The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly.

The present

Oxygenic photosynthesis accounts for virtually all of the production of organic matter form non-organic ingredients. Production is split about evenly between land and marine plants, and phytoplankton are the dominant marine producers. [author=Christopher B. Field, C.B., Behrenfeld, M.J., Randerson, J.T., and Falkowski, P. | date=July 1998
title=Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components
journal=Science | volume=281 |issue=5374 | pages=237-240 | doi=10.1126/science.281.5374.237
url=http://www.sciencemag.org/cgi/content/full/sci;281/5374/237 | accessdate=2008-09-13
]

The processes that drive evolution are still operating. Well-known examples include the changes in coloration of the peppered moth over the last 200 years and the more recent appearance of pathogens that are resistant to antibiotics. [cite journal
author=Grant, B.S., and Wiseman, L.L. date=2002
title=Recent History of Melanism in American Peppered Moths
journal=Journal of Heredity | volume=93 | issue=2 | pages=86–90 | issn=1465-7333
url=http://jhered.oxfordjournals.org/cgi/content/abstract/93/2/86 | accessdate=2008-09-11
doi=10.1093/jhered/93.2.86
year=2002
pmid=12140267

] [cite journal
author=Levin, B.R., Perrot, V., and Walker, N. | date=March 2000
title=Compensatory Mutations, Antibiotic Resistance and the Population Genetics of Adaptive Evolution in Bacteria
journal=Genetics | volume=154 | pages=985–997
url=http://www.genetics.org/cgi/content/abstract/154/3/985 | accessdate=2008-09-11
pmid=10757748
] There is even evidence that humans are still evolving, and possibly at an accelerating rate over the last 40,000 years. [cite journal
author=Hawks, J., Wang, E.T., Cochran, G.M., Harpending, H.C. and Moyzis, R.K. | date=December 2007
title=Recent acceleration of human adaptive evolution
journal=Proceedings of the National Academy of Sciences | volume=104| issue=52 | pages=20753–20758
doi=10.1073/pnas.0707650104
url=http://www.pnas.org/content/104/52/20753.full | accessdate=2008-09-11
pmid=18087044
]

See also

Footnotes

References

Further reading

* cite book
first = Richard | last = Cowen | title = History of Life | edition = 4th edition
publisher = Blackwell Publishing Limited | year = 2004 | id = ISBN 978-1405117562

* cite book
first = Richard | last = Dawkins | authorlink = Richard Dawkins
title = The Ancestor's Tale, A Pilgrimage to the Dawn of Life
publisher = Houghton Mifflin Company | location = Boston | year = 2004 | id = ISBN 0-618-00583-8

* cite book
first = Richard | last = Dawkins | authorlink = Richard Dawkins
title=The Selfish Gene | year=1990
publisher=Oxford University Press | isbn=0192860925

* cite book
author = Smith, John Maynard; Eörs Szathmáry
title = The Major Transitions in Evolution
publisher = Oxford University Press
location = Oxfordshire
year=1997
isbn=0-198-50294-X

External links

General information
* [http://www.fossilmuseum.net/Evolution.htm General information on evolution- Fossil Museum nav.]
* [http://evolution.berkeley.edu/ Understanding Evolution from University of California, Berkeley]
* [http://nationalacademies.org/evolution/ National Academies Evolution Resources]
* [http://tellapallet.com/tree_of_life.htm Evolution poster- PDF format "tree of life"]
* [http://www.newscientist.com/channel/life/evolution Everything you wanted to know about evolution by "New Scientist"]
* [http://science.howstuffworks.com/evolution.htm/printable Howstuffworks.com — How Evolution Works]
* [http://anthro.palomar.edu/synthetic/ Synthetic Theory Of Evolution: An Introduction to Modern Evolutionary Concepts and Theories]

History of evolutionary thought
* [http://darwin-online.org.uk The Complete Work of Charles Darwin Online]
* [http://www.rationalrevolution.net/articles/understanding_evolution.htm Understanding Evolution: History, Theory, Evidence, and Implications]


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