Editing Precambrian (section)

==Life forms==
{{Further|Origin of life|Avalon explosion|Earliest known life forms}}
A specific date for the origin of life has not been determined. [[Carbon]] found in 3.8 billion-year-old rocks (Archean Eon) from islands off western [[Greenland]] may be of organic origin. Well-preserved microscopic fossils of [[bacteria]] older than 3.46 billion years have been found in [[Western Australia]].<ref>{{cite book |last1=Brun |first1=Yves |author-link=Yves Brun |first2=Lawrence J. |last2=Shimkets |title=Prokaryotic development |publisher=[[ASM Press]] |date=January 2000 |page=114 |isbn=978-1-55581-158-7 |url=https://books.google.com/books?id=wRxrAAAAMAAJ}}</ref> Probable fossils 100 million years older have been found in the same area. However, there is evidence that life could have evolved over 4.280 billion years ago.<ref name="NAT-20170301">{{cite journal |author=Dodd, Matthew S. |author2=Papineau, Dominic |author3=Grenne, Tor |author4=slack, John F. |author5=Rittner, Martin |author6=Pirajno, Franco |author7=O'Neil, Jonathan |author8=Little, Crispin T. S. |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates|journal=Nature |volume=543 |issue=7643 |pages=60–64 |date=2 March 2017 | doi=10.1038/nature21377|pmid=28252057 |bibcode=2017Natur.543...60D |doi-access=free }}</ref><ref name="NYT-20170301">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest |url=https://www.nytimes.com/2017/03/01/science/earths-oldest-bacteria-fossils.html |date=1 March 2017 |work=[[The New York Times]] |access-date=2 March 2017 }}</ref><ref name="BBC-20170301">{{cite web |last=Ghosh |first=Pallab |title=Earliest evidence of life on Earth 'found' |url=https://www.bbc.co.uk/news/science-environment-39117523 |work=[[BBC News]] |date=1 March 2017 |access-date=2 March 2017}}</ref><ref name="4.3b oldest">{{cite news |last1=Dunham |first1=Will |title=Canadian bacteria-like fossils called oldest evidence of life |url=http://ca.reuters.com/article/topNews/idCAKBN16858B?sp=true |archive-url=https://web.archive.org/web/20170302114728/http://ca.reuters.com/article/topNews/idCAKBN16858B?sp=true |url-status=dead |archive-date=March 2, 2017 |date=1 March 2017 |work=[[Reuters]] |access-date=1 March 2017 }}</ref> There is a fairly solid record of bacterial life throughout the remainder (Proterozoic Eon) of the Precambrian.

Complex multicellular organisms may have appeared as early as 2100 Ma.<ref>{{cite journal |last1=Albani |first1=Abderrazak El |last2=Bengtson |first2=Stefan |last3=Canfield |first3=Donald E. |last4=Bekker |first4=Andrey |last5=Macchiarelli |first5=Roberto |last6=Mazurier |first6=Arnaud |last7=Hammarlund |first7=Emma U. |last8=Boulvais |first8=Philippe |last9=Dupuy |first9=Jean-Jacques |last10=Fontaine |first10=Claude |last11=Fürsich |first11=Franz T. |last12=Gauthier-Lafaye |first12=François |last13=Janvier |first13=Philippe |last14=Javaux |first14=Emmanuelle |last15=Ossa |first15=Frantz Ossa |last16=Pierson-Wickmann |first16=Anne-Catherine |last17=Riboulleau |first17=Armelle |last18=Sardini |first18=Paul |last19=Vachard |first19=Daniel |last20=Whitehouse |first20=Martin |last21=Meunier |first21=Alain |title=Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago |journal=Nature |date=July 2010 |volume=466 |issue=7302 |pages=100–104 |doi=10.1038/nature09166|pmid=20596019 |bibcode=2010Natur.466..100A |s2cid=4331375 }}</ref> However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers."<ref>{{cite journal |last1=Donoghue |first1=Philip C. J. |last2=Antcliffe |first2=Jonathan B. |title=Origins of multicellularity |journal=Nature |date=July 2010 |volume=466 |issue=7302 |pages=41–42 |doi=10.1038/466041a|pmid=20596008 |s2cid=4396466 }}</ref> Other possible early complex multicellular organisms include a possible 2450 Ma red alga from the [[Kola Peninsula]],<ref>{{cite journal |last1=Rozanov |first1=A. Yu. |last2=Astafieva |first2=M. M. |title=A unique find of the earliest multicellular algae in the Lower Proterozoic (2.45 Ga) of the Kola Peninsula |journal=Doklady Biological Sciences |date=1 March 2013 |volume=449 |issue=1 |pages=96–98 |doi=10.1134/S0012496613020051|pmid=23652437 |s2cid=15774804 }}</ref> 1650 Ma carbonaceous biosignatures in north China,<ref>{{cite journal |last1=Qu |first1=Yuangao |last2=Zhu |first2=Shixing |last3=Whitehouse |first3=Martin |last4=Engdahl |first4=Anders |last5=McLoughlin |first5=Nicola |title=Carbonaceous biosignatures of the earliest putative macroscopic multicellular eukaryotes from 1630 Ma Tuanshanzi Formation, north China |journal=Precambrian Research |date=1 January 2018 |volume=304 |pages=99–109 |doi=10.1016/j.precamres.2017.11.004|bibcode=2018PreR..304...99Q }}</ref> the 1600 Ma ''[[Rafatazmia]]'',<ref>{{cite journal |last1=Bengtson |first1=Stefan |last2=Sallstedt |first2=Therese |last3=Belivanova |first3=Veneta |last4=Whitehouse |first4=Martin |title=Three-dimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae |journal=PLOS Biology |date=14 March 2017 |volume=15 |issue=3 |pages=e2000735 |doi=10.1371/journal.pbio.2000735|pmid=28291791 |pmc=5349422 |doi-access=free }}</ref> and a possible 1047 Ma ''[[Bangiomorpha]]'' red alga from the Canadian Arctic.<ref>{{cite journal|doi=10.1130/G39829.1|title=Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis|journal=Geology|volume=46|issue=2|pages=135–138|year=2017|last1=Gibson|first1=Timothy M|last2=Shih|first2=Patrick M|last3=Cumming|first3=Vivien M|last4=Fischer|first4=Woodward W|last5=Crockford|first5=Peter W|last6=Hodgskiss|first6=Malcolm S.W|last7=Wörndle|first7=Sarah|last8=Creaser|first8=Robert A|last9=Rainbird|first9=Robert H|last10=Skulski|first10=Thomas M|last11=Halverson|first11=Galen P|url=https://authors.library.caltech.edu/83811/3/2018030.pdf}}</ref> The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period.<ref>{{cite journal |last1=Laflamme |first1=M. |title=Modeling morphological diversity in the oldest large multicellular organisms |journal=Proceedings of the National Academy of Sciences |date=9 September 2014 |volume=111 |issue=36 |pages=12962–12963 |doi=10.1073/pnas.1412523111|pmid=25114212 |pmc=4246935 |bibcode=2014PNAS..11112962L |doi-access=free }}</ref><ref>{{cite journal |last1=Kolesnikov |first1=Anton V. |last2=Rogov |first2=Vladimir I. |last3=Bykova |first3=Natalia V. |last4=Danelian |first4=Taniel |last5=Clausen |first5=Sébastien |last6=Maslov |first6=Andrey V. |last7=Grazhdankin |first7=Dmitriy V. |title=The oldest skeletal macroscopic organism Palaeopascichnus linearis |journal=Precambrian Research |date=October 2018 |volume=316 |pages=24–37 |doi=10.1016/j.precamres.2018.07.017|bibcode=2018PreR..316...24K |s2cid=134885946 }}</ref> A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as [[Ediacaran biota|Ediacaran or Vendian biota]]. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic Eon. By the middle of the following Cambrian Period, a very diverse fauna is recorded in the [[Burgess Shale]], including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the [[Cambrian explosion]] of life.<ref>{{cite book|last1=Fedonkin|first1=Mikhail A.|author-link=Mikhail A. Fedonkin|last2=Gehling|first2=James G.|last3=Grey|first3=Kathleen|last4=Narbonne|first4=Guy M.|last5=Vickers-Rich|first5=Patricia|author5-link=Patricia Vickers-Rich|others=Foreword by [[Arthur C. Clarke]]|date=2007|title=The Rise of Animals: Evolution and Diversification of the Kingdom Animalia|location=Baltimore, Maryland|publisher=[[Johns Hopkins University Press]]|isbn=978-0-8018-8679-9|lccn=2007061351|oclc=85162342|ol=17256629M}}</ref><ref>{{cite book |last1=Dawkins |first1=Richard |author-link=Richard Dawkins |title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution |year=2005 |publisher=[[Houghton Mifflin Harcourt]] |isbn=9780618619160 |first2=Yan |last2=Wong  |url=https://archive.org/details/ancestorstale00rich_0 |url-access=registration |pages=[https://archive.org/details/ancestorstale00rich_0/page/n696 673]}}</ref>

While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic [[Algal mat|mats]] that covered terrestrial areas.<ref>{{Cite encyclopedia |url=http://www.paulselden.net/uploads/7/5/3/2/7532217/elsterrestrialization.pdf |title=Terrestrialization (Precambrian–Devonian) |last=Selden |first=Paul A. |year=2005 |encyclopedia=[[Encyclopedia of Life Sciences]] |publisher=[[John Wiley & Sons, Ltd.]] |doi=10.1038/npg.els.0004145 |isbn=978-0470016176 }}</ref>

Tracks from an animal with leg-like appendages have been found in what was mud 551 million years ago.<ref>{{cite web| url = https://www.independent.co.uk/news/science/archaeology/oldest-fossil-footprints-china-found-discovered-yangtze-virginia-tech-a8386911.html| title = Scientists discover 'oldest footprints on Earth' in southern China dating back 550 million years| website = [[Independent.co.uk]]| date = 7 June 2018}} ''[[The Independent]]''</ref><ref>{{cite journal |last1=Chen |first1=Zhe |last2=Chen |first2=Xiang |last3=Zhou |first3=Chuanming |last4=Yuan |first4=Xunlai |last5=Xiao |first5=Shuhai |title=Late Ediacaran trackways produced by bilaterian animals with paired appendages |journal=Science Advances |date=June 2018 |volume=4 |issue=6 |pages=eaao6691 |doi=10.1126/sciadv.aao6691|pmid=29881773 |pmc=5990303 |bibcode=2018SciA....4.6691C }}</ref>

===Emergence of life===

The [[RNA world]] hypothesis asserts that RNA evolved before coded proteins and DNA genomes.<ref>{{cite journal |vauthors=Fine JL, Pearlman RE |title=On the origin of life: an RNA-focused synthesis and narrative |journal=RNA |volume=29 |issue=8 |pages=1085–98 |date=August 2023 |pmid=37142437 |pmc=10351881 |doi=10.1261/rna.079598.123 }}</ref> During the Hadean Eon (4,567–4,031 Ma) abundant [[Geothermal activity|geothermal]] [[Microenvironment (ecology)|microenvironment]]s were present that may have had the potential to support the synthesis and replication of [[RNA]] and thus possibly the evolution of a primitive life form.<ref name = Salditt2023>{{cite journal |vauthors=Salditt A, Karr L, Salibi E, Le Vay K, Braun D, Mutschler H |title=Ribozyme-mediated RNA synthesis and replication in a model Hadean microenvironment |journal=Nat Commun |volume=14 |issue=1 |pages=1495 |date=March 2023 |pmid=36932102 |pmc=10023712 |doi=10.1038/s41467-023-37206-4 |bibcode=2023NatCo..14.1495S }}</ref> It was shown that porous rock systems comprising heated air-water interfaces could allow [[ribozyme]]-[[Catalysis|catalyzed]] RNA replication of sense and antisense strands that could be followed by strand-dissociation, thus enabling combined synthesis, release and folding of active ribozymes.<ref name = Salditt2023/> This primitive RNA replicative system also may have been able to undergo template strand switching during replication ([[genetic recombination]]) as is known to occur during the RNA replication of extant [[coronavirus]]es.<ref>{{cite journal |vauthors=Su S, Wong G, Shi W, Liu J, Lai AC, Zhou J, Liu W, Bi Y, Gao GF |title=Epidemiology, Genetic Recombination, and Pathogenesis of Coronaviruses |journal=Trends Microbiol |volume=24 |issue=6 |pages=490–502 |date=June 2016 |pmid=27012512 |pmc=7125511 |doi=10.1016/j.tim.2016.03.003 }}</ref>