Editing Symbiogenesis (section)

=== Date ===

The question of when the transition from prokaryotic to eukaryotic form occurred and when the first [[crown group]] eukaryotes appeared on earth is unresolved. The oldest known body fossils that can be positively assigned to the Eukaryota are acanthomorphic [[acritarch]]s from the 1.631 [[Gya (unit)|Gya]] Deonar Formation of India.<ref>{{Cite journal |last=Prasad |first=Pijai |date=August 2005 |title=Organic-walled microfossils from the Proterozoic Vindhyan Supergroup of Son Valley, Madhya Pradesh, India |url=http://14.139.63.228:8080/pbrep/bitstream/123456789/1084/1/PbV54_13.pdf |journal=Paleobotanist |volume=54 }}</ref> These fossils can still be identified as derived post-nuclear eukaryotes with a sophisticated, morphology-generating [[cytoskeleton]] sustained by mitochondria.<ref>{{Cite journal |last=Butterfield |first=Nicholas J. |date=2014-11-26 |title=Early evolution of the Eukaryota |journal=Palaeontology |volume=58 |issue=1 |pages=5–17 |doi=10.1111/pala.12139|doi-access=free }}</ref> This fossil evidence indicates that endosymbiotic acquisition of [[alphaproteobacteria]] must have occurred before 1.6 Gya. Molecular clocks have also been used to estimate the last eukaryotic common ancestor, however these methods have large inherent uncertainty and give a wide range of dates. Reasonable results include the estimate of c. 1.8 Gya.<ref>{{cite journal |last1=Parfrey |first1=Laura Wegener |last2=Lahr |first2=Daniel J. G. |last3=Knoll |first3=Andrew H. |last4=Katz |first4=Laura A. |title=Estimating the timing of early eukaryotic diversification with multigene molecular clocks |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=108 |issue=33 |pages=13624–9 |date=August 2011 |pmid=21810989 |pmc=3158185 |doi=10.1073/pnas.1110633108 |bibcode=2011PNAS..10813624P |doi-access=free }}</ref> A 2.3 Gya estimate<ref>{{cite journal |last1=Hedges |first1=S. Blair |last2=Blair |first2=Jaime E. |last3=Venturi |first3=Maria L. |last4=Shoe |first4=Jason L. |title=A molecular timescale of eukaryote evolution and the rise of complex multicellular life |journal=BMC Evolutionary Biology |volume=4 |pages=2 |date=January 2004 |pmid=15005799 |pmc=341452 |doi=10.1186/1471-2148-4-2 |doi-access=free }}</ref> also seems reasonable, and has the added attraction of coinciding with one of the most pronounced biogeochemical perturbations in Earth history, the early Palaeoproterozoic [[Great Oxygenation Event]]. The marked increase in atmospheric oxygen concentrations at that time has been suggested as a contributing cause of eukaryogenesis, inducing the evolution of oxygen-detoxifying mitochondria.<ref>{{cite journal |last1=Gross |first1=Jeferson |last2=Bhattacharya |first2=Debashish |title=Uniting sex and eukaryote origins in an emerging oxygenic world |journal=Biology Direct |volume=5 |pages=53 |date=August 2010 |pmid=20731852 |pmc=2933680 |doi=10.1186/1745-6150-5-53 |doi-access=free }}</ref> Alternatively, the Great Oxidation Event might be a consequence of eukaryogenesis, and its impact on the export and burial of organic carbon.<ref>{{Cite journal |last=Butterfield |first=Nicholas J. |date=1997 |title=Plankton ecology and the Proterozoic-Phanerozoic transition |journal=Paleobiology |volume=23 |issue=2 |pages=247–262 |doi=10.1017/S009483730001681X|bibcode=1997Pbio...23..247B |s2cid=140642074 }}</ref>