Editing Extinction event (section)

==Uncertainty in the Proterozoic and earlier eons==
Because most diversity and [[biomass (ecology)|biomass]] on Earth is [[microbial]], and thus difficult to measure via fossils, extinction events placed on-record are those that affect the easily observed, biologically complex component of the [[biosphere]] rather than the total diversity and abundance of life.<ref>{{cite journal |vauthors=Nee S |date=August 2004 |title=Extinction, slime, and bottoms |journal=PLOS Biology |volume=2 |issue=8 |pages=E272 |doi= 10.1371/journal.pbio.0020272 |pmc=509315 |pmid=15314670 |doi-access=free }}</ref> For this reason, well-documented extinction events are confined to the [[Phanerozoic eon]] – with the sole exception of the [[Great Oxidation Event|Oxygen Catastrophe]] in the [[Proterozoic]] – since before the Phanerozoic, all living organisms were either microbial, or if multicellular then soft-bodied. Perhaps due to the absence of a robust microbial fossil record, mass extinctions might only ''seem'' to be mainly a Phanerozoic phenomenon, with merely the ''observable'' extinction rates appearing low before large complex organisms with hard body parts arose.<ref name=Butterfield2007>{{cite journal |vauthors=Butterfield NJ |year=2007 |title=Macroevolution and macroecology through deep time |journal= Palaeontology |volume=50 |issue=1 |pages=41–55 |doi=10.1111/j.1475-4983.2006.00613.x |bibcode=2007Palgy..50...41B |s2cid=59436643 |url= http://eprints.esc.cam.ac.uk/174/1/Butterfield__Palaeontolgy_50_Pt_1_2007_.pdf |access-date=6 October 2019 |url-status=dead |archive-url= https://web.archive.org/web/20220721114458/http://eprints.esc.cam.ac.uk/174/1/Butterfield__Palaeontolgy_50_Pt_1_2007_.pdf |archive-date=21 July 2022 }}</ref>

Extinction occurs at an uneven rate. Based on the [[fossil record]], the [[background extinction rate|background rate of extinctions]] on Earth is about two to five [[taxonomy (biology)|taxonomic]] [[family (biology)|families]] of [[marine animal]]s every million years.{{efn|
Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to [[land animal]]s.
}}

The Oxygen Catastrophe, which occurred around 2.45&nbsp;billion years ago in the [[Paleoproterozoic]], is plausible as the first-ever major extinction event. It was perhaps also the worst-ever, in some sense, but with the Earth's ecology just before that time so poorly understood, and the concept of [[prokaryote]] genera so different from genera of complex life, that it would be difficult to meaningfully compare it to any of the "Big Five" even if Paleoproterozoic life were better known.<ref>{{cite news |vauthors=Plait P |date=28 July 2014 |title=Poisoned planet |website= Slate.com |url= https://slate.com/technology/2014/07/the-great-oxygenation-event-the-earths-first-mass-extinction.html |access-date=8 July 2019}}</ref>

Since the [[Cambrian explosion]], five further major mass extinctions have significantly exceeded the background extinction rate. The most recent and best-known, the [[Cretaceous–Paleogene extinction event]], which occurred approximately {{period start|Paleogene}}&nbsp;Ma (million years ago), was a large-scale mass extinction of animal and plant species in a geologically short period of time.<ref name="Ward 2006">{{cite magazine |vauthors=Ward PD |date=October 2006 |title=Impact from the deep |magazine=[[Scientific American]] |volume=295 |issue=4 |pages=64–71 |bibcode=2006SciAm.295d..64W |doi=10.1038/scientificamerican1006-64 |doi-broken-date=1 November 2024 |pmid=16989482}}</ref> In addition to the five major Phanerozoic mass extinctions, there are numerous lesser ones, and the ongoing mass extinction caused by human activity is sometimes called the [[Holocene extinction|sixth mass extinction]].<ref>{{cite magazine |vauthors=Kluger J |date=25 July 2014 |title=The sixth great extinction is underway – and we're to blame |magazine=[[Time (magazine)|Time]] |url=https://time.com/3035872/sixth-great-extinction/ |access-date=December 14, 2016 }}
:
{{cite news |date=June 22, 2015 |title=Earth is on brink of a sixth mass extinction, scientists say, and it's humans' fault |newspaper=[[The Washington Post]] |url=https://www.washingtonpost.com/news/morning-mix/wp/2015/06/22/the-earth-is-on-the-brink-of-a-sixth-mass-extinction-scientists-say-and-its-humans-fault/ |access-date=December 14, 2016 |vauthors=Kaplan S}}
:
{{cite news |date=October 20, 2015 |title=How humans are driving the sixth mass extinction |newspaper=[[The Guardian]] |url=https://www.theguardian.com/environment/radical-conservation/2015/oct/20/the-four-horsemen-of-the-sixth-mass-extinction |access-date=December 14, 2016 |vauthors=Hance J}}
:
{{cite news |title=Vanishing: The Earth's 6th mass extinction |website=[[CNN]] |url=http://www.cnn.com/specials/world/vanishing-earths-mass-extinction |access-date=December 19, 2016}}
:
{{cite journal |vauthors=Mason R |date=2015 |title=The sixth mass extinction and chemicals in the environment: our environmental deficit is now beyond nature's ability to regenerate |journal=J. Biol. Phys. Chem. |volume=15 |issue=3 |pages=160–176 |doi=10.4024/10MA15F.jbpc.15.03}}
:
{{cite news |date=January 17, 2022 |title=Study confirms sixth mass extinction is currently underway, caused by humans |newspaper=[[The Independent]] |location= |url=https://www.independent.co.uk/climate-change/news/sixth-mass-extinction-global-biodiversity-b1994346.html |access-date=January 18, 2022 |vauthors=Sankaran V}}</ref>