Editing Precambrian (section)

==Subdivisions==
{{main|Timetable of the Precambrian}}
{{Life timeline}}
A terminology has evolved covering the early years of the Earth's existence, as [[radiometric dating]] has allowed absolute dates to be assigned to specific formations and features.<ref>{{cite web| url = https://www.geosociety.org/science/timescale/| title = Geological Society of America's "2009 GSA Geologic Time Scale."| access-date = 2019-08-29| archive-date = 2020-11-05| archive-url = https://web.archive.org/web/20201105231837/https://www.geosociety.org/science/timescale/| url-status = dead}}</ref> The Precambrian is divided into three eons: the [[Hadean]] ({{Period start|Hadean}}–{{Period start|Archean}} Ma), [[Archean]] ({{Period start|Archean}}-{{Period start|Proterozoic}} Ma) and [[Proterozoic]] ({{Period start|Proterozoic}}-{{Period start|Cambrian}} Ma). See [[Timetable of the Precambrian]].
* [[Proterozoic]]: this eon refers to the time from the lower [[Cambrian]] boundary, {{Period start|Cambrian}} Ma, back through {{Period start|Proterozoic}} Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary.<ref>{{Cite journal |last1=Plumb |first1=Kenneth A. |last2=James |first2=Harold L. |date=1986-07-15 |title=Subdivision of precambrian time: recommendations and suggestions by the subcommission on precambrian stratigraphy |url=https://www.sciencedirect.com/science/article/abs/pii/0301926886900318 |journal=Precambrian Research |volume=32 |issue=1 |pages=65–92 |doi=10.1016/0301-9268(86)90031-8 |bibcode=1986PreR...32...65P |issn=0301-9268}}</ref> The Proterozoic Eon is divided into three eras: the [[Neoproterozoic]], [[Mesoproterozoic]] and [[Paleoproterozoic]].
** [[Neoproterozoic]]: The youngest [[geologic era]] of the Proterozoic Eon, from the [[Cambrian]] Period lower boundary ({{Period start|Cambrian}} Ma) back to {{Period start|Neoproterozoic}} Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American stratigraphy.
*** [[Ediacaran]]: The youngest [[geologic period]] within the Neoproterozoic Era. The "2012 Geologic Time Scale" dates it from {{Period start|Cambrian}} to {{Period start|Ediacaran}} Ma. In this period the [[Ediacaran biota]] appeared.
*** [[Cryogenian]]: The middle period in the [[Neoproterozoic]] Era: {{Period start|Ediacaran}}-{{Period start|Cryogenian}} Ma.
*** [[Tonian]]: the earliest period of the Neoproterozoic Era: {{Period start|Cryogenian}}-{{Period start|Tonian}} Ma.
** [[Mesoproterozoic]]: the middle era of the [[Proterozoic]] Eon, {{Period start|Neoproterozoic}}-{{Period start|Mesoproterozoic}} Ma. Corresponds to "Precambrian Y" rocks of older North American stratigraphy.
** [[Paleoproterozoic]]: oldest era of the Proterozoic Eon, {{Period start|Mesoproterozoic}}-{{Period start|Paleoproterozoic}} Ma. Corresponds to "Precambrian X" rocks of older North American stratigraphy.
* [[Archean]] Eon: {{Period start|Proterozoic}}-{{Period start|Archean}} Ma.
* [[Hadean]] Eon: {{Period start|Archean}}–{{Period start|Hadean}} Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although some [[zircon]] crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the [[Geology of the Moon|Moon]] and [[meteorite]]s.<ref>{{cite journal |last1=Harrison |first1=T. Mark |title=The Hadean Crust: Evidence from >4 Ga Zircons |journal=Annual Review of Earth and Planetary Sciences |date=27 April 2009 |volume=37 |issue=1 |pages=479–505 |doi=10.1146/annurev.earth.031208.100151|bibcode=2009AREPS..37..479H }}</ref><ref>{{cite journal |last1=Abramov |first1=Oleg |last2=Kring |first2=David A. |last3=Mojzsis |first3=Stephen J. |title=The impact environment of the Hadean Earth |journal=Geochemistry |date=October 2013 |volume=73 |issue=3 |pages=227–248 |doi=10.1016/j.chemer.2013.08.004|bibcode=2013ChEG...73..227A }}</ref>
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by [[Global Boundary Stratotype Section and Point|GSSPs]]. The Precambrian could be divided into five "natural" eons, characterized as follows:<ref>{{cite book | last = Bleeker | first = W. | editor = [[Felix M. Gradstein]] | editor2 = James G. Ogg | editor3 = [[Alan Gilbert Smith|Alan G. Smith]] | title = A Geologic Time Scale 2004 | orig-year = 2004 | publisher = Cambridge University Press | isbn = 978-0-521-78673-7 | chapter = Toward a "natural" Precambrian time scale | date = 2004}} also available at [http://www.stratigraphy.org/bak/precambrian/Bleeker_2004.pdf Stratigraphy.org: Precambrian subcommission]</ref>
# Accretion and differentiation: a period of planetary formation until [[Giant impact hypothesis|giant Moon-forming impact event]].
# Hadean: dominated by heavy bombardment from about 4.51 Ga (possibly including a [[cool early Earth]] period) to the end of the [[Late Heavy Bombardment]] period.
# Archean: a period defined by the first crustal formations (the [[Isua greenstone belt]]) until the deposition of [[banded iron formation]]s due to increasing atmospheric oxygen content.
# Transition: a period of continued banded iron formation until the first continental [[red beds]].
# Proterozoic: a period of modern [[plate tectonics]] until the first [[animal]]s.