Thescientists found their evidence by analyzing chromium (Cr) isotopes in ancientsediments from China,Australia,Canada,and the United States.Chromium isfound in the Earths continental crust,and chromium oxidation is directlylinked to the presence of free oxygen in the atmosphere.
Specifically,the team studied samples deposited in shallow,iron-rich ocean areas,near theshore.They compared their data with other samples taken from younger localesknown to have higher levels of oxygen.
Oxygensrole in controlling the first appearance of animals has long vexed scientists.We were missing the right approach until now,Planavsky said.Chromium gaveus the proxy.Previous estimates put the oxygen level at 40% of todaysconditions during pre-animal times,leaving open the possibility that oxygenwas already plentiful enough to support animal life.
Inthe new study,the researchers acknowledged that oxygen levels were highlydynamicin the early atmosphere,with the potential for occasional spikes.However,they said,It seems clear that there is a first-order difference inthe nature of Earth surface Cr cyclingbefore and after the rise of animals.
If we are right,ourresults will really change how people view the origins of animals and othercomplex life,and their relationships to the co-evolving environment,saidco-author Tim Lyons of the University of California-Riverside.This could be agame changer.
Fundingsources for the research included the NASA Exobiology Program and the NationalScience Foundations Earth-Life Transitions program,awarded to Planavsky,Reinhard,and Lyons.
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