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銆€銆€Lackof Oxygen Delayed the Rise of Animals on Earth

銆€銆€Scientistshave long speculated as to why animal species didn鈥檛 flourish sooner, oncesufficient oxygen covered the Earth鈥檚 surface. Animals began to prosper at theend of the Proterozoic period, about 800 million years ago 鈥?but what about thebillion-year stretch before that, when most researchers think there also wasplenty of oxygen?

銆€銆€Well,it seems the air wasn鈥檛 so great then, after all.

銆€銆€Ina study published Oct. 31 in Science, Yale researcher Noah Planavsky and hiscolleagues found that oxygen levels during the 鈥渂oring billion鈥?period wereonly 0.1% of what they are today. In other words, Earth鈥檚 atmosphere couldn鈥檛have supported a diversity of creatures, no matter what genetic advancementswere poised to occur.

銆€銆€鈥淭here is no questionthat genetic and ecological innovation must ultimately be behind the rise ofanimals, but it is equally unavoidable that animals need a certain level ofoxygen,鈥?said Planavsky, co-lead author of the research along with ChristopherReinhard of the Georgia Institute of Technology. 鈥淲e鈥檙e providing the firstevidence that oxygen levels were low enough during this period to potentiallyprevent the rise of animals.鈥?

銆€銆€Thescientists found their evidence by analyzing chromium (Cr) isotopes in ancientsediments from China, Australia, Canada, and the United States. Chromium isfound in the Earth鈥檚 continental crust, and chromium oxidation is directlylinked to the presence of free oxygen in the atmosphere.

銆€銆€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.

銆€銆€Oxygen鈥檚role in controlling the first appearance of animals has long vexed scientists.鈥淲e were missing the right approach until now,鈥?Planavsky said. 鈥淐hromium gaveus the proxy.鈥?Previous estimates put the oxygen level at 40% of today鈥檚conditions during pre-animal times, leaving open the possibility that oxygenwas already plentiful enough to support animal life.

銆€銆€Inthe new study, the researchers acknowledged that oxygen levels were 鈥渉ighlydynamic鈥?in the early atmosphere, with the potential for occasional spikes.However, they said, 鈥淚t seems clear that there is a first-order difference inthe nature of Earth surface Cr cycling鈥?before and after the rise of animals.

銆€銆€鈥淚f 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,鈥?saidco-author Tim Lyons of the University of California-Riverside. 鈥淭his could be agame changer.鈥?

銆€銆€Fundingsources for the research included the NASA Exobiology Program and the NationalScience Foundation鈥檚 Earth-Life Transitions program, awarded to Planavsky,Reinhard, and Lyons.

銆€銆€Theother members of the research team included Xiangli Wang, a postdoctoral fellowat Yale; Thomas Johnson, of the University of Illinois; Danielle Thomson, ofCarleton University; Peter McGoldrick, of the University of Tasmania; andWoodward Fischer, of the California Institute of Technology.

銆€銆€16.The study discovered the rise of animals occurred earlier than the Proterozoicperiod.