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Lackof Oxygen Delayed the Rise of Animals on Earth

Scientistshave long speculated as to why animal species didn’t flourish sooner, oncesufficient oxygen covered the Earth’s surface. Animals began to prosper at theend of the Proterozoic period, about 800 million years ago — but what about thebillion-year stretch before that, when most researchers think there also wasplenty of oxygen?

Well,it seems the air wasn’t so great then, after all.

Ina study published Oct. 31 in Science, Yale researcher Noah Planavsky and hiscolleagues found that oxygen levels during the “boring billion” period wereonly 0.1% of what they are today. In other words, Earth’s atmosphere couldn’thave supported a diversity of creatures, no matter what genetic advancementswere poised to occur.

“There 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,” said Planavsky, co-lead author of the research along with ChristopherReinhard of the Georgia Institute of Technology. “We’re providing the firstevidence that oxygen levels were low enough during this period to potentiallyprevent the rise of animals.”

Thescientists found their evidence by analyzing chromium (Cr) isotopes in ancientsediments from China, Australia, Canada, and the United States. Chromium isfound in the Earth’s 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.

Oxygen’srole 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 today’sconditions 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 “highlydynamic” in 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 cycling” before 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 Foundation’s Earth-Life Transitions program, awarded to Planavsky,Reinhard, and Lyons.

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.

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

A.Right B. Wrong C. Not mentioned

17.Many researchers believe the oxygen level was high during pre-animal times.

A. Right B. Wrong C.Not mentioned

18. The teamwas funded by several research institutes.

A.Right B. Wrong C. Not mentioned

19. Geneticadvancements triggered the rise of animals.

A.Right B. Wrong C. Not mentioned

20. Thesamples studied in the research were collected in ocean areas.

A. Right B. Wrong C.Not mentioned

21. Thestudy revealed that chromium found in Earth’s continental crust remained stablebefore and after the rise of animals.

A.Right B. Wrong C. Not mentioned

22. TimLyons liked to play computer games in his spare time.

A.Right B. Wrong C. Not mentioned

參考答案：BACBABC

（責(zé)任編輯：vstara）

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