共用题干Lack of Oxygen Delayed the Rise of Animals on

共用题干

Lack of Oxygen Delayed the Rise of Animals on Earth

Scientists have long speculated as to why animal species didn’t flourish sooner,once sufficient oxygen

covered the Earth’s surface. Animals began to prosper at the end of the Proterozoic period,about 800

million years ago-but what about the billion-year stretch before that,when most researchers think there

also was plenty of oxygen?

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

In a study published Oct. 3 1 in Science,Yale researcher Noah Planavsky and his colleagues found that

oxygen levels during the"boring billion"period were only 0.1%of what they are today .In other

words,Earth’s atmosphere couldn’t have supported a diversity of creatures,no matter what genetic

advancements were poised to occur.

"There is no question that genetic and ecological innovation must ultimately be behind the rise of

animals,but it is equally unavoidable that animals need a certain level of oxygen,"said Planavsky,colead

author of the research along with Christopher Reinhard of the Georgia Institute of Technology.“We’re

providing the first evidence that oxygen levels were low enough during this period to potentially prevent

the rise of animals."

The scientists found their evidence by analyzing chromium(Cr)isotopes in ancient sediments from

China,Australia,Canada,and the United States.Chromium is found in the Earth’s continental crust,and

chromium oxidation is directly linked to the presence of free oxygen in the atmosphere.

Specifically,the team studied samples deposited in shallow,iron-rich ocean areas,near the shore.They

compared their data with other samples taken from younger locales known to have higher levels of oxygen.

Oxygen’s role in controlling the first appearance of animals has long vexed scientists."We were missing

the right approach until now,"Planavsky said."Chromium gave us the proxy."

Previous estimates put the oxygen level at 40%of today’s conditions during pre-animal times. leaving open

the possibility that oxygen was already plentiful enough to support animal life.

In the new study,the researchers acknowledged that oxygen levels were"highly dynamic"in the early

atmosphere,with the potential for occasional spikes. However,they said,"It seems clear that there is a

first-order difference in the nature of Earth surface Cr cycling"before and af- ter the rise of animals.

"If we are right,our results will really change how people view the origins of animals and other complex

life,and their relationships to the co-evolving environment,"said co-author Tim

Lyons of the University of California-Riverside."This could be a game changer."

"There’s a lot’of interest right now in a broader discussion surrounding the role that environ- mental

stability played in the evolution of complex life,and we think our results are a significant contribution

to that,"Reinhard said.

The study revealed that chromium found in Earth’s continental crust remained stable before and after

the rise of animals.

A..Right

B. Wrong

C. Not mentioned