2015年职称英语真题：理工A(文字版部分) 第2部分 阅读判断
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 o