题型:完形填空 题类:模拟题 难易度:困难
上海市崇明区2021届高三英语质量抽查试卷
Around 13000 years ago North America had a wider variety of mammals (哺乳动物) than modem Africa. There were multiple horse species,camels and some now-extinct animals. And such1creatures were not just found in North America. On every continent mammals on average were a lot larger from around 2.5 million until about 11700 years ago.
Scientists have long debated what caused all these large creatures to go extinct while many of the smaller ones2.A team of researchers led by biologist Felisa Smith analyzed evidence from millions of years' worth of mammalian extinctions and found that on each continent large mammals started to3Ground the same time humans first showed up.
If the extinction trend continues, many more large mammals will soon disappear as well, as the primary threats from humans have4from overhunting to indirect processes such as habitat loss. In their new study Smith and her team analyzed a database of all land mammals that lived from 65 million years ago until today. "We found5no effect of climate on mammalian extinction over 65 million years," she says. But starting around 125000 years ago and continuing until today, large mammals have been more likely to go extinct than smaller ones, the researchers found. The average6of surviving mammals has decreased as a result. And those large-mammal extinctions are tightly coupled with the7of humans. For most of mammalian8history, an animal's size was not predictive of its extinction risk. That9only appeared once humans began to live alongside large mammals.
This finding does not mean climate-related changes could not have10some wildlife populations, enabling humans to more easily bring about their eventual downfall.11, it suggests the greater likelihood of large mammals going extinct is tied to human activities. The animals that evolved without the risk of hunting from humans were suddenly faced with a new12. They simply could not13fast enough to survive the invasion of humans.
Smith says the lesson to be learned from the new findings is that our ancestors prepared us to be extremely skillful killers. "What's14now," she says, "is that some of us are comfortable enough, have a high enough standard of living, that we can start thinking about our use of the Earth," Rather than simply behaving as15, many of us are now in a position to become environmental protectors.
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