In 200 Years Cows May Be the Biggest Land Mammals (哺乳动物) on the Planet
Around 13,000 years ago North America had a wider variety of mammals than modern Africa. There were multiple horse species, camels and some now-extinct animals. And such _________ creatures were not just found in North America. On every continent mammals on average were a lot larger from around 2.5 million until about 11,700 years ago.
Scientists have long debated what caused all these large creatures to go extinct while many of the smaller ones _________. 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 to _________ around 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 have _________ from overhunting to indirect processes such as habitat loss. The largest land mammal 200 years from now could well be the domestic cow, Smith’s research suggests. 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 found _________ no effect of climate on mammalian extinction over 65 million years,” she says. But starting around 125,000 years ago and continuing until today, large mammals have been more likely to go extinct than smaller ones, the researchers found.
The average _________ of surviving mammals has decreased as a result. And those large-mammal extinctions are tightly coupled with the _________ of humans. For most of mammalian _________ history, an animal’s size was not predictive of its extinction risk. That _________ only appeared once humans began to live alongside large mammals.
This finding does not mean climate-related changes could not have __________ some wildlife populations, enabling humans to more easily bring about their eventual downfall. __________, 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 new __________. They simply could not __________ fast 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’s __________ now,” 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 as __________, many of us are now in a position to become environmental protectors.
【小题1】A.smart | B.massive | C.marine | D.ancient |
【小题2】A.survived | B.shrank | C.escaped | D.returned |
【小题3】A.hide away | B.bring up | C.break up | D.die out |
【小题4】A.faded | B.restored | C.improved | D.expanded |
【小题5】A.absolutely | B.predictably | C.exclusively | D.potentially |
【小题6】A.weight | B.speed | C.size | D.appetite |
【小题7】A.production | B.appearance | C.exploration | D.cruelty |
【小题8】A.recorded | B.contemporary | C.evolutionary | D.conclusive |
【小题9】A.link | B.contact | C.adaptation | D.distinction |
【小题10】A.enlarged | B.stressed | C.impressed | D.dominated |
【小题11】A.Moreover | B.Otherwise | C.Meanwhile | D.Rather |
【小题12】A.species | B.hope | C.rule | D.threat |
【小题13】A.grow | B.digest | C.withdraw | D.adapt |
【小题14】A.instructive | B.tremendous | C.different | D.marvelous |
【小题15】A.sponsors | B.creators | C.consumers | D.producers |