A. Theories about how large animals were able to survive the most recent ice age
B. Environmental changes that occur following an ice age
C. Theories about the hunting methods used by early humans in North America
D. Explanations for the disappearance of large animals in North America
NARRATOR:Listen to part of a lecture in an archaeology class.
FEMALE PROFESSOR:Between 11,000 and 10,000 B.C.E., North America was populated by a wide variety of great beasts like mammoths and mastodons— both elephant-like creatures with big tusks— and camels, giant sloths… mm, the list goes on…By about 10,000 B.C.E., all those giant creatures, the megafauna of North America, were gone.Uh, we don’t know exactly what happened to them, but there are some theories.
One theory is that they were hunted to extinction by humans.The humans who coexisted with these giant species in North America at that time were what we today call the Clovis people.
And there’s a Clovis site in a valley in Southern California where the remains of thirteen mammoths were found.And spear points, tools for processing meat, and fireplaces.That would appear to be some pretty compelling evidence.Mammoth bones have also been found at some other Clovis sites…But then, at other Clovis sites, uh, there’s also a lot of evidence that the Clovis people mostly gathered plants and hunted small game, like rabbits and wild turkeys…Also, there are several places in North America where you have natural accumulations of mammoth bones— uh, that look very similar to the accumulations at the Clovis sites, except there’s no human debris— where the mammoths almost certainly died as the result of some kind of natural disaster…So I think it’s quite likely that those thirteen mammoths in Southern California also died of natural causes… and that the Clovis people simply took advantage of the situation.
Um, OK, that’s the hunting theory.Now let’s look at another theory— uh, an alternative to the hunting theory— the climate change theory.
At around 11,500 B.C.E., the world was coming out of an ice age.And with that came increased seasonality; that is, the summers became warmer and the winters actually became colder.These extreme shifts would have put a lot of stress on the bodies of animals that were used to a more moderate range of temperatures.But the most important impact of this increased seasonality may very well have been its effect on the distribution of plants.Today we take for granted that there are horizontal bands of plant communities: in the far north is tundra, which gives way to forest as you move southward, and even farther south, grasslands take over.But during the ice age, these plant communities actually grew together, mixed with one another.So ice-age animals had access to many different types of plants, different types of food.But when the seasons became more distinct, the plant communities were pulled apart. That meant that in any given area there was less plant diversity.And as a result, uh, so the theory goes, the ice-age animals that depended on plant diversity couldn’t survive… and the great beasts were the ones that needed the most diversity in their diet.
Again, we have what at first seems like a pretty attractive theory…But then, how do you explain the fact that this had happened before— you know, global cooling followed by global warming— and there was no extinction then…
[Shifting gears] Uh, you know, I recently read an interesting article about an archaeologist who tried to solve this puzzle with the help of his computer.What he did was, he wrote a computer program to simulate what would happen to mammoths under certain conditions; um, say, for example, there’s a drought for a couple of decades; or, uh, hunters are killing off 5 percent of the population, and so on…One thing he found was that humans didn’t necessarily have to kill these animals in great numbers in order to nudge them toward extinction.That’s because very large animals have a slow rate of reproduction, so all you have to do is remove a few young females from the herd, and you can—fairly quickly— significantly reduce the population…And then he came up with a scenario that combined some hunting by humans with some environmental stress—and bang!The simulated mammoths were extinct within decades.So it seems the mixture of hunting and climate change is a likely scenario…Uh, of course, computer simulations are not a substitute for hard evidence…
教授：在公元前 11000 年到 10000 年前，北美洲生活着各种大型的生物，比如猛犸和乳齿象（两种都是类似大象的动物，都有巨大的长牙），还有骆驼和大地懒等大型生物。但是大约在公元前 10000年时，所有的巨型生物，北美的猛犸都消失了。我们不能确定到底发生了什么事情，但是对此有许多理论。
在南加利福尼亚的山谷中发现了一个克洛维斯遗址，这里发现了 13 只猛犸象的遗迹。同时还发现了矛头，加工肉的工具和篝火。这看起来是十分令人信服的证据。在其他的克洛维斯遗址中也发现了猛犸象的骨头。但是在其他的克洛维斯遗址中，也有不少证据证明克罗维斯人大部分是以采集植物和猎杀小型动物为生，如兔子和野驴...而且在南美的一些地方还发现了许多自然聚集的猛犸象的骨头，看起来与克洛维斯遗址中的一样，只不过没有人类的遗迹，看起来猛犸象像是死于某种自然灾害。所以我认为在南加利福尼亚克洛维斯遗址中发现的猛犸象也有可能是死于自然灾害，而克洛维斯人只是坐享渔翁之利。
在公元前 11500 年前，地球走出了冰河世纪。随之而来的是季节性的增加，夏天变得更加温暖，而冬天则变得更加寒冷。这种极端的变化可能给一直处于气候变化不大的环境中的动物的身体带来了很大的压力。而季节性增加造成最大的冲击可能是对植物分布的影响。现在我们认为植物群体的水平分布理所当然，最北面是冻土带，接下来往南是森林带，而到了最南边是草原带。但是在冰河世纪，这些不同的植物实际上是长在一起的，相互融合。所以冰河世纪的动物能够吃到不同的植物，各种各样的食物。但是当季节的变化越来越明显，植物群便逐渐分开，也就是说在一定的区域，植物的种类多样性降低。按照这种理论，结果那些依赖于植物多样性的动物便无法生存，而那些巨型哺乳动物的饮食是最需要多样性的。
Between 11,000 and 10,000 B.C.E., North America was populated by a wide variety of great beasts, like mammoth and mastodons, both elephant-like creatures with big tusks, and camels, giant sloths, the list goes on. By about 10,000 B.C.E., all those giant creatures, the Metgauna of North America were gone. We don't know exactly what happened to them, but there are some theories.
在文章开头部分先介绍相关的背景知识，提到物种灭绝，然后信号词but后再给出今天的主旨，提到some theories. 后文介绍了两种，一种是hunted to extinction by human 另一种是climate change theory. 最后说到两者的相互作用
D选项中的explanation 还有disappearance 与原文最为吻合，是对全文主旨最准确的概括