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旁白：请听一段考古学的课堂讲解。

Narrator: Listen to part of a lecture in an archaeology class.

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，两种都是象类的生物，都有巨大的长牙，驼峰和大地懒和其它特征。

Professor: 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.

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。

By about 10,000 B.C.E., all those giant creatures, the Metgauna of North America were gone.

我们不能确定到底发生了什么事情，但是对此有许多理论。

We don't know exactly what happened to them, but there are some theories.

一种理论是说它们被人类猎杀以至于灭绝了。

One theory is that they were hunted to extinction by humans.

当时在北美洲与这些巨型生物同时存在的是我们现在所说的克洛维斯人。

The humans who coexisted with these giant species in North America at that time were what we today called the Clovis People.

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头猛犸象的遗迹。

And there is a Clovis site in a valley in southern California where the remains of thirteen mammoths were found.

同时还发现了矛头，加工肉的工具和篝火。

And spear points, tools for processing meat and fire places.

这看起来是十分令人信服的证据。

That would appear to be some pretty compelling evidences.

在其它的克洛维斯遗址中也发现了猛犸象的骨头。

Mammoth bones have also been found at some other Clovis sites.

但是在其它的克洛维斯遗址中，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，如兔子和野火鸡。

But then at other Clovis sites, there's also a lot of evidence that the Clovis people mostly gather plants and hunted small game, like rabbits and wild turkeys.

而且在南美的一些地方还发现了许多自然急剧的猛犸象的骨头，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。

Also there are several places in North America where you have natural accumulations of mammoth bones that look very similar to the accumulations at the Clovis site, except there's no human debris, where the mammoth almost certainly died as a result of some kind of natural disaster.

所以我认为在南加利福尼亚克洛维斯遗址中发现的13具猛犸象的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。

So I think it is 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.

现在我们来讨论另一个理论，它是猎杀理论的一个可替代理论，气候变化理论。

Now let's look at another theory, uh, an alternative to the hunting theory, the climate change theory.

在公元前11500年前，地球走出了冰河世纪。

At around 11,500 B.C.E.,the world was coming out of an Ice Age.

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。

And with that came increased seasonality, that is, the summers became warmer, and the winters actually became colder.

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体能 带来了很大的压力。

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.

而其中最大的冲击可能是对植物分布的影响。

But the most important impact of this increased seasonality may very well have been its effect on the distribution of plants.

现在我们认为植物的水平分布理所当然。

Today we take for granted that there horizontal bands of plant communities.

最北面是冻土带，接下来往南是森林带。

In the far north, it is tundra, which gives way to forest as you move southward.

And even farther south, grasslands take over.

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。

But during the Ice Age, these plant communities actually grew together, mixed with one another.

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。

So Ice Age animals had access to many different types of plants, different types of food.

但是当季节的变化越来越明显，植物群变逐渐分开，也就是说在一定的区域，植物的种类多样性降低。

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.

结果，按照这种理论，那些依赖于植物多样性的动物便无法生存。

And as a result, uh, so the theory goes, the Ice Age animals that depended on plant diversity couldn't survive.

而那些巨型哺乳动物的饮食是最需要多样性的。

And the great beasts were the ones that needed the most diversity in their diet.

还是，这个理论开始的时候看起来很有吸引力，但是接下来，你要如何解释以前也发生过这种情况呢？

Again, we have what at first seems like a pretty attractive theory, but then, how do you explain the fact that this has happened before?

要知道，全球变冷接着又全球变暖，但是之前却没有发生灭绝。

You know, global cooling followed by global warming, and there was no extinction then.

我最近读了一篇有趣的文章，一位考古学家试图在计算机的帮助下解决这个谜题。

Uh, you know, I recently read an interesting article about an archaeologist who tried to solve this puzzle with the help of his computer.

他用电脑程序模拟在某种情况下猛犸的情况。

What he did was, he wrote a computer program to simulate what would happen to mammoth under certain conditions.

例如，长达几十年的干旱，或者是捕猎者猎杀了百分之五的象群。

Say, for example, there is a drought for a couple of decades, or hunters are killing or five percent of the population, and so on.

他发现了一点，人们并不需要猎杀大量的这种生物就能将这种生物推向灭绝。

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.

因为大型生物的繁殖速度十分缓慢，只要除去象群中的几头年轻的母象，它们的数量就会大幅的下降。

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.

接着他提出了一个假想，人们的猎杀再加上一些环境的压力。

And then he came up with a scenario that combined some hunting by humans with some environmental stress, and...Bang!

这些模拟的猛犸象群在几十年间便灭绝了。

The simulated mammoths were extinct within decades.

由此可见，猎杀和气候变化的相互作用可能是猛犸象灭绝的一个假想。

So it seems the mixture of hunting and climate change is a likely scenario.

当然，电脑模拟并非是一个有力的证据。

Uh, of course, computer simulations are not a substitute for hard evidence.

Why does the professor mention an archaeological site in Southern California?

What is the professor's opinion about the Clovis people?

According to the climate change theory, how did the climate change at the end of the ice age make it more difficult for large animals to survive?

What does the professor imply is one problem with the climate change theory?

The professor mentions a series of computer simulations. What did the simulations suggest?

Narrator: Listen to part of a lecture in an archaeology class. 
Professor: 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. 
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 called the Clovis Peopple. And there is 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 fire places. 
That would appear to be some pretty compelling evidences. Mammoth bones have also been found at some other Clovis sites. 
But then at other Clovis sites, there's also a lot of evidence that the Clovis people mostly gather 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 that look very similar to the accumulations at the Clovis site, except there's no human debris, where the mammoth almost certainly died as a result of some kind of natural disaster. So I think it is 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 horizontal bands of plant communities. In the far north, it 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, 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 has happened before? You know, global cooling followed by global warming, and there was no extinction then. 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 mammoth under certain conditions. Say, for example, there is a drought for a couple of decades, or hunters are killing or five 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.