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1 .<-NARRATOR:->Listen to part of a lecture in an archaeology class.
1 .<-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…
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。
2 .By about 10,000 B.C.E., all those giant creatures, the megafauna of North America, were gone.
3 .Uh, we don’t know exactly what happened to them, but there are some theories.
1 .One theory is that they were hunted to extinction by humans.
2 .The humans who coexisted with these giant species in North America at that time were what we today call the Clovis people.
1 .And there’s a Clovis site in a valley in Southern California where the remains of thirteen mammoths were found.
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。
2 .And spear points, tools for processing meat, and fireplaces.
3 .That would appear to be some pretty compelling evidence.
4 .Mammoth bones have also been found at some other Clovis sites…
5 .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…
6 .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…
7 .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.
1 .Um, OK, that’s the hunting theory.
2 .Now let’s look at another theory— uh, an alternative to the hunting theory— the climate change theory.
1 .At around 11,500 B.C.E., the world was coming out of an ice age.
在公元前 11500 年前，地球走出了冰河世纪。
2 .And with that came increased seasonality; that is, the summers became warmer and the winters actually became colder.
3 .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.
4 .But the most important impact of this increased seasonality may very well have been its effect on the distribution of plants.
5 .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.
6 .But during the ice age, these plant communities actually grew together, mixed with one another.
7 .So ice-age animals had access to many different types of plants, different types of food.
8 .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.
9 .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.
1 .Again, we have what at first seems like a pretty attractive theory…
2 .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…
1 .[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.
2 .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…
3 .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.
4 .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…
5 .And then he came up with a scenario that combined some hunting by humans with some environmental stress—and bang!
6 .The simulated mammoths were extinct within decades.
7 .So it seems the mixture of hunting and climate change is a likely scenario…
8 .Uh, of course, computer simulations are not a substitute for hard evidence…