Thousands of years ago, in North America's past, all of its megafauna—large mammals such as mammoths and giant bears—disappeared. One proposed explanation for this event is that when the first Americans migrated over from Asia, they hunted the megafauna to extinction.These people, known as the Clovis society after a site where their distinctive spear points were first found, would have been able to use this food source to expand their population and fill the continent rapidly.Yet many scientists argue against this "Pleistocene overkill" hypothesis. Modern humans have certainly been capable of such drastic effects on animals, but could ancient people with little more than stone spears similarly have caused the extinction of numerous species of animals?Thirty-five genera or groups of species (and many individual species) suffered extinction in North America around 11,000 B.C., soon after the appearance and expansion of Paleo-lndians throughout the Americas (27 genera disappeared completely, and another 8 became locally extinct, surviving only outside North America).

Although the climate changed at the end of the Pleistocene, warming trends had happened before. A period of massive extinction of large mammals like that seen about 11,000 years ago had not occurred during the previous 400,000 years, despite these changes. The only apparently significant difference in the Americas 11,000 years ago was the presence of human hunters of these large mammals. Was this coincidence or cause-and-effect?

We do not know.Ecologist Paul S. Martin has championed the model that associates the extinction of large mammals at the end of the Pleistocene with human predation.With researcher J. E. Mosimann, he has co-authored a work in which a computer model showed that in around 300 years, given the right conditions, a small influx of hunters into eastern Beringia 12,000 years ago could have spread across North America in a wave and wiped out game animals to feed their burgeoning population.

The researchers ran the model several ways, always beginning with a population of 100 humans in Edmonton, in Alberta, Canada, at 11,500 years ago.Assuming different initial North American big-game-animal populations (75-150 million animals) and different population growth rates for the human settlers (0.65%-3.5%), and varying kill rates, Mosimann and Martin derived figures of between 279 and 1,157 years from initial contact to big-game extinction.

Many scholars continue to support this scenario.For example, geologist Larry Agenbroad has mapped the locations of dated Clovis sites alongside the distribution of dated sites where the remains of wooly mammoths have been found in both archaeological and purely paleontological contexts.These distributions show remarkable synchronicity (occurrence at the same time).

There are, however, many problems with this model.Significantly, though a few sites are quite impressive, there really is very little archaeological evidence to support it.Writing in 1982, Martin himself admitted the paucity of evidence;for example, at that point, the remains of only 38 individual mammoths had been found at Clovis sites. In the years since, few additional mammoths have been added to the list;there are still fewer than 20 Clovis sites where the remains of one or more mammoths have been recovered, a minuscule proportion of the millions that necessarily would have had to have been slaughtered within the overkill scenario.

Though Martin claims the lack of evidence actually supports his model—the evidence is sparse because the spread of humans and the extinction of animals occurred so quickly—this argument seems weak. And how could we ever disprove it?As archaeologist Donald Grayson points out, in other cases where extinction resulted from the quick spread of human hunters—for example, the extinction of the moa, the large flightless bird of New Zealand—archaeological evidence in the form of remains is abundant. Grayson has also shown that the evidence is not so clear that all or even most of the large herbivores in late Pleistocene America became extinct after the appearance of Clovis. Of the 35 extinct genera, only 8 can be confidently assigned an extinction date of between 12,000 and 10,000 years ago.Many of the older genera, Grayson argues, may have succumbed before 12,000 B.C., at least half a century before the Clovis showed up in the American West.

数千年前，在过去的北美，所有巨型动物——猛犸象和巨熊这些大型动物——都消失了。对此事件的一个解释是，当第一批美国人从亚洲迁移过来时，他们狩猎巨型动物以至于使其灭绝。人们在这些人所使用的独特的矛头被首次发现后，把这喜人称为克洛维斯族人，这些人能够使用这些食物源来扩展他们的人口，并且很快的遍布整个大陆。然而，许多科学家反对这种“更新世过度捕杀”假说。现代人对动物确实具有如此剧烈的影响，但难道只有石矛等武器的古代人也同样能造成无数种动物的灭绝吗？在公元前11000年前后，北美洲有35个属或种群（以及许多单个物种），在古印第安人在美洲出现和扩张后不久就灭绝了。（27个属全部消失，另有8个属于局部灭绝，只幸存于北美以外的地区）。尽管更新世末期气候发生了变化，但变暖趋势之前也出现过。在过去的40万年中，尽管发生了这些变化，但像大约11，000年前这样大型哺乳动物大规模灭绝的时期，并没有出现。11,000年前的美洲与以前相比，唯一明显的差异是，这些大型哺乳动物的猎人的存在。这是巧合还是有因果关系？我们不知道。 生态学家保罗•马丁（Paul S. Martin）提出将更新世末期大型哺乳动物灭绝与人类捕食相关联的模式。 研究人员J.E Mosimann与他合作建立了一个计算机模型，展示在大约300年的时间里，条件正确的话，12,000年前在Beringia东部的小量涌入的猎人会波浪式地快速传播到北美洲， 消除野兽以养活其新增的人口。研究人员以多种方式运行该模型，他们总是从11,500年前的加拿大艾伯塔省埃德蒙顿的100人开始分析。评估了不同的北美大型野生动物独立种群（0.75-1.5亿只动物）和不同的人类定居者的不同人口增长率（0.65％-3.5％），以及不断变化的捕杀率，Mosimann和Martin得出了数据，人类从最初开始接触到大型生物到其灭绝的这段时间是在279年到1157年之间。许多学者继续支持这种说法。例如，地质学家Larry Agenbroad已经在地图上标识出了那个时期的克洛维斯遗址的位置以及在考古学和生物学立场下发现羊毛猛犸一体的遗址分布。 这些分布显示出明显的同步性（同时发生）。然而，这种模式存在很多问题。值得注意的是，虽然有几个遗址令人印象深刻，但实际上很少有考古证据能支持它。1982年，马丁写到，他自己承认自己缺乏证据;例如，在那个时候，在克洛维斯遗址仅发现了38头猛犸象的遗骸。在那之后，几乎没有其它的猛犸象被发现；但是仍然有不到20个克洛维斯遗址，其中一头或多头猛犸象的遗体已经被发现，这只是数百万在过度屠杀的情况下被杀害的猛犸象的一些小部分。尽管马丁声称缺乏支持他模型的证据——因为人类的扩张和动物的灭绝发生的太快，故现有的证据较少——但这个论点似乎站不住脚。我们怎么能反驳它呢？正如考古学家唐纳德格雷森指出的那样，在其他因人类猎人快速传播而灭绝的情况下——例如，新西兰一种大型不能飞行的鸟类恐鸟的灭绝——以遗骸形式出现的考古证据非常丰富。格雷森还表明，更新世时期末期美洲的所有甚至大部分大型食草动物在克洛维斯出现后都灭绝了的证据不是很清楚。在35个已灭绝的属中，只有8个可以确切地指定其是在12,000至10,000年前灭绝的。格雷森认为，许多更老的属可能在12,000年前就已经灭绝了，至少在克洛维斯出现在美国西部之前半个世纪就已经灭绝了。