TOEFL Academic Passage Practice 031American Megafauna

Thousands of years ago, all of North America’s megafauna—large mammals such as mammoths and giant bears—disappeared. One proposed explanation is that when the first Americans migrated from Asia, they hunted these animals to extinction. These people, known as the Clovis society after a site where their distinctive spear points were first found, could have used this food source to expand their population and spread rapidly across the continent. Yet many scientists question this “Pleistocene overkill” hypothesis. Modern humans have certainly caused drastic effects on animal populations, but could ancient people with little more than stone spears have caused the extinction of numerous species? Thirty-five genera, or groups of species, became extinct in North America around 11,000 B.C., soon after Paleo-Indians, late-Pleistocene hunters, appeared and expanded throughout the Americas. Twenty-seven genera disappeared completely, while another eight became locally extinct in North America but survived elsewhere.

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

We do not know. Ecologist Paul S. Martin has championed a model linking the extinction of large mammals at the end of the Pleistocene to human predation. With researcher J. E. Mosimann, he co-authored a study in which a computer model showed that, under 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 a growing population.

The researchers ran the model several ways, always beginning with 100 humans in Edmonton, Alberta, 11,500 years ago. Assuming initial North American big-game populations of 75–150 million animals, human population growth rates of 0.65–3.5 percent, and varying kill rates, Mosimann and Martin calculated that big-game extinction could have occurred 279–1,157 years after initial human contact.

Many scholars continue to support this scenario. Geologist Larry Agenbroad mapped the locations of dated Clovis sites alongside dated sites where woolly mammoth remains had been found in both archaeological and purely paleontological contexts. The distributions showed remarkable synchronicity, or occurrence at the same time.

There are, however, many problems with this model. Although a few sites are impressive, there is little archaeological evidence to support it. Writing in 1982, Martin himself admitted the paucity of evidence: at that point, remains of only 38 individual mammoths had been found at Clovis sites. Few additional mammoths have been added since then; remains of one or more mammoths have been recovered at fewer than 20 Clovis sites, a minuscule proportion of the millions that would have had to be slaughtered under the overkill scenario.

Martin claims that the lack of evidence supports his model because human expansion and animal extinction occurred so quickly. But this argument seems weak. As archaeologist Donald Grayson points out, in other cases of extinction caused by the rapid spread of human hunters—for example, the extinction of the moa, New Zealand’s large flightless bird—archaeological evidence is abundant. Grayson has also shown that the evidence does not establish that all, or even most, large herbivores in late-Pleistocene America became extinct after the appearance of Clovis. Of the 35 extinct genera, only eight can confidently be assigned an extinction date between 12,000 and 10,000 years ago. Many older genera may have disappeared before 12,000 B.C., at least half a century before the Clovis people appeared in the American West.