The extinction of the Pleistocene megafauna—large animals such as woolly mammoths, giant sloths, and saber-toothed cats—has long puzzled scientists. While many factors have been proposed, the most compelling explanation is overhunting by early humans. First, the timing of extinctions correlates strongly with human arrival. In North America, megafauna disappeared roughly 13,000 years ago, soon after the Clovis people crossed the Bering land bridge. In Australia, large marsupials vanished about 45,000 years ago, coinciding with the arrival of Aboriginal settlers. Second, archaeological evidence supports human predation. At sites like the Murray Springs in Arizona, mammoth bones have been found with spear points embedded, and butchery marks on bones indicate systematic killing. Third, the selectivity of extinction points to human hunting. Species that were large, slow-moving, and had low reproductive rates—easy targets for hunters—were the most affected, while smaller, faster-breeding animals survived. This pattern is consistent with overhunting, not climate change, which would have impacted all species equally. Therefore, human overhunting, not climate shifts, was the primary driver of the Pleistocene megafaunal extinctions.
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The reading’s claim that human overhunting caused megafaunal extinctions is overly simplistic. First, the timing argument is flawed. In North America, some megafauna, like the Shasta ground sloth, disappeared as early as 15,000 years ago, well before Clovis hunters arrived, while others survived until 10,000 years ago, long after humans were present. This lack of synchrony undermines a single cause. Second, the archaeological evidence is ambiguous. At many so-called kill sites, the association between human artifacts and animal bones is questionable. For example, tool marks may be from scavenging, not hunting, and some bones show no signs of human processing. Moreover, the number of known kill sites is minuscule compared to the vast populations of these animals—hardly evidence of systematic overkill. Third, the selectivity argument can be better explained by ecology. Large species were more vulnerable to climate-induced habitat fragmentation, as they required vast territories and specific vegetation. As the climate warmed and glaciers retreated, their habitats shrank, and smaller, adaptable species thrived. This pattern matches climate models, which show dramatic vegetation shifts during the late Pleistocene. In sum, the evidence points to climate change as the primary driver, with human hunting playing only a minor role, if any.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
