The extinction of the megafauna—large mammals such as mammoths, giant sloths, and saber-toothed cats—at the end of the Pleistocene epoch has long puzzled scientists. While many factors have been proposed, the most compelling explanation is overhunting by early humans. First, the timing of megafaunal extinctions correlates strongly with human migration. In North America, for example, the arrival of Clovis hunters around 13,000 years ago coincides with the rapid disappearance of nearly 35 genera of large mammals. Second, archaeological evidence supports human predation. At sites like Murray Springs in Arizona, spear points have been found embedded in the bones of extinct mammoths, indicating direct hunting. Third, the selectivity of extinction points to human impact. Species that were most vulnerable—large, slow-reproducing, and easy to hunt—were precisely those that vanished, whereas smaller and faster-breeding species survived. This pattern is consistent with overkill, not with climate change, which would affect a broader range of species regardless of size or reproductive rate. Therefore, human hunting, not climate, was the primary driver of the megafaunal extinction.
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The reading's argument that humans alone caused megafaunal extinction is not convincing. First, the timing correlation is weak. In many regions, such as South America and Australia, megafaunal extinctions occurred thousands of years before humans arrived, or they persisted long after human presence. For instance, in Australia, giant marsupials disappeared roughly 45,000 years ago, but human colonization dates to at least 50,000 years ago, leaving a gap that undermines a direct causal link. Second, the archaeological evidence is sparse and ambiguous. The spear points found with mammoth bones are rare, and many such sites show no clear butchering marks. It is possible that humans scavenged already dead animals, as fresh kills are unlikely to outnumber natural carcasses. Third, the selectivity pattern can be better explained by environmental factors. The late Pleistocene witnessed dramatic climate shifts and habitat fragmentation. Large-bodied species, which require vast territories and specific vegetation, would be more vulnerable to these changes than smaller, adaptable generalists. In fact, paleoclimate data reveal that many extinct species had already been declining before human arrival, suggesting climate stress was the primary cause. Thus, the reading overestimates human impact.
📝 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.
