TOEFL統合ライティング

📖 読解文章

The extinction of the megafauna—large animals 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 extinctions correlates strongly with human arrival. In North America, for example, the megafauna disappeared roughly 13,000 years ago, shortly after the Clovis people crossed the Bering land bridge. Second, archaeological evidence reveals kill sites where megafauna bones are found with human-made projectile points, indicating direct hunting. For instance, at the Clovis type site in New Mexico, spear points were discovered among mammoth remains. Third, the selective extinction of large-bodied animals supports the overhunting hypothesis, as these species would have provided the most meat and thus been prime targets. Smaller species, which were less attractive to hunters, survived. Therefore, human predation, not climate change, was the primary driver of this mass extinction.

🎧 リスニング講義

The lecture challenges the overhunting theory, offering alternative explanations. First, the timing argument is flawed. While human arrival and megafauna extinctions often overlap, in some regions like Australia, extinctions occurred thousands of years before any evidence of human presence. Conversely, in parts of Africa and Asia, humans coexisted with large animals for much longer, yet extinctions were less severe. Second, the archaeological evidence is not definitive. Kill sites are rare, and many purported examples are ambiguous—bones may have been scavenged after natural death, or the projectile points could be later intrusions. Moreover, if humans had caused extinction through hunting, we would expect far more kill sites than actually exist. Third, the selective extinction of large animals is better explained by ecological factors. Large species have slower reproductive rates, making them more vulnerable to any environmental disruption, including subtle climate shifts. Paleoclimate data show significant warming and vegetation changes during the Pleistocene-Holocene transition, which would have reduced the habitats and food sources for megafauna. Thus, the lecture concludes that climate change, not human overhunting, was the principal cause.