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 human hunters. First, the timing of megafaunal extinctions correlates strongly with human arrival on continents. In North America, for example, most large mammals disappeared roughly 13,000 years ago, just after the Clovis culture's appearance. Similarly, in Australia, extinctions followed human colonization around 45,000 years ago. Second, archaeological evidence supports human predation. At several sites, such as the Clovis kill sites in the American Southwest, spear points have been found embedded in mammoth skeletons, and butchery marks on bones indicate that humans actively hunted these animals. Third, the selectivity of the extinctions points to human hunting rather than climate change. Extinct species were predominantly large-bodied, slow-reproducing animals—prime targets for hunters—whereas smaller, faster-reproducing species survived. Climate shifts would have affected a broader range of species, not just the large ones. Thus, the pattern of extinction aligns more closely with human activity than with any natural environmental change.

🎧 リスニング講義

The reading's claim that human overhunting drove megafaunal extinctions is not supported by the evidence. First, the timing argument is flawed. In North America, some megafauna, such as the Shasta ground sloth, disappeared before Clovis hunters arrived, while others, like the bison, survived long after. This mismatch undermines the notion that human arrival caused a sudden extinction event. Second, the archaeological evidence is limited and ambiguous. The few kill sites with embedded spear points are exceptions; the vast majority of megafaunal remains show no signs of human butchery. Moreover, many of the butchered bones could have been scavenged from animals that died of other causes, not hunted. Third, the selectivity pattern can be explained by climate and ecological factors. Large animals are more vulnerable to environmental stress because they require vast territories and specific vegetation. During the rapid climate shifts at the Pleistocene's end, habitats fragmented, disrupting food sources and migration routes, which would have disproportionately affected large species. In contrast, smaller animals with flexible diets and faster reproduction could adapt more readily. Therefore, the extinction pattern is better explained by climate change than by human hunting.