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 on different continents. In North America, for example, the megafauna disappeared shortly after the Clovis people crossed the Bering land bridge around 13,000 years ago, whereas in Africa, where humans evolved alongside large animals for millennia, extinctions were far less severe. Second, archaeological evidence supports the overkill hypothesis. Numerous kill sites have been found with spear points embedded in the bones of mammoths and other large prey, indicating that humans actively hunted these animals. Third, the selectivity of the extinctions points to human hunting rather than climate change. The species that vanished tended to be large, slow-reproducing, and easy to target—such as the woolly mammoth—while smaller, agile animals survived. Climate shifts would have affected all species regardless of size, so the pattern is best explained by human predation.

🎧 リスニング講義

The reading presents overhunting as the primary cause of megafauna extinction, but this view is seriously flawed. First, the timing argument is not as clear-cut as claimed. In many regions, such as Siberia and Australia, megafauna populations declined or disappeared thousands of years before humans arrived in significant numbers. Conversely, in parts of Africa and Asia, some large animals coexisted with humans for tens of thousands of years without extinction. Thus, human arrival alone cannot explain the pattern. Second, the archaeological evidence is far weaker than the reading suggests. Only a handful of kill sites have been discovered, and many of these are ambiguous—for instance, bones near stone tools might not have been hunted but scavenged, or the tools might have been used for other purposes. If overhunting were the main cause, we would expect far more abundant and unequivocal evidence of mass hunting. Third, the selectivity argument actually supports climate change. During glacial-interglacial transitions, habitats shifted dramatically, favoring smaller, adaptable species while larger animals, which require extensive territory and specific food sources, struggled to cope. For example, the extinction of the woolly mammoth coincided with the loss of its grassland habitat, which was replaced by forests and tundra. This ecological change would have affected large species disproportionately, matching the observed pattern without invoking human hunters.