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 has long puzzled scientists. While many factors have been proposed, the most compelling explanation is overhunting by early humans. First, the timing of extinctions closely matches human migration patterns. In North America, for example, the arrival of Clovis hunters around 13,000 years ago coincides with the rapid demise of nearly all large mammal species. Second, archaeological evidence supports human predation. At sites like the Murray Springs Clovis site in Arizona, researchers have found spear points embedded in mammoth skeletons, along with butchering marks on bones. Such direct evidence indicates that humans actively hunted these animals, not merely scavenged. Third, the selectivity of extinction—where only large-bodied species disappeared while smaller animals survived—is consistent with human hunting preferences. Large animals provide more meat per kill and are easier to target due to their slower reproduction rates, making them vulnerable to even modest hunting pressure. In contrast, climate change at the end of the Ice Age would have affected both large and small species equally, yet small mammals persisted. Therefore, human overhunting stands as the primary driver of megafaunal extinction.

🎧 リスニング講義

The reading's argument that humans alone caused megafaunal extinction is flawed. First, the timing correlation is not as tight as claimed. In Australia, megafaunal extinctions occurred around 46,000 years ago, but human arrival is dated to at least 65,000 years ago—a lag of nearly 20,000 years. If hunting were the cause, extinction should have followed settlement more quickly. Second, the archaeological evidence is ambiguous. Many 'spear points' found with mammoth bones may actually be projectile points used for smaller game or even for hunting birds. Moreover, cut marks on bones could result from scavenging carcasses that died of natural causes, not from active hunting. In fact, no kill sites show unambiguous mass kills of megafauna. Third, the selectivity argument ignores that climate change can also be selective. Large animals have lower population densities and longer generation times, making them more vulnerable to environmental stress, such as habitat fragmentation or shifts in vegetation. During the Younger Dryas, a sudden cold snap drastically reduced plant communities, which disproportionately affected large herbivores and their predators. Small species, with faster reproductive rates, could recover quickly. Thus, the evidence better supports climate-induced extinction, not human overhunting.