TOEFL統合ライティング

📖 読解文章

The extinction of the woolly mammoth has long been attributed to overhunting by early humans, who migrated across the Bering Land Bridge and spread throughout North America and Eurasia. While this "Pleistocene overkill" hypothesis is popular, a closer examination reveals that climate change, not human activity, was the primary driver of their demise. First, the timing of mammoth extinctions correlates more strongly with periods of rapid warming than with human arrival. For example, on Wrangel Island, mammoths survived until 4,000 years ago, long after humans arrived, but disappeared soon after a dramatic shift to a warmer, wetter climate. Second, mammoths were adapted to cold, dry steppe environments, which provided their primary food sources—grasses and sedges. As temperatures rose, these steppes were replaced by forests and bogs, fragmenting their habitat and reducing available forage. Evidence from fossil pollen and ancient DNA shows that these vegetation changes occurred just before the last mammoth populations vanished. Third, human hunting was likely unsustainable on a large scale due to low human population densities and the difficulty of killing such massive beasts. Archaeological sites with mammoth bones often show evidence of scavenging, not active hunting, suggesting humans primarily used carcasses that had already died from other causes. Thus, while humans may have contributed to localized declines, the overwhelming evidence points to climate-induced habitat loss as the decisive factor in the woolly mammoth's extinction.

🎧 リスニング講義

The reading's argument that climate change alone drove the woolly mammoth to extinction is flawed. While climate shifts did occur, they were not catastrophic enough to explain the sudden disappearance of a species that had survived multiple glacial-interglacial cycles over millions of years. In fact, mammoths thrived during previous warm periods, so why would the last one be fatal? The key difference is the presence of highly skilled human hunters. First, the timing argument is misleading. On Wrangel Island, humans did not arrive until about 3,700 years ago, but recent evidence shows that mammoth populations there had already been in decline due to inbreeding and poor health—a classic sign of a population already stressed by human hunting elsewhere, not by climate. Second, the vegetation change was not as abrupt as claimed. Paleoecological data indicate that open steppe persisted in many areas until after the mammoths were gone, and mammoths could have adapted by shifting their diet to other plants. Indeed, isotopic analyses of mammoth bones show they consumed a variety of vegetation, including shrubs, in later periods. Third, the claim that humans only scavenged is contradicted by kill sites with projectile points embedded in mammoth bones, and by the development of sophisticated hunting weapons like the Clovis points, designed for large game. Moreover, human populations were not uniformly low; they expanded rapidly, and even modest hunting pressure can drive slow-reproducing species like mammoths to extinction. Therefore, the most parsimonious explanation remains that human overhunting, perhaps combined with climate stress, delivered the final blow.