TOEFL統合ライティング

📖 読解文章

The extinction of the woolly mammoth has long been attributed to human overhunting, but recent evidence suggests that climate change was the primary driver. First, the timing of mammoth extinctions aligns with the end of the last Ice Age, when global temperatures rose dramatically. Paleoclimatic data show that between 15,000 and 10,000 years ago, vast grasslands—the mammoth’s main food source—were replaced by forests and tundra, reducing their habitat. Second, mammoth populations on isolated islands, such as Wrangel Island, survived until around 4,000 years ago, long after humans had arrived but when sea levels rose and the island shrank. This suggests that habitat loss due to climate change, not hunting, was the critical factor. Third, genetic studies of late-surviving mammoths reveal a loss of genetic diversity and an increase in deleterious mutations, indicating that small, isolated populations became vulnerable to environmental stress. These populations likely collapsed due to climate-induced changes in vegetation and water availability, not human predation.

🎧 リスニング講義

The reading’s claim that climate change alone drove mammoth extinction is flawed. First, while temperature rose, mammoths had survived earlier interglacial periods with similar warmth. For example, they thrived during the Eemian interglacial 120,000 years ago, so warmth itself was not fatal. Second, the Wrangel Island case actually supports human involvement: archaeological evidence shows that humans had not reached the island until about 3,600 years ago, and mammoth bones show cut marks from hunting. The population’s final collapse coincides with human arrival, not just sea-level rise. Third, the genetic diversity loss cited in the reading is a consequence of small population size, but what caused the small size? In many mainland sites, mammoth bones are found with spear points and butchery marks, indicating that humans reduced populations before climate stress became lethal. Thus, while climate may have been a stressor, overhunting was the decisive factor that pushed mammoths over the edge.