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 their decline coincides with the end of the last Ice Age, when global temperatures rose rapidly. This warming caused dramatic shifts in vegetation, replacing the nutritious steppe grasses that mammoths relied on with less digestible forests and tundra. Second, isotopic analysis of mammoth bones from the late Pleistocene shows a sharp increase in nitrogen-15 levels, a marker of nutritional stress, indicating that populations were starving long before human hunters became widespread. Third, the geographic pattern of extinction does not match human settlement. Mammoths survived on remote islands like Wrangel Island for thousands of years after humans had reached mainland Siberia and North America, but these island populations eventually died out during periods of extreme climatic fluctuation, not during times of peak human activity. Thus, while humans may have delivered a final blow, the primary cause was an environmental shift that destroyed their habitat and food supply.

🎧 リスニング講義

The reading’s argument that climate change was the primary cause of mammoth extinction is unconvincing. First, the correlation between warming and decline is weak. During previous interglacial periods, mammoths survived similar warming events without extinction, and their range actually expanded during some warm phases. The real tipping point came only after humans developed advanced hunting tools, like Clovis points, around 13,000 years ago. Second, the nitrogen-15 evidence is misleading. Elevated nitrogen levels can also result from drought or migration into new areas, not necessarily starvation. In fact, bone collagen from the same period shows no signs of disease or reduced fertility, which would be expected if malnutrition were severe. Third, the island survival argument actually supports human hunting. On Wrangel Island, mammoths persisted until ~4,000 years ago, but that is exactly when the first human settlers arrived there, as evidenced by stone tools and butchered remains. The final extinction of island populations coincided with human colonization, not with any notable climate shift. Therefore, the evidence points to human overhunting as the decisive factor, with climate playing only a minor role.