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 correlates with rapid warming periods, not with human migration patterns. For example, populations on Wrangel Island survived until 2000 BCE, long after humans arrived, but disappeared shortly after a dramatic shift in vegetation. Second, mammoth diets were highly specialized, relying on dry, cold-adapted grasses and sedges. As the climate warmed, these plants were replaced by shrubs and trees, reducing the nutritional quality of forage. Isotopic analysis of mammoth bones shows a marked decline in nitrogen-15 levels, indicating malnutrition during their final millennia. Third, genetic studies reveal a severe loss of genetic diversity in late-surviving populations, leading to inbreeding and reduced fertility. This 'genetic meltdown' would have made them vulnerable to disease and environmental stress, independent of human hunting pressure. Consequently, while humans may have delivered the final blow in some regions, the overarching cause of extinction was climate-driven habitat loss and genetic collapse.

🎧 リスニング講義

The reading presents a compelling case for climate change, but it overlooks key evidence that human hunting was decisive. First, the correlation with warming periods is misleading. On Wrangel Island, the mammoth population persisted through multiple warming events, but the island was also visited by humans at the time of their demise. Archaeological sites show butchered mammoth remains with cut marks dated to 2000 BCE, directly linking human activity to their extinction. Second, while vegetation changed, mammoths were not as specialized as claimed. Fossilized dung and stomach contents reveal they consumed a variety of plants, including birch and willow twigs, which thrived in warmer conditions. The isotopic decline in nitrogen-15 may reflect changes in soil composition rather than starvation, as similar patterns appear in modern herbivores that are not endangered. Third, the genetic diversity loss is a symptom, not a cause. Small populations always suffer inbreeding, but they can recover if pressure is removed. However, paleogenomic data show that mammoth populations collapsed precisely when human hunters expanded into their habitats, not during earlier warming phases. The final extinction event on Wrangel Island coincides with the arrival of a small human group, who likely hunted the remaining individuals to extinction. Thus, the evidence points to humans as the primary agent, with climate playing a secondary role.