The Extinction of the Megafauna: A Climate-Driven Event
During the late Pleistocene, approximately 50,000 to 10,000 years ago, many large terrestrial mammals, such as mammoths, giant sloths, and saber-toothed cats, went extinct. While some scientists argue that human hunting was the primary cause, a closer examination of the evidence suggests that climate change was the dominant factor. First, the timing of the extinctions correlates strongly with major climatic shifts. For example, in North America, most megafauna disappeared during the Younger Dryas, a sudden return to glacial conditions, not during periods of stable climate. This temporal link implies that rapid temperature changes disrupted ecosystems, reducing the availability of food and water, which would have stressed large animals regardless of human activity. Second, many megafaunal species had survived earlier human arrivals in other regions. In Africa and southern Asia, where hominins had existed for over a million years, megafauna did not suffer mass extinctions until much later, suggesting that human hunting alone was insufficient to cause collapse. Third, the extinction selectively affected the largest animals, which are particularly vulnerable to environmental stress due to their low reproductive rates and high metabolic demands. Climate-driven habitat loss would have hit these species hardest, whereas human hunters, who likely targeted easier prey, would have had less impact. Thus, the pattern of extinction is more consistent with climate change than with overhunting.
🎧 听力讲座
🎯 模拟考试环境：先点「播放讲座」听完内容，再作答。听力原文默认隐藏。
Now listen to a lecture on the same topic.
Professor: The reading argues that climate change, not human hunting, drove the extinction of megafauna. However, the evidence actually points more strongly to human overhunting. First, the alleged correlation with climate is misleading. In North America, the megafauna disappeared not during the Younger Dryas itself, but at the very beginning of that cold period—and this timing coincides exactly with the arrival of Clovis hunters, who left distinctive spear points in kill sites. Moreover, in Australia, megafauna went extinct about 45,000 years ago, when climate was relatively stable, but humans had just arrived. So the pattern is not about climate shifts but about human arrival. Second, the claim that megafauna survived earlier human presence in Africa is flawed. In Africa and southern Asia, megafauna did not vanish because they co-evolved with hominins, becoming wary of humans. But when modern humans later reached new continents, the animals had no fear, making them easy prey. This explains why extinctions were rapid after human arrival, not gradual as climate change would predict. Third, the focus on large animals actually supports the hunting hypothesis. Hunters preferentially targeted large game because they provided more meat and fat per kill, and archaeological sites show cut marks on megafaunal bones. In contrast, climate-induced stress would have affected all large herbivores uniformly, but extinction was selective—even among large mammals, species that were less accessible to hunters survived. Therefore, the most parsimonious explanation is that humans, not climate, caused the extinctions.
📝 ライティング指示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
📊 TOEFL Integrated Writing Rubric
Content Accuracy: Accurately capture the relationship between lecture and reading
Information Completeness: Cover all key counter-arguments from the lecture
Structure & Coherence: Logical organization and progression
Language Use: Grammar, vocabulary, expression accuracy
Scored 0-30 based on holistic scoring (original 0-5, scaled to 30).
