The Extinction of the Megafauna: A Climate-Driven Event
During the late Pleistocene, roughly 50,000 to 10,000 years ago, many large mammals—such as woolly mammoths, saber-toothed cats, and giant ground sloths—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 more strongly with climatic shifts than with human arrival. In North America, the megafauna extinctions occurred around 13,000 years ago, during a period of rapid warming at the end of the last ice age. Human hunters, however, had already been present for at least 2,000 years before this event. If hunting were the main cause, we would expect extinctions to follow immediately after human arrival, not millennia later.
Second, climate change directly affected the habitats and food sources of these large animals. The warming climate led to the replacement of open grasslands with dense forests, which would have reduced the available forage for grazing megafauna. For example, woolly mammoths relied on dry, grassy steppes; as these environments shrank, their populations would have declined naturally, independent of any human impact.
Third, the fossil record shows that many megafauna species experienced population bottlenecks—severe reductions in numbers—well before humans reached their habitats. Genetic studies of ancient mammoth remains reveal a significant decline in genetic diversity starting around 30,000 years ago, a time when human populations were sparse and had limited hunting technology. This pre-human population stress points to environmental factors, not overhunting, as the root cause.
🎧 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 part of a lecture on the same topic.
Professor: The reading argues that climate change drove the megafauna extinctions, but this view is seriously flawed. Let me address each point.
First, the timing argument doesn't hold up. While humans were present in North America for 2,000 years before the mass extinction, that doesn't mean they weren't the cause. In fact, human populations were growing exponentially during that time, and their hunting techniques became more sophisticated. The extinction spike at 13,000 years ago coincides not just with warming, but also with the emergence of the Clovis culture—known for their effective spear points. So the delay could simply reflect when humans became efficient enough to overhunt large animals.
Second, the habitat change theory is oversimplified. Many megafauna species were generalists, not specialists. For instance, mammoths could eat a variety of vegetation, not just grasses. Additionally, during previous interglacial warming periods, these same animals survived similar habitat changes. So why would they suddenly go extinct during this particular warming event? That inconsistency suggests that an additional factor—like human hunting—was necessary to push them over the edge.
Third, the genetic bottleneck evidence is misinterpreted. A decline in genetic diversity 30,000 years ago does not prove that climate was the cause. It could be the result of a natural population fluctuation that later recovered. In fact, some mammoth populations did recover and survived until much later, only to disappear when humans arrived on their final refuges, such as Wrangel Island. That pattern strongly implicates human hunting as the decisive factor.
📝 Writing Prompt 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).
