The Extinction of the Megafauna: A Climate-Driven Event
Around 12,000 years ago, many large terrestrial mammals, such as woolly mammoths, saber-toothed cats, and giant ground sloths, disappeared from North America. While some scientists attribute this extinction to human hunting, a closer examination of the evidence suggests that climate change was the primary driver. First, the timing of the extinctions coincides with a period of rapid warming known as the Bølling-Allerød interstadial, which occurred roughly 14,700 years ago. This warming caused dramatic shifts in vegetation, reducing the open grasslands that megafauna depended on and fragmenting their habitats. Second, fossil records show that many megafauna species experienced population declines well before humans arrived in large numbers. For example, populations of the shrub-ox and the American camel began to dwindle thousands of years before the Clovis people, who are often linked to overhunting, appeared. Third, the megafauna that survived, such as bison and elk, were generalist feeders that could adapt to diverse environments, whereas the extinct species were largely specialists, unable to cope with rapid ecological change. This pattern strongly implies that climate-induced habitat alteration, not human predation, was the decisive factor in their demise.
🎧 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’s argument that climate change alone drove the megafauna extinction is problematic. While it’s true that a warming period occurred, the timing doesn’t match for all species. Many extinctions happened later, around 11,000 years ago, which aligns with the arrival of the Clovis hunters, not with the earlier warming. Moreover, the claim that populations declined before humans arrived is misleading. In many sites, we find evidence of healthy, stable populations right up until human contact. For instance, at the Rancho La Brea tar pits, radiocarbon dating shows that saber-toothed cats and dire wolves coexisted with humans for centuries before disappearing. The most damaging evidence comes from the fossil record itself: we find megafauna bones with embedded projectile points and cut marks from butchering, which is direct proof of hunting. But perhaps the strongest counterpoint is the survival of generalist species. If climate alone were responsible, why did bison and elk, which are also generalists, survive? The key difference is that these species were smaller and reproduced more quickly, making them less vulnerable to hunting pressure. In contrast, large, slow-reproducing megafauna were easy targets for human hunters, who likely overexploited them. So, the evidence points to humans, not climate, as the primary cause.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
