The extinction of the megafauna—large animals such as woolly mammoths, giant sloths, and saber-toothed cats—at the end of the Pleistocene epoch has long puzzled scientists. While many factors have been proposed, the most compelling explanation is overhunting by early humans. First, the timing of extinctions closely correlates with human migration. In North America, for example, the arrival of Clovis hunters around 13,000 years ago coincides with the rapid disappearance of over 30 genera of large mammals. Second, archaeological evidence supports human predation. At sites like Murray Springs in Arizona, remains of mammoths have been found with projectile points embedded in their bones, indicating active hunting. Third, the selectivity of extinction supports the overhunting hypothesis. Species that went extinct were typically large-bodied and slow-reproducing, making them especially vulnerable to human hunting pressure, whereas smaller, fast-reproducing species survived. This pattern is consistent with what we know of human impact on island ecosystems, where hunting led to similar selective losses. Therefore, human hunters, not climate change or disease, were the primary cause of the megafauna extinction.
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The reading's argument that humans alone drove megafauna extinction is flawed. First, the timing correlation is not as tight as claimed. In many regions, such as South America and Australia, megafauna disappeared thousands of years before humans arrived, or survived for millennia after human contact. For instance, in Tasmania, humans arrived 40,000 years ago, yet some large species persisted until 20,000 years ago. Second, the archaeological evidence is ambiguous. Many 'hunting' sites show signs of scavenging, not hunting. The mammoth bones at Murray Springs bear cut marks that may result from butchering animals that had already died of natural causes. Moreover, the number of kill sites is far too small to explain the extinction of entire populations. Third, the selectivity argument actually contradicts human overhunting. If humans were the main predators, they would have targeted the largest, fattiest animals, which were often the biggest species. Yet some very large animals, like the African elephant, survived, while smaller species, such as the dwarf mammoth, went extinct. This selectivity aligns better with climate-induced habitat changes, which disproportionately affected species with narrow ecological tolerances. Thus, the evidence points to a more complex scenario, likely involving climate change and other factors.
📝 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.
