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 more closely with major climatic shifts than with human expansion. Radiocarbon dating shows that most mammoth populations vanished during the Pleistocene-Holocene transition (approximately 12,000 years ago), a period of rapid warming and vegetation change, whereas human hunters had already occupied Siberia and North America for millennia without causing extinction. Second, climate change altered the mammoth’s habitat drastically. The warming climate caused the replacement of the dry, grassy steppe—the mammoth’s primary food source—with wet, forested tundra and peatlands. This dietary shift led to nutritional stress and reduced reproductive success, as evidenced by the decline in average body size and fertility in late populations. Third, genetic studies of ancient mammoth remains reveal a loss of genetic diversity and an increase in harmful mutations before extinction. These genetic signatures are typical of populations shrinking due to environmental pressure, not sudden overhunting. Furthermore, isolated mammoth populations on Wrangel Island survived until 4,000 years ago, long after humans arrived, but they eventually died out due to inbreeding and habitat loss—again pointing to climate as the decisive factor.
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The reading’s argument that climate change caused mammoth extinction is flawed. First, the correlation between climate and extinction is not as tight as claimed. While warming occurred 12,000 years ago, mammoths survived in mainland Siberia until 10,000 years ago and on Wrangel Island until 4,000 years ago. If climate were the main driver, why did they persist through the warmest periods? In contrast, human hunting pressure increased precisely during these later survival windows. Second, the habitat change argument is overstated. The replacement of steppe by tundra did occur, but mammoths were generalist feeders—their dung and teeth show they consumed a wide variety of plants, including sedges and shrubs. Modern elephants also adapt to diverse diets. Moreover, the decline in body size may reflect natural variation, not nutritional stress, and similar size reductions occurred during earlier interglacial periods when mammoths thrived. Third, the genetic data are misinterpreted. Loss of genetic diversity and harmful mutations are expected in any small, isolated population, regardless of cause. Such genetic patterns are often the result of population decline, not the cause. In fact, human hunting could have fragmented populations, leading to the same genetic signatures. Finally, the Wrangel Island example actually undermines the climate hypothesis—those mammoths survived the postglacial warming for 6,000 years, and their extinction coincides with the arrival of seafaring humans on the island, not with any dramatic climate event.
📝 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.
