The extinction of the Pleistocene megafauna—large animals such as woolly mammoths, saber-toothed cats, and giant ground sloths—has long puzzled scientists. While multiple factors have been proposed, the most compelling explanation is overhunting by early humans. First, the timing of extinctions correlates strongly with human arrival on different continents. In North America, megafauna disappeared shortly after the Clovis people crossed the Bering land bridge around 13,000 years ago. In Australia, large marsupials vanished soon after human colonization roughly 45,000 years ago. Second, archaeological evidence supports human predation. At sites like the Murray Springs in Arizona, researchers have found spear points embedded in mammoth skeletons, along with butchering tools and fire pits. Such kill sites demonstrate that humans actively hunted these animals, not merely scavenged them. Third, the selectivity of extinction—whereby only large-bodied species disappeared while smaller animals survived—matches the hunting preferences of humans, who would target high-yield prey for meat and hides. Moreover, species that evolved in isolation, such as island giants, were especially vulnerable because they lacked fear of humans. Together, these lines of evidence make human overhunting the most parsimonious explanation for the megafaunal collapse.
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The lecture challenges the overhunting hypothesis, arguing that climate change and other factors better explain megafaunal extinctions. First, the correlation between human arrival and extinction is not consistent. For instance, in Africa and Eurasia, where humans evolved and coexisted with megafauna for hundreds of thousands of years, extinctions were far less severe. In contrast, the most dramatic die-offs occurred during periods of rapid climatic warming at the end of the Pleistocene, such as the Younger Dryas, which drastically altered habitats. Second, the archaeological evidence is limited. Only a handful of kill sites have been confirmed, and many purported spear points are actually too small to have killed a fully grown mammoth. Instead, such implements may have been used for scavenging or hunting smaller game. Third, the selectivity of extinction does not favor humans. Large animals are more sensitive to environmental changes because they have lower reproductive rates and require vast territories. As climate shifted, their food sources—like specific grasslands and forests—disappeared, leading to population crashes. In contrast, smaller, generalist species could adapt. Furthermore, island extinctions often lagged behind human arrival by centuries, suggesting that habitat loss from sea-level rise, not hunting, was the primary driver.
📝 작문 지시 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).
