TOEFL統合ライティング

📖 読解文章

The extinction of the woolly mammoth (Mammuthus primigenius) at the end of the Pleistocene has long been debated. While some scientists attribute their demise to human overhunting, others propose climate change or a combination of factors. However, a closer examination of the evidence suggests that a single, catastrophic extraterrestrial impact—a comet or asteroid strike—was the primary cause. This hypothesis is supported by three key lines of evidence. First, a distinct sedimentary layer dating to approximately 12,800 years ago, known as the Younger Dryas boundary, contains elevated concentrations of iridium, magnetic spherules, and nanodiamonds—materials typical of cosmic impacts. These markers have been found at multiple sites across North America and Europe, indicating a widespread event. Second, the timing of the mammoth extinction correlates precisely with this impact event. Radiocarbon dating of the last known mammoth remains in mainland Alaska and Siberia clusters around 12,800 years ago, coinciding with the onset of the Younger Dryas cooling period, which was likely triggered by the impact's dust and soot blocking sunlight. Third, the impact would have generated massive wildfires and a ‘nuclear winter’ effect, destroying the mammoths' grassland habitats and causing a sudden collapse in their populations. This scenario explains the abrupt extinction pattern better than gradual climate change or human hunting, which would have left a more protracted decline.

🎧 リスニング講義

The reading's argument for an extraterrestrial impact as the sole cause of mammoth extinction is intriguing, but it suffers from serious flaws. While the sedimentary markers are real, they are not unique to impacts. Volcanic eruptions, such as the massive Laacher See eruption in Germany around 12,900 years ago, can also produce iridium, spherules, and even nanodiamonds through shock metamorphism. Thus, the presence of these materials does not prove a cosmic origin. Second, the correlation of timing is overstated. The last mammoth remains on mainland continents are dated to around 12,800 years ago, but many isolated island populations, such as those on Wrangel Island in the Arctic Ocean, survived until about 4,000 years ago. This indicates that the extinction was not a single synchronous event, but a gradual process that continued for thousands of years after the supposed impact. Third, the impact scenario fails to account for the actual ecological context. The Younger Dryas period saw a return to cold, dry conditions, but this did not lead to a sudden loss of grasslands. In fact, mammoths are adapted to cold steppe environments. Moreover, paleontological evidence shows that human hunting, with the use of advanced Clovis spear points, was widespread and intensified at this time. The extinctions across North America align more closely with human hunting pressure than with a single catastrophic event.