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 intrigued scientists. While climate change and human hunting are often cited, a less discussed but compelling hypothesis is that a catastrophic extraterrestrial impact—specifically a comet or asteroid airburst—triggered a rapid sequence of events leading to their demise. Three lines of evidence support this cosmic impact theory. First, a distinct sedimentary layer dating to roughly 12,800 years ago, known as the Younger Dryas boundary, contains anomalously high concentrations of iridium, a rare element in Earth's crust but common in asteroids and comets. This iridium spike is analogous to the one found at the Cretaceous-Paleogene boundary, which is linked to the dinosaur extinction impact. Such a geochemical signature strongly suggests an extraterrestrial object collided with Earth at that time. Second, the same boundary layer contains microscopic carbon spherules and nanodiamonds, which form under extreme heat and pressure typical of an impact. These materials are not produced by volcanic activity or normal wildfires, making them unique markers of a high-energy cosmic event. Their presence in multiple sites across North America and Europe points to a widespread impact or airburst. Third, the timing aligns perfectly with the onset of the Younger Dryas, a sudden return to near-glacial conditions that lasted about 1,200 years. This abrupt cooling would have drastically altered the mammoth's steppe-tundra habitat, reducing food availability and fragmenting populations, ultimately driving them to extinction. The impact theory thus provides a plausible mechanism for the rapid environmental change, linking the cosmic event to the mammoth's demise.

🎧 リスニング講義

The reading's cosmic impact theory for the woolly mammoth extinction is intriguing but deeply flawed. Let me address each point. First, the iridium spike. While the Younger Dryas boundary does show elevated iridium in some sites, the concentrations are far lower than at the K-Pg boundary and are highly variable geographically. In fact, many contemporaneous layers lack any iridium anomaly. Volcanic eruptions, particularly large-scale basaltic floods, can also produce localized iridium enrichments. The iridium evidence is thus not unique to an impact. Second, the carbon spherules and nanodiamonds. These are not exclusive to impacts. Recent experiments show that they can form in ordinary wildfires that burn at high temperatures, especially when organic-rich soils are involved. Moreover, the nanodiamonds identified in some studies have been re-examined and found to be misidentified—they are actually graphene or other carbon forms. So the microstructural evidence is equivocal at best. Third, the timing. The extinction of woolly mammoths was not synchronous with the Younger Dryas onset. Radiocarbon dating shows that mammoth populations persisted on mainland Siberia and Alaska for thousands of years after 12,800 years ago, with the last mainland population dying out around 4,000 years ago on Wrangel Island. If an impact had caused their extinction via rapid cooling, they would have disappeared everywhere at once. The gradual, staggered extinction pattern is far more consistent with a combination of human hunting and habitat loss due to post-glacial warming, not a single catastrophic event.