This much we know: About 20,000 years ago the great continental ice sheet of the Wisconsin glaciation in North America was receding and biomes were shifting as the climate warmed. This was the end of the most recent “ice age” and it marked the beginnings of the relatively stable climate that has prevailed over the last several thousand years in the Holocene. But there was a little dip along the way: a 1000-year cold snap beginning about 12,900 years ago known as the “Younger Dryas.” Although declines in Pleistocene “megafauna” began well before the Younger Dryas, it appears that the last of those large mammals gave up the ghost around 11,000 years ago.
The decline in large mammals coincided not just with changes in climate, but also with the arrival of humans in North America. There is global evidence for the “overkill hypothesis” to be at the root of large mammal extinctions. In other words, where humans ventured, extinction quickly followed. In their struggle to survive, our ancestors hunted out most of the big mammal species.
But that idea does not hold universal appeal, and a good, old-fashioned scientific controversy has bubbled up regarding a competing hypothesis to explain the extinction of the Pleistocene megafauna: the explosion of an asteroid over Canada that triggered the rapid climate change associated with the Younger Dryas.
Richard Kerr’s News Focus column on the Science Magazine website explores the controversy in light of a new paper challenging the asteroid hypothesis. Now, a team of impact scientists are challenging the data that supported the asteroid hypothesis in the first place: the team has tried repeatedly to replicate the searches that led to the formation of that hypothesis, focusing on trace geologic features one might use to pin down the occurrence and timing of such an impact. Their inability to find “nanodiamonds” among the sediments claimed to have been produced by the asteroid has cast doubts on the asteroid hypothesis.
Great explanation of the issue here as well.