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(...) The study reviewed the existing research on the Younger Dryas impact hypothesis, which, in various forms, argues that a comet caused significant extinctions, climate changes, and shifts in human culture around 10,900 BCE.
(...) In other words, prehistoric humans might have witnessed a catastrophic cosmic impact that triggered widespread wildfires, global cooling, and the extinction of large animals, some of which might have preyed on humans. It’s still unclear whether the impact occurred. But in the centuries after the comet allegedly struck Earth, human cultures made broad transitions away from hunter-gatherer societies toward agricultural-based civilizations.
What caused Earth’s most recent cooling?
One mystery this hypothesis aims to solve is the cause of the Younger Dryas, which is the brief ice age that Earth experienced about 12,900 to 11,700 years ago. An “impact winter” might have been the culprit: it’s a hypothesized scenario where a cosmic impact causes global cooling because the impact and its resulting ground fires would eject enough material into the atmosphere to significantly block radiation from the sun.
This alleged comet left no crater; proponents of the impact hypothesis argue that much of the comet likely fragmented high in the atmosphere and might have struck a large ice sheet on Earth’s surface. But crater aside, the comet did seem to leave behind other evidence. One example is the Younger Dryas “black mats,” which are distinctive soil layers (or “YD boundaries”) that have been discovered at more than 100 sites across four continents.
A smoking gun?
It is unclear how the mats got their blackish color. One explanation is that it came from charcoal produced by wildfires. But what is clear is that the mats serve as a dividing line between epochs on Earth, mainly because only certain materials and fossils are found above or below this layer. For example, no spearheads or other archaeological evidence of the prehistoric PaleoAmerican Clovis culture has been found above the black mats.
“More generally, many extinct megafaunal species are found below the black mat, but not within or above, including horse, camel, mastodon, direwolf, American lion,” the researchers wrote.
