The theory that the comet won't die.

The reappearance in southerly fossil deposits of tundra plants was marked by scientists as the cause of the rapid climatic reversal. The wildflower Dryas integrifolia has a 1,200-year time span. The impact caused the sudden cooling and contributed to the demise of the mammoths, steppe bison and other large mammals along with the people who pursued them. The Younger Dryas impact may have caused a turn toward agriculture in Eurasia and eventually civilization. If the impact had not led to the extinction of horses in the Americas, Native Americans would have domesticated them and presented more formidable opposition to European conquistadors. He says the setup of human culture would have been different.