" The cause of these great extinctions, the most extensive in the seventy-million-year record for mammals, is a mystery. The two prevailing guesses, climatic environmental pressure or the destruction caused by human immigration in these regions, are at a stalemate. Nearly all regions of extinction were briefly inhabited by early humans, which is why some feel they played roles. I was not there at the time, so I can only speculate, but surely a global catastrophe of some sort triggered the cataclysm. "

― , Tears of the Cheetah: The Genetic Secrets of Our Animal Ancestors