Through some combination of climatic change and the spread of highly skilled hunters, almost all of the largest mammals rapidly died out in the Americas. The extinctions comprised two-thirds of all New World species that weighed more than one hundred pounds at maturity—including the giant beaver, giant ground sloth, mammoth, mastodon, and horses and camels. It is ironic that horses and camels first evolved in North America and migrated westward into Asia, where they were eventually domesticated, while those that remained in the Americas became extinct.
