Around 12,000 years ago many of the largest animals in North America,such as mammoths and giant sloths,became extinct.Numerous fossilized remains attest to the millions of years that these species existed in relative stability on the continent; other roof of this group's former existence survives in traits of still-living species. Pronghorn antelopes of the American West run far faster than any of their predators; their speed might well be a remnant of their need to escape pursuit by the American cheetahs that are now extinct. Likewise,avocado seeds are uncomfortably large to be eaten and dispersed by current species, but avocados evolved in habitats populated by tree-foraging giants like the sometimes dinosaur-proportioned ground sloths. The timing of

the North American extinction of these large animals, which are collectively termed megafauna coincides with the arrival of humans to the continent, which led paleontologist Paul Martin to hypothesize in the 1960s that the overhunting of these animals by human populations led to their demise.

Critics of the overhunting theory pointed to climate change at the end of the last ice age as an alternative explanation for the extinctions,freeing these early pioneering humans from blame. And North America did indeed experience dramatic climate changes during the transition out of the most recent ice age. But it had done so countless times before during the Pleistocene (2.6 million to 11,700 years ago),and the climate changes at the end of the most recent ice age were certainly no larger or more intense than the many previous oscillations (fluctuations)between glacial and interglacial times. The megafauna of the Pleistocene had easily responded to the earlier changes,shifting their ranges to maintain their preferred habitat. The changing climate might have added an extra element of instability, making the biosphere more vulnerable to the sort of disruption imposed by multiplying bands of skilled hunters transforming the landscape with fire as they spread. But there is no reason to think that the megafauna of North America would have gone extinct without the introduction of humans. It is also nearly impossible to appeal to climate change to explain why nocturnal animals tended to fare better in the extinctions. The same can be said for plants, few of which went extinct. Ground sloth dung that Martin encountered in the Grand Canyon revealed a diet of plants that still thrive in the arid landscapes of North America and are happily eaten by bighorn sheep and wild burros. It seems unlikely that the slow, defenseless giant ground sloths disappeared for lack of food.

Finally, there existed comparison groups that could test Martin's theories. On islands and landmasses that remained undiscovered by humans for thousands of years, megafauna survived the climate changes at the end of the Pleistocene,as they had many times before,only to be destroyed when humans eventually arrived on their shores. The last ground sloths might have vanished from mainland North America 10,000 years ago,but in 2005 Martin's former student, University of Florida paleontologist David Steadman,found fossils of a species that stayed on in the Caribbean islands of Hispaniola and Cuba for an additional 5,000 years. When the West Indies islands were first settled by humans, these Caribbean ground sloths quickly disappeared there as well.

Amazingly, woolly mammoths also persisted unseen onremote islands,even as late as the golden age of Egyptian pyramid construction (about 2686 to 2325 BCE). The mammoths found themselves isolated,but safe,on Wrangel Island off Siberia,and on Saint Paul in the secluded Pribilof lslands in the Bering Sea,far north of the Aleutian Islands. These refuges went undiscovered by humans and the mammoths survived,while their mainland relatives became extinct thousands of years before.

Similarly, Steller's sea cow,a gigantic 30-foot-long (9 meter- long) manatee cousin,was eradicated from the North Pacific coast around 12,000years ago but managed to survive unmolested as a smal remnant population on the uninhabited Commander Islands off Russia until the eighteenth century. The Commander Islands were discovered in 1741by fur traders.The 12-ton giant was hunted to extinction on this, its final refuge—and thus its entire world-within three decades of human discovery.