Who Killed Woolly?

Hard as it may be to imagine, 20,000 years ago--when the last Ice Age reached its climax--much of North America looked like something out of Africa's teeming Serengeti Plain. Roaming through grasslands and forests were mammoths and mastodons with huge, curved tusks, ground sloths the size of rhinos and bison with sharp-tipped horns that measured more than 6 ft. from tip to tip. Bear-size beavers roamed the forests. Large-headed llamas grazed in rocky meadows. And giant armadillos maneuvered across the landscape like living armored tanks.

Stalking these massive creatures were predators of equally awesome aspect: muscular jaguars, saber-toothed cats and thick-bodied dire wolves with bone-crunching teeth. Most horrible of all were the short-faced bears, huge animals that weighed twice as much as modern grizzlies and could chase down and kill anything except perhaps a full-grown mastodon. But about 13,000 years ago, not long after the first humans arrived in the New World, all but a few of these remarkable creatures--collectively known as megafauna--had vanished.

Why? Some scientists blame climate change; others finger a mega-outbreak of disease. Still others subscribe to what Australian paleontologist Tim Flannery refers to as the "black-hole theory of extinction." In this case, as Flannery wryly explains in his just published ecological history of North America, The Eternal Frontier (Atlantic Monthly Press; $27.50), the black hole lay between the nose and chin of our Stone Age ancestors.

To many, the notion that primitive hunters could have killed off more than 100 species of large animals has long seemed preposterous. While Homo sapiens certainly killed and ate the likes of mammoths and mastodons, notes Ross MacPhee, an expert on mammalian extinctions at New York City's American Museum of Natural History, it must have done so with great caution. As he puts it: "If some guy walked up to a mammoth armed only with a pointy stick, chances are he would have been road pizza within minutes!"

Yet when John Alroy, an evolutionary biologist at the University of California at Santa Barbara, put the question of what happened to the megafauna to a computer model, he got back a surprising answer. As he reported in the journal Science last week, the overkill hypothesis is actually quite plausible. That's because it was not necessary for human hunters to do in every last animal. All people had to do was kill slightly more animals than were born, a process that Alroy's model suggests could easily have taken place over a span of 1,000 years. Ecological upheavals owing to climate change and outbreaks of epidemic disease could then have sped up the process, although Alroy's model did not factor these in.

Or then again, it could have been the other way around, argues MacPhee, who believes disease offers a more credible instrument of decline. He suspects that some disease-causing microbe--one to which New World organisms lacked resistance--tagged along with the Stone Age hunters who first ventured into the New World. Once again this pathogen (which could have been carried by dogs or vermin) need not have killed every single animal to have set the wheels of extinction in motion. During the late 19th century in Africa, for example, native antelope, wildebeests and other ungulates were decimated by rinderpest, a disease spread by imported cattle.

For many years, these three competing hypotheses--climate change, overhunting, hyperdisease--seemed to be little more than intriguing ideas. But at long last scientists are beginning to buttress them with hard data. Within the past year, for example, a team led by MacPhee and his colleague, molecular biologist Alex Greenwood, has managed to isolate viral material from frozen mammoth tissues and bone marrow. So far they haven't identified a microbial culprit for the extinctions, but candidates under consideration include the species-crossing viruses that cause rabies and distemper.

For his part, vertebrate paleontologist Russ Graham of the Denver Museum of Nature and Science is collecting evidence of climate-driven habitat changes that occurred during the critical time frame. Among other things, he notes, the forests and savannas in which mastodons and mammoths foraged not only became less diverse as glacial conditions wound down but also dwindled in size. As the preferred habitat shrank, so did the numbers of these huge animals, rendering the animals vulnerable to all sorts of bad influences, including disease and overhunting.

Proponents of the overkill hypothesis have also been busy. In another article published in Science last week, Tim Flannery and his colleagues describe how they used a sophisticated new dating technique to pin down the timing of a megafauna extinction that occurred in Australia. A total of 55 species of vertebrates, including giant kangaroos and an ostrich-size bird known as Genyornis newtoni, all disappeared about 46,400 years ago, they report--within 5,000 to 10,000 years of the presumed arrival of humans. The same close correlation between the appearance of humans and the disappearance of large-bodied animals has been documented in New Zealand and Madagascar.