End of the Big Beasts
by Peter Tyson

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Who or what snuffed out the mammoths and other megafauna 13,000 years ago?

It takes a certain kind of person to take on this question as his or her life's work. You have to be itching to know the answer yet patient as a Buddha, for the answer is frustratingly elusive. I know I'm not the type. I'm intrigued by the question but far too anxious to calmly accept, as some experts suggest, that it might be years or decades, if ever, that a definitive, widely accepted solution will come.

The three people I spoke to about the so-called megafaunal extinctions possess this sort of edgy sangfroid. They also stand in three decidedly different camps regarding why America's rich complement of big animals went extinct quite suddenly at the end of the Ice Age. The three camps are known tongue in cheek as "overkill," "overchill," and "overill":

• Archeologist Gary Haynes of the University of Nevada Reno thinks that the continent's first human hunters, fresh from Siberia, killed the big beasts off as they colonized the newly discovered land.

• Donald Grayson, an archeologist at the University of Washington, Seattle, believes that climate changes at the end of the Pleistocene epoch triggered the collapse.

• And mammalogist Ross MacPhee of the American Museum of Natural History has advanced the idea, with virologist Preston Marx, that a virulent "hyperdisease" brought with the first people might have raced through species with no natural immunity, bringing about their demise.

Despite their differing views, these researchers sound remarkably similar over the phone. Each is convincing about the merits of his chosen hypothesis, even as he acknowledges its limitations. Each is adamant as to the flaws that he thinks dooms one or both of the other theories, even as he is gracious toward those who side with that theory. Finally, each reveals that spark of deep passion for the subject, and the choke needed to rein it for the long haul.

So why is the answer so elusive? As often happens in paleontology, it all comes down to lack of empirical evidence, something all three hypotheses arguably suffer from. (There's a fourth hypothesis, actually—that a combination of overkill and overchill did it.)

Overkill

In the early 1960s ecologist Paul Martin of the University of Arizona advanced the idea that the first Americans, who as every schoolchild knows are thought to have crossed from Siberia to America across the Bering Land Bridge, hunted the megafauna to extinction. For many years, "overkill" became the leading contender. The timing seemed more than coincidental: humans were thought to have arrived no earlier than about 14,000 years ago, and by roughly 13,000 years ago, most of the megafaunal species abruptly vanish from the fossil record. (See a list of all 35 vanished genera of North American Ice Age mammals.)

But some skeptics, Grayson among them, have asked where's the evidence? Grayson and archeologist David Melzer of Southern Methodist University have noted that late-Ice Age sites bearing megafaunal remains that show unequivocal evidence of slaughter by humans number just 14. Moreover, they stress, only two types of giants were killed at those 14 sites, mammoth and mastodon. No signs have turned up that early hunters preyed on giant ground sloths, short-faced bears, camels, or any of the other large mammals that went extinct. That's hardly enough evidence, they argue, to lay blame for a continent's worth of lost megafauna at the foot of the first Americans.

Gary Haynes begs to differ. "I don't care what anybody else says, 14 kill sites of mammoth and mastodon in a very short time period is extraordinary," he told me. It's one thing to find a campsite with some animal bones in it, he says, quite another to find the actual spot where an animal was downed and butchered—where, say, a spearpoint turns up still sticking in bone. "It's very, very rare to find a kill site anywhere in the world," he says. And absence of other megafauna in kill sites doesn't mean they weren't hunted. "There is no doubt Native Americans were eating deer and bear and elk," Haynes says, citing several megafauna that pulled through. "But you cannot find a single kill site of them across 10,000 years."

Could what scholars agree must have been a relatively modest initial population of hunters have emptied an entire continent of its megafauna virtually overnight, geologically speaking? (In fact, it's three continents: South America and, to a lesser extent, Northern Eurasia also lost many large creatures at the end of the Ice Age.)

MacPhee, for his part, finds it hard to swallow. "I just don't think it's plausible, especially if we're also talking about collapses for megafauna that didn't actually go extinct." (Researchers have found evidence that certain populations of surviving megafauna, including musk oxen in Asia, fell precipitously at the end of the Ice Age.) "It gets a little bit beyond probability in my view that people could have been so active as to hunt every animal of any body size, in every context, in every possible environment, over three continents."

Overchill

Could climate change have done it? Scholars generally agree that North America witnessed some rapid climate adjustments as it shook off the Ice Age beginning about 17,000 years ago. The most significant swing was a cold snap between about 12,900 and 11,500 years ago. Known as the Younger Dryas, this partial return to ice-age conditions may have stressed the megafauna and their habitats sufficiently to cause widespread die-offs, Grayson and others believe.

Detractors, again, point to the lack of evidence. "There aren't any deposits of starved or frozen or somehow naturally killed animals that are clearly non-cultural in origin that you would expect if there was an unusual climate swing," says Haynes. "I don't think that evidence exists." Another question anti-overchillers have is how the megafauna survived numerous glaciations and deglaciations during the past two million years only to succumb to the one that closed the Pleistocene.

The dearth of evidence doesn't deter researchers working in this area. In fact, it's a spur.

MacPhee points to yet another problem: the geography of the extinctions. For one thing, both North and South America suffered them. "This is to me practically the most decisive evidence there is that it could not have been what we conventionally think of as climate change," he says. "If the entire continental part of the Western Hemisphere was affected at roughly the same time, as good as we can tell with the carbon-14 record, then what force of nature are we talking about? The guys who support climate change are silent on that point." And why was Africa spared? (Elephants, giraffes, and many other African megafauna made it through just fine.) "There's nothing that we know of in nature, climatically speaking, that works in that fashion, as to affect one half of the world and not the other," MacPhee says.

Grayson admits that overchill advocates have failed to develop the kind of records that are needed to test climate hypotheses in detail. But he focuses on climate change, he says, because he sees absolutely no evidence that people were involved. "You can't look at climate and say climate didn't do it for the simple reason that we don't really know what to look for," Grayson told me. "But what you can do fairly easily is look at the evidence that exists for the overkill position. That position would seem to make fairly straightforward predictions about what the past should have been like, and when you look to see if it was that way, you don't find it."

Overill

Not finding supportive evidence has particularly plagued the "overill" hypothesis. This is the notion that diseases brought unwittingly by newly arriving people, either in their own bodies or in those of their dogs or perhaps rats, could have killed off native species that had no natural immunity. MacPhee devised this hypothesis after realizing that the link between initial human arrival and subsequent large-animal extinctions was strong not just in North America but in many other parts of the world (see map at right), but that in his opinion, convincing evidence for hunting as the trigger simply did not exist.

Despite what he calls "prodigious effort" using DNA techniques and immunological probes, however, MacPhee and his colleagues have failed to detect clues to any pathogens in megafaunal bones, much less nail down a specific disease, like rabies or rinderpest, that could have jumped species boundaries and wiped out all the big beasts. "[MacPhee's hyphothesis] doesn't even have circumstantial evidence," says Haynes, "because we can't prove there was hyperdisease. We can prove people were here, and we can prove climates were changing." Fair enough, says MacPhee, though he points out that the burgeoning ability of Asian bird flu to infect across species boundaries seems to suggest that some diseases are ecologically and genetically preordained to, as he puts it, "go hyper."

Soldiering on

The dearth of evidence—seemingly significant in all three camps—doesn't deter researchers working in this area. In fact, it's a spur. MacPhee may be speaking for all scholars involved in this famously contentious debate when he says: "What's of interest here for me personally is that these Pleistocene extinctions have occupied the minds of some very able thinkers over the last half century or so, and nobody's come up with anything that's drop-dead decisive. So it's attractive as an intellectual problem."

Granted. But hey, aren't you just dying to know what happened?