The four people I spoke to about the megafaunal or "large-animal" extinctions possess this sort of edgy sangfroid. While keeping an open mind, they also stand in four decidedly different camps regarding why America's rich complement of big beasts went extinct quite suddenly at the end of the Ice Age. The four camps are known tongue-in-cheek as "overkill," "overchill," "overill," and "overgrill"*:
- Archeologist Gary Haynes, University of Nevada Reno, and others think that the continent's first human hunters, fresh from Siberia, killed the megafauna off as they colonized the newly discovered land.
- Donald Grayson, an archeologist at the University of Washington, Seattle, along with colleague David Meltzer of Southern Methodist University, believes that climate changes at the end of the Pleistocene epoch triggered the collapse.
- Mammalogist Ross MacPhee of the American Museum of Natural History has advanced the idea, with virologist Preston Marx, that a virulent "hyperdisease" brought by the first Americans might have raced through species with no natural immunity, bringing about their demise.
- And, in the newest hypothesis advanced, geologist James Kennett, U.C. Santa Barbara, and colleagues propose that a comet impact or airburst over North America did it.
So why is the answer so elusive? As often happens in the paleosciences, it largely comes down to lack of empirical evidence, something all four hypotheses arguably suffer from. (There's a fifth hypothesis, actually—that a combination of overkill and overchill did it.)
In the early 1960s, ecologist Paul Martin of the University of Arizona postulated that the first Americans, after crossing into the Americas over 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.
But skeptics have asked, Where's the evidence? Grayson and Meltzer (overchill) have noted that late-Ice Age sites bearing megafaunal remains that show unequivocal sign of slaughter by humans number just 14. Moreover, they stress, only two types of giants were killed at those 14 sites, mammoth and mastodon. There's no sign that early hunters preyed on giant ground sloths, short-faced bears, or the massive, armadillo-like glyptodonts, for instance. (Forensic studies of a cache of Clovis tools found in 2008 suggest the Clovis people did hunt now-extinct camels and horses.) That's hardly enough evidence, Grayson and Meltzer argue, to lay blame for a continent's worth of lost megafauna at the foot of the first Americans.
Gary Haynes (overkill) 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 ancient hunter felled and butchered an animal—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 large mammals that pulled through. "But you cannot find a single kill site of them across 10,000 years."
The dearth of widely convincing evidence only serves as a spur.
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 species at the end of the Ice Age.) For his part, Ross MacPhee (overill) 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." Certain populations of surviving big beasts, including bison in North America and musk oxen in Asia, are known to have fallen 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."
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 dissenters have is how the megafauna survived many abrupt glacial and deglacial shifts during the past two million years only to succumb to the one that closed the Pleistocene. "It just doesn't hold water," Jim Kennett (overgrill) told me.
Grayson admits that overchill advocates have failed to develop the kind of records needed to test climate hypotheses in detail. But he focuses on climate change, he says, because he sees absolutely no sign 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."
A lack of data 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 with Preston Marx 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), but that in his opinion, convincing evidence for hunting as the culprit 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 from one type of animal to another and wiped out all the big beasts. "There's no evidence, and there's virtually no possibility of getting any evidence," Kennett told me.
"[Overill] doesn't even have circumstantial evidence, because we can't prove there was hyperdisease," Haynes says. "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."
The most recent hypothesis, advanced by Kennett and 25 other scientists in a 2007 Proceedings of the National Academy of Sciences paper, concerns the proposed cosmic impact. Right about the time the Younger Dryas began and at least 15 of those 35 extinct mammals and arguably the Clovis culture itself appear to vanish abruptly from the fossil record—that is, right about 12,900 years ago—Kennett et al see markers of a major catastrophe. The markers lie in a thin layer at the base of a "black mat" of soil that archeologists have identified at over 50 Clovis sites across North America.
According to Kennett, fieldworkers have uncovered fossils of the 15 genera of mammals that survived right up to Younger Dryas times just beneath—but neither within nor above—this black mat. (Some fossil bones butt up against this layer so closely that the mat has blackened them, Kennett told me.) Stone-tool remains of the Clovis culture also end just beneath the mat, he says. Moreover, Kennett and the team he works with have identified charcoal, soot, microscopic diamonds, and other trace materials at the base of the mat. These materials indicate, he says, that a comet (not an asteroid—different constituents) exploded in the atmosphere or struck the surface, likely in pieces. This triggered widespread wildfires and extinctions, changed ocean circulation, and coughed up sun-blocking ash and dust, all of which helped unleash the Younger Dryas. Tokens of this cosmic cataclysm have shown up in the Greenland ice sheet as well, Kennett says.
Aren't you just dying to know what happened?
Where then, skeptics ask, is the crater? Unlike the asteroid strike at the end of the Cretaceous, the one thought to have ended the reign of the dinosaurs, this 12,900-year-old event currently has no hole or holes definitively linked to it. Kennett says it's still early, noting that it took nearly a decade for scientists to discover the dinosaur-ending impact crater after evidence for a cosmic collision 65 million years ago first turned up in sedimentary layers around the world. Then again, there may be no crater, Kennett says. He cites Tunguska: In 1908, an object that scholars believe was a meteor or comet exploded high above the Tunguska River in Siberia, leveling trees over 800 square miles but leaving no crater.
Critics also take issue with the black-mat evidence. Haynes (overkill) argues that the mat's charcoal-rich layer could as likely be from human-caused fires as from comet-caused wildfires, while Grayson (overchill) questions the purported collapse of Clovis populations, for which he and many other archeologists see very little evidence.
Finally, there are the extinctions themselves. Of the 35 extinct genera, 20 or so cannot be shown to have survived up to the Younger Dryas. The youngest date, for example, for fossils of Eremotherium, a giant ground sloth, is 28,000 years ago. "So the idea that this impact could have caused the extinctions of all these animals just does not make sense," Grayson says. In response, Kennett points out that the fossil record is imperfect, and one would not expect to see the most recent occurrence of rare forms like Eremotherium to extend right up to the Younger Dryas, as the remains of more common animals like mammoths, horses, and camels do.
If there's one thing all scholars involved in this famously contentious debate would welcome it's more data. For in science, as Kennett put it to me, "data eventually rules." Grayson, for one, feels the field would benefit from a better understanding of just when each of those 20 rarer genera of big beasts went extinct. "Until we know when these extinctions occurred, I think we're wasting our time in trying to explain them," he says.
In the meantime, the dearth of widely convincing evidence only serves as a spur. MacPhee may be speaking for all researchers working on this mystery 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?
*Gary Haynes offered this sobriquet when I asked him if a playful term for the comet hypothesis had caught on yet.