End of the Big Beasts
- By Peter Tyson
- Posted 03.01.15
- NOVA
Who or what killed off North America's mammoths and other megafauna 13,000 years ago?
It takes a certain kind of person to tackle this question in earnest.
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 giant ground sloth is just one of numerous large mammals that
vanished forever as the Ice Age wound down in North America. The
question is,
Why? Enlarge © Karen Carr [www.karencarr.com]
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"
*:
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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.
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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.)
No evidence has turned up that the giant short-faced bear, the
sabertooth cat, or any of the other extinct big beasts shown in this
painting (save for the mastodon) were ever hunted by early Americans.
Enlarge © Karen Carr [www.karencarr.com]
Overkill
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."
This map shows how catastrophic animal extinctions occurred around the
world not long after humans first arrived in a geographical region.
Numbers indicate percentages of extinct genera during the past 100,000
years.
Enlarge © Linda Huff
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 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."
If climate change was the culprit, why didn't animals like the
now-vanished dire wolf (shown here menacing a peccary) go extinct in any
of numerous switches from glacial to non-glacial conditions in the past
two million years?
Enlarge © Karen Carr [www.karencarr.com]
Overill
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."
Did a hyperdisease race through animals as diverse as armadillo-like
glyptodonts (here), mammoths, camels, and cheetahs? The jury is still
very much out.
Enlarge © Karen Carr [www.karencarr.com]
Overgrill
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.
Archeologist Vance Haynes, who first discovered the "black mat" in
this Arizona riverbed in the 1960s, points it out to geophysicist Allen
West.
Enlarge Courtesy Doug Hamilton
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.
Soldiering on
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?
