Investigating a Mega-Mystery
When, at least 12,000 years ago, human beings first crossed into
North America from Siberia, the continent teemed with large animals.
Today, of course, our only encounters with giant short-faced bears,
enormous sloths and dozens of other such extinct species come in
museums. On this much, archaeologists and paleontologists agree. The
causes of this mass extinction, however, remain clouded by
conflicting findings and holes in the archaeological record.
The mystery extends far beyond North America. Between about 50,000
and 10,000 years ago, near the end of the Pleistocene, much of the
world's megafauna (usually defined as animals weighing at least 100
pounds) disappeared. At the same time, Homo sapiens was
expanding from Africa into Eurasia, Australia and the Americas. The
late Pleistocene also witnessed dramatic climate change, especially
during the period of warming and deglaciation that followed the Last
Glacial Maximum some 20,000 years ago.
This convergence of events makes for exciting—and sometimes
contentious—science. High-impact human hunting, referred to by
archaeologists as "overkill," and climate change are the
two most cited possible causes of the extinctions, but the role of
each remains contested.
The debate began to heat up in the late 1960s after Paul S. Martin,
a professor of geosciences at the University of Arizona, first
proposed a "blitzkrieg" model of human overkill for North
America—basically, overkill on fast forward. In this scenario,
humans moved rapidly through the continent, slaughtering mammoths,
mastodons and other large prey as they went. Within about 1,000
years, most North American endemic megafauna were gone.
The blitzkrieg hypothesis has since been applied elsewhere, but it
remains controversial. Criticism has focused on the lack of
archaeological evidence, a charge Martin has responded to by arguing
that, if the extinctions occurred quickly, there would be little
trace of the massacre in the fossil record. Archaeologists Donald
Grayson of the University of Washington and David Meltzer of
Southern Methodist University have been particularly critical of
Martin's response, calling it "faith-based" science.
Two recent papers, both published in Proceedings of the National
Academy of Sciences of the U.S.A., try to help settle the
question. Todd Surovell and Nicole Waguespack of the University of
Wyoming and P. Jeffrey Brantingham of the University of California,
Los Angeles, studied the timing and location of Pleistocene
encounters between humans and proboscideans (the order that includes
mammoths, mastodons and elephants) and found evidence supporting the
overkill hypothesis. Meanwhile, Clive N. G. Trueman of the
University of Portsmouth and Judith H. Field of the University of
Sydney were part of a multinational team that confirmed the age of
megafauna fossils at a site in eastern Australia, concluding that
their work weakens claims for overkill in the land Down Under.
Surovell, Waguespack and Brantingham outlined two possible
extinction scenarios, one based on human overkill and the other on
climate change. They then plugged into their models data from 41
archaeological sites in Africa, Europe, Asia and the Americas that
contain remains of proboscideans hunted or scavenged by humans. If
people hunted these animals to extinction, the authors argue, the
kill sites should appear along the border between proboscidean and
human ranges. So, as humans expanded south across North America, for
example, the sites would also be located farther and farther south.
If climate was the culprit, then people and proboscideans should
have shared some of the same territory, at least until climate
change shrunk proboscidean habitat. Thus, kill sites would be found
both along and behind the frontier of human expansion.
The authors concluded that the location and age of the sites
correlate closely with an overkill model. As humans moved north into
Eurasia from Africa and, later, south from Alaska across the
Americas, proboscidean range contracted correspondingly. Climate
change, then, cannot account for proboscidean extinction
"unless one were to invoke serial climatic change that
perfectly tracks human global colonization." The odds, they're
saying, aren't good.
Although the authors do not claim to have proved that humans drove
other species to extinction, Surovell is skeptical of arguments for
climate change. "I would like to see somebody explain how
climate change could cause mass extinction on such a large
geographical scale," he says. "Climate is constantly changing."
In Australia, much of the evidence for overkill relies on proving
that many large animals became extinct within several millennia of
the first appearance of humans, usually estimated at about 50,000
years ago. Unlike other parts of the world, nothing in Australia's
fossil record proves that humans hunted megafauna. As Trueman and
Field note in their paper, there aren't even any sites with evidence
that early inhabitants had the tools to kill large animals.
Trueman and Field discuss the dates of a controversial
archaeological site, Cuddie Springs, that might prove that at least
some Australian megafauna survived much longer than previously
thought, dealing a blow to arguments for overkill. The site includes
remains of several extinct animals, including Diprotodon, a
two-ton marsupial, and Genyornis, a large, flightless bird.
Previous efforts, made using radiocarbon dating and other methods,
have concluded that some megafauna remains found there are 36,000 to
30,000 years old, but the findings have been disputed. Trueman and
Field used a newer technique in their recent work, an analysis of
rare earth elements (REEs) in bone fragments, and confirmed these
dates. As they're buried, bones adsorb REEs, leaving a
"fingerprint" that links the bones to their original layer
Proving that people coexisted with large animals for 10,000 years or
more would not necessarily remove humans from the extinction
equation, but it would make it more likely that other factors, such
as climate, also played a key part. Field, for one, is convinced
that the findings at Cuddie Springs disprove the possibility of
blitzkrieg in Australia and cast doubt on the overkill hypothesis.
It's about time, she says, "to start entertaining other ideas
about the extinction process."
Not everyone is convinced. In a 2001 paper published in
Science, Richard G. Roberts of the University of Wollongong
and a team of investigators found evidence of widespread Australian
megafauna extinctions by about 46,000 years ago, concluding that
humans must have played an important role. Roberts says that he
still has "some strong reservations" about the recent
paper. He notes that REEs are usually used to date much older bones,
for which an error of thousands of years one way or the other would
be insignificant. Although he himself is not entirely persuaded by
blitzkrieg, he does think that it remains a possibility.
If Pleistocene humans hunted some large animals to extinction but
blitzkrieg is ruled out as a possibility in Australia, as the recent
findings suggest, the search for an overarching theory may be
futile. In a review of recent research on the extinctions, published
in the October 1, 2004, issue of Science, coauthors Anthony
D. Barnosky, Paul L. Koch, Robert S. Feranec, Scott L. Wing and Alan
B. Shabel argued that it will be more productive to look for
localized, species-by-species explanations than a single cause. Some
combination of climate change and human activity, they think,
probably determined the fate of much of the world's megafauna.
Implicating multiple factors might not be as satisfying as
convicting a single perpetrator, but it may better explain the
evidence at hand. And as the Science authors point out, the
combination of climate change and human action can have a much
greater effect on the world's animal species than either factor
alone. There's no debate that both are today affecting the viability
of the remaining megafauna.—Amos Esty