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New Theory on Old Debate: Comet Killed the Mammoth

By Christopher Lee
Washington Post Staff Writer
Monday, June 11, 2007; A06

There are intriguing new clues in the mystery of how the woolly mammoth met
its demise in North America more than 10,000 years ago.

For decades, scientists have debated whether the giant, elephant-like
beasts were driven to extinction by the arrival of overzealous human
hunters or by global warming at the end of the Pleistocene era, the last
great Ice Age. Some say it was a combination of the two.

Recently, a group of more than two dozen scientists offered a new
explanation. They have found signs that a comet -- or multiple fragments of
one -- exploded over Canada about 12,900 years ago with the force
equivalent to millions of nuclear weapons. That unleashed, they said, a
tremendous shock wave that destroyed much of what was in its path and
ignited wildfires across North America.

Another group, with the help of DNA evidence extracted from mammoth bones,
teeth and ivory, has for the first time identified two distinct genetic
groups among mammoths. They found that one group had died out by 40,000
years ago for unknown reasons, leaving the second to continue until the
species went extinct.

The comet blast and firestorm could have dealt that death blow to the
mammoth and more than 15 other species of large mammals, or "mega fauna,"
including the mastodon, the saber-toothed cat, the American camel and the
giant ground sloth, the other researchers said. They presented their
findings last month at a meeting of the American Geophysical Union in
Acapulco.

"The shock wave would have spread across the whole continent," said Richard
B. Firestone, a nuclear chemist at Lawrence Berkeley National Laboratory in
California who helped do the research. "This event was large enough to
directly kill most everything instantly. Those that survived would have
found their food sources devastated, their water polluted, all kinds of
things that would have made it difficult to go on much longer."

In more than 20 locations from Arizona to Canada and California to the
Carolinas, the scientists found glass-like carbon, microscopic diamonds,
enriched iridium and other materials that they say are indicative of an
extraterrestrial impact lying in a sediment layer corresponding to the time
period. Just above that layer they found charcoal soot, decayed plant life
and other debris consistent with widespread burning.

Above that, the remarkable thing is what they did not find: further
evidence of the mammoth and the other large animals.

"The mammoths come up to the line and not beyond it," said James Kennett, a
marine geologist and professor emeritus at the University of California at
Santa Barbara. "At some sites, the black layer with impact material shrouds
the bones."

The explosion may also have spelled the end for the Clovis culture, the
prehistoric North Americans who hunted with distinctive stone spearheads
that have been found in the bones of the fossils of mammoths and other
animals, researchers said. While humans as a species survived the
cataclysm, the Clovis culture and its relatively advanced stone tools did
not endure.

"At many Clovis sites, like in Arizona and New Mexico, you get the Clovis
tools up to the impact layer, and then they never go beyond it," Kennett
said.

The comet theory, while adding a new twist to the tale, is not wholly
incompatible with earlier explanations for how the mammoth met its end.
Researchers said it is possible that the Ice Age beasts, which stood about
9 feet tall and weighed three tons, struggled as the climate warmed, that
increasingly skilled and numerous human hunters dramatically thinned their
numbers, and that the exploding comet finished them off.

"Our theory is that if this event had not happened, that mammoths would
still most likely -- not certainly, but most likely -- be wandering around
North America now," said Allen West, a retired geophysicist who is a leader
of the research team. "Almost certainly, humans hunting animals can have a
major effect on populations. It seems like there was, in a sense, a perfect
storm going on -- of overkill, the comet, climate change, possibly disease.
I don't think this theory negates any of the other theories. It's just one
more of a mix of things that were absolutely lethal to these animals."

The scientists have not published their findings, although two papers are
under review by the National Academy of Sciences, Kennett said. Firestone
said the lack of a distinctive impact crater -- the airborne explosion did
not leave one -- has generated controversy. Even some who accept that the
explosion occurred question whether it was the definitive blow, he said.

The second group of researchers, in a study published last week in the
journal Current Biology, analyzed mitochondrial DNA from 41 mammoths from
Europe, Asia and North America. Radiocarbon dating found the oldest of the
mammoths lived about 50,000 years ago, and the most recent specimens were
from about 12,000 years ago.

Scientists found two distinct genetic groups among mammoths in northeast
Siberia, indicating that the animals probably had existed in isolation
during a warm phase thousands of years earlier and had come together when
the glaciers -- and their habitat -- expanded again. Why one group died out
they do not know, but the loss of genetic diversity theoretically leaves a
species more vulnerable because the remaining population may be less able
to adapt to changing conditions, the researchers said.

They will discuss their research this week at the Fourth International
Mammoth Conference in Yakutsk, Russia.

"In terms of understanding the process of extinction, we've learned
something -- that it's not something that just happens in a flash
everywhere and they're all gone," said Adrian Lister, a professor of
paleobiology at the Natural History Museum in London. "It seems to have
been a progressive reduction in the genetic diversity of the species over
tens of thousands of years."

Colleague Ian Barnes, a senior lecturer in biological sciences at Royal
Holloway, part of the University of London, said it is likely that there
was "a slow grinding down" of genetic diversity.

"What we're now starting to think is . . . there is no single event that
causes their extinction," Barnes said. "What's important is that we seem to
have the conditions for extinction set up a long time before the actual
extinction occurs."