New Theory on Old Debate: Comet Killed the Mammoth

"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."