Ancient DNA Traces The Wooly Mammoth's Disappearance

Some ancient-DNA evidence has offered new clues to a very cold case: the disappearance of the last woolly mammoths, one of the most iconic of all Ice Age giants, according to a June 7th report published online in Current Biology.

DNA lifted from the bones, teeth, and tusks of the extinct mammoths revealed a “genetic signature” of a range expansion after the last interglacial period. After the mammoths’ migration, the population apparently leveled off, and one of two lineages died out.

“In combination with the results on other species, a picture is emerging of extinction not as a sudden event at the end of the last ice age, but as a piecemeal process over tens of thousands of years involving progressive loss of genetic diversity,” said Dr. Ian Barnes, of Royal Holloway, University of London. “For the mammoth, this seems much more likely to have been driven by environmental rather than human causes, even if humans might have been responsible for killing off the small, terminal populations that were left.”

Barnes, along with Dr. Adrian Lister of the University College London and the Natural History Museum in London and others, had earlier found evidence that bison, bears, and lions underwent major population shifts twenty-five to fifty thousand years ago. Those results came as a surprise, the researchers said, because scientists tended to think that the major environmental changes happened about fifteen to twenty-five thousand years ago, when the glaciers reached their fullest extent. The findings also offered early human hunters a potential alibi; they didn’t come on the scene in large numbers until even later.

In search of a general pattern in the new study, Barnes and Lister’s team looked to the extinct woolly mammoth. What they found, however, was an “interesting pattern, not like those of the other species.”

Their genetic data indicate that Siberian mammoths expanded from a small base some time before sixty thousand years ago. Moreover, they found two distinct genetic groups, implying that mammoths had diverged in isolation for some time before merging back into a single population. The DNA further suggests that no later than forty thousand years ago, one of the groups died out, leaving only the second alive at the time of the mammoth’s last gasp.

“At a time when we should be very concerned about the potential extinction of many existing large mammals, studying those that occurred in the geologically recent past can provide many insights,” Lister said. “Our work, together with that of others, shows that the conditions for extinction can be set up long before the actual extinction event.”

(I'll put it here)

Monday 17 December 2007

How forests wiped out woolly mammoths

Last Updated: 12:01am GMT 16/12/2007

Woolly mammoths were among the biggest mammals to have walked the earth, but it appears they were driven into extinction by nothing more dangerous than trees.

A leading expert on the ice age will claim this week that, rather than being wiped out by human hunters, the giant creatures were doomed by the spread of forests around the world at the end of the last ice age 10,000 years ago.

Professor Adrian Lister, a palaeobiologist at University College London, has found that the extensive areas of frozen grassland on which mammoths thrived were gradually replaced by forests, leaving the animals nothing to eat.

Analysis on the DNA extracted from hundreds of fossils has revealed that the genetic differences between individual mammoths were so slight that the animals were unable to adapt to the changes in their environment.

It contrasts with previous theories that humans hunted the woolly beasts into extinction or that rising temperatures left them unable to cope. Prof Lister will present his findings at the annual meeting of the Palaeontological Association in Sweden on Monday.

He will reveal a detailed picture of how mammoths first evolved about seven million years ago in tropical Africa, migrated north and adapted to the cold that allowed them to thrive in the ice age before dying out.

“In the middle of the last ice age, around 30,000 years ago, there were millions of mammoths roaming over a huge area,” said Prof Lister. “Around 20,000 years later there were hardly any left.

“As the forests moved in, the mammoths were pushed out of their normal habitat. These animals are mostly governed by vegetation rather than climate and so they were squeezed into very small populations as the forests took over the cold grasslands.

“I don’t think that people played a major role in wiping them out, although they may have pushed those final populations over the edge. The major impact factor was the change in the vegetation from grassland to trees.”

Prof Lister, with colleagues at the Natural History Museum and the Royal Holloway in London, analysed hundreds of mammoth fossils, extracting and comparing DNA sequences.

He said: “The DNA we have been able to extract from mammoth bones is like a clock and allows us to trace the evolutionary story in great detail now.”

American research revealed last week that mammoths and other great beasts from the last ice age were blasted by meteorites. Tusks from Alaska, 35,000 years old, were found to have been peppered with fragments.

Richard Firestone, of the Lawrence Berkeley National Laboratory, believes that this impact, along with another 13,000 years ago, caused big mammals such as the mammoth to disappear.

He said: “These large animals were particularly exposed to the shockwave produced by a large impact and had few places of refuge where they might have survived.

“There are probably strong parallels to the demise of the dinosaurs.”

Woolly giants: grazers of the frozen grassland

Weighed eight tons and reached 11ft tall.

Tusks of up to 11ft long were used to clear snow.

Ate more than 300lb of grass a day.

Four inches of fat, and hair up to 3ft long insulated them from the cold.

Closest living relative is the Asian elephant.

Perfectly preserved baby mammoth was found in Siberia last year.

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