Did virus kill off mammoth?

Evidence of the first viral infection identified in an extinct animal has been recovered from the remains of three woolly mammoths.

The discovery marks the start of an attempt to solve one of the mysteries of prehistory - did a virus wipe out the giant mammals at the end of the last Ice Age?

Alex Greenwood, a researcher at the American Museum of Natural History in New York, told the Society of Vertebrate Paleontology meeting in Mexico yesterday that his team had isolated genetic material from an ERVL retrovirus, a distant relative of HIV, from fragments of mammoth DNA.

Using new techniques to analyse ancient DNA, proviral sequences were found in samples taken from mammoth remains ranging from 26,000 to 4,500 years old, recovered from mainland Siberia, Alaska and their final refuge, Wrangel Island, Russia.

Retroviruses inject messenger RNA into cells, convert it to DNA and incorporate it in the cell's genetic code. The cell is thus converted to a factory for more retroviruses.

If the infection targets reproductive cells, the viral DNA can be passed on to future generations. Up to 10 per cent of the human genome is believed to consist of viral DNA fragments, most of which have mutated into non-harmful forms.

Scholars have long debated the cause of the extinction of the megafauna of the Pleistocene epoch, including mammoths, sabretooth cats and giant ground sloths. Extinction is thought to have taken about 400 years across four continents.

Suggestions that post-Ice Age human hunters or climate change led to their demise have been challenged by Ross MacPhee's idea that a viral infection carried by humans or animals associated with them - such as domesticated dogs - was responsible.

The virus that Greenwood has identified is not responsible for the extinction, but he hopes to study more remains. This may produce evidence of invading viruses.

"We have a chance to gather evidence for the theory," said Greenwood.

But he admitted that the task was enormous and that he may well become extinct before finding the answer.