Did climate change or humans cause the extinctions of the large-bodied Ice Age mammals (commonly called megafauna) such as the woolly rhinoceros and woolly mammoth?

Scientists have for years debated the reasons behind the Ice Age mass extinctions, which caused the loss of a third of the large mammals in Eurasia and two thirds of the large mammals in North America, and now, an inter-disciplinary team from more than 40 universities around the world led by Professor Eske Willerslev and his group from the Centre for GeoGenetics, University of Copenhagen, have tried to answer the contentious question in one of the biggest studies of its kind ever.

The study by the team, which includes two Texas A and M University professors, was published online in the journal Nature and reveals dramatically different responses of Ice Age species to climate change and human impact.

Using ancient DNA, species distribution models and the human fossil record, the findings indicate that neither climate nor humans alone can account for the Ice Age mass extinctions.

"Our findings put a final end to the single-cause theories of these extinctions," says Willserslev.

"Our data suggest care should be taken in making generalizations regarding past and present species extinctions; the relative impacts of climate change and human encroachment on species extinctions really depend on which species we're looking at."

The study reports that climate alone caused extinctions of woolly rhinoceros and musk ox in Eurasia, but a combination of climate and humans played a part in the loss of bison in Siberia and wild horse.

While the reindeer remain relatively unaffected by any of these factors, the reasons causes of the extinction of the mammoth remain unresolved.

The study also reports that climate change has been intrinsically linked with major population size changes over the past 50,000 years, supporting the view that populations of many species will decline in the future owing to climate change and habitat loss.

Finally, the authors find no clear pattern in their data distinguishing species that went extinct from species that survived.

Eline Lorenzen, professor at the University of Copenhagen and lead author of the study, said, "The fact that we couldn't pinpoint what patterns characterize extinct species - despite the large and varying amount of data analyzed - suggests that it will be challenging for experts to predict how existing mammals will respond to future global climate change. Which species will go extinct and which will survive?

"The bottom line is that we really don't know why some of these ancient species became extinct," adds Ted Goebel, researcher in the Department of Anthropology at Texas A and M and affiliated with the Center for the Study of First Americans.

"Now we can better predict what might happen to animals in the future as climate change occurs. What happens to species when their ranges are significantly diminished, and why do some animals adapt successfully while others become extinct?

"We now have a genetic roadmap to follow in our efforts to protect sensitive animal populations - especially in drastically impacted regions like the Arctic."