Study finds dual causes for mass extinction in Ice Age

— -- Ice Age beasts likely fell victim to a changing climate and for some, prehistoric hunters, researchers concluded in a study published Wednesday.

Woolly mammoths and woolly rhinos disappeared more than 6,000 years ago, along with a third of large mammal species in Asia and more than two-thirds in North America. Scholars have long debated whether "overkill" by prehistoric hunters or "overchill" by a shifting climate was the culprit in this mass extinction.

"Blaming one or the other has always been somewhat of a simplistic hypothesis," says genetics expert Thomas Gilbert of Denmark's University of Copenhagen, a co-author on a Nature journal analysis that concluded the "megafaunal" extinction started at the end of the last severe Ice Age. Ultimately, a changing climate combined with the sudden appearance of human hunters likely led to the species' declines, and in some cases extinctions, he says.

Mammoths and other species, such as cave lions, disappeared around the time that people invaded North America in significant numbers 14,000 years ago. That coincidence led to the conclusion that prehistoric hunters caused the extinctions, rather than a shifting environment.

Seeking to answer the question, researchers studied 846 fossil DNA samples, nearly 3,000 fossil bones and 6,300 archaeological-site records tracing populations of woolly mammoths, woolly rhinos and four species still around — wild horses, bison, reindeer and musk ox — over the past 50,000 years.

Overall, the study finds populations of these species ebbed and flowed with warm and cold periods. About 20,000 years ago, human hunters seem to have driven a decline in genetic diversity among horses, reindeer and bison.

Researchers also conclude prehistoric people didn't often hunt musk ox or woolly rhinos, pointing to climate changes as the culprit behind the species' respective decline and extinction. Woolly mammoths appear to have been hunted out of their southern ranges in Siberia, but not enough to directly drive their extinction.

"People come along and they look like just one more, last added stress on these species," says paleobiologist Anthony Barnosky of the University of California-Berkeley, who was not part of the study. "What the study shows…is that extinctions don't happen gradually, they reach 'tipping points' and then species are gone."

The "jury is still out" on whether early Native Americans killed off the woolly mammoth in North America, says study co-author Beth Shapiro of Penn State. "We're showing that animals respond differently to changing climate, and perhaps that's not so surprising, especially when people change the landscape on them."