Ice Age Horses May Have Been Killed Off by Humans, Study Finds

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Previous research had suggested that Alaska's caballoid horse species became extinct some 500 years before the first humans arrived.

Those dates would mean that overhunting could not have contributed to the extinction of Alaska's ancient horses—though humans could have contributed to the demise of North American mammoths, which stayed on the scene for perhaps another thousand years.

But Andrew Solow, a geostatistician at Woods Hole Oceanographic Institution in Massachusetts, and colleagues have published a statistical evaluation of the fossil record that suggests that humans shouldn't be exonerated just yet.

Their data, to be published in tomorrow's issue of the journal Proceedings of the National Academy of Sciences, reveal that horses did disappear before mammoths, though only by perhaps 200 years.

Their findings also suggest that both species may not have gone extinct until after human arrival—so human hunting may well have played a role in their demise.

"You can't just take the latest fossil remains [and assign their date] as the time of extinction," Solow said. "There's a sampling issue.

"We constructed a confidence region—that's the set of dates that you can't rule out with confidence as the extinction times."

Those dates suggested the possibility that both caballoid horses and mammoths survived well past the generally accepted arrival dates for humans.

The results don't identify the cause of the extinctions, and experts say a fossilized "smoking gun" seems unlikely.

"Even if a fossil told you that [species] survived past the arrival of humans, it's still the case that there was climate change going on as well as hunting pressure," Solow said.

"I think the notion that there was a single cause is probably not right. It's probably more complicated than that."

The Smithsonian's Wing believes that the complicated circumstances leave paleobiologists and others with their work cut out for them to determine just why so many of the world's large animals vanished.

"I think that leaves everyone with a big job to do to investigate new sites, date remains, date human occupations, and try to do the best that they can," he said.

"It may take a long time to accumulate enough evidence. But this is the kind of thing that has to happen."

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