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

Brian Handwerk
for National Geographic News
May 1, 2006
Some 12,000 years ago, North American mammoths, ancient horses, and many other large mammals vanished within the short span of perhaps 400 years.

Scientists cannot be sure what killed them, but a new study suggests that humans aren't off the hook just yet.

The large animals' disappearance at the end of the Pleistocene era (50,000 to 10,000 years ago) happened at about the same time that many large animals, or megafauna, went extinct around the globe.

Victims included species such as the saber-toothed cat and the diprotodon—a rhinolike beast that was the world's largest marsupial.

Now a new study of the fossil record fuels the debate about the cause of the creatures' fate.

In North America two major events occurred at about the same time as the megafaunal extinctions: The planet cooled, and early humans arrived from Asia to populate the continent.

For decades scientists have debated which of these factors was responsible for widespread megafaunal extinctions.

Was the climate change simply too much for the animals to withstand? Or did the ancient mammals succumb to human hunting pressure?

Many experts suggest a combination of these factors and perhaps others, such as disease.

"It's hard to see this as one of those things where a single piece of evidence will make it obvious what happened," said Scott Wing, a paleobiologist at the Smithsonian Institution's National Museum of Natural History.

"The phenomenon that people are trying to explain is not something that happened in one place at one time. It happened across the globe, at different times on different continents. I think that there are clearly multiple factors involved."

Humans Not Exonerated

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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