When climate took a turn toward the cold tens of thousands of years ago, both Neanderthals and early humans started traveling further distances to find food, found a new study.
As a result, the two groups encountered each other often.. And a consequent boom in inter-species liaisons eventually led to the extinction of Neanderthals.
While much of the theory remains controversial, the study adds to growing evidence that Neanderthals developed advanced cultures and that they adapted to changes in the environment just like early humans did.
The study also hints at what's to come if climate change forces modern cultures to blend, as their homes become inhospitable from drought, flooding or severe weather.
"We are increasingly finding evidence of sophisticated behavior among Neanderthals, and now the question is: If they were so smart, why did they become extinct?" said Michael Barton, an anthropologist at Arizona State University in Tempe.
"Our answer is that they became extinct because they were so smart, not in spite of it," he said. "They were doing what everyone else was doing, and how they dealt with worldwide environmental change made their population and probably other endemic populations disappear."
To see how ancient groups of people moved around as the climate changed, Barton and colleagues analyzed stone tools from 167 cave sites that spanned Eurasia from the Near East to Gibraltar. Fossils indicated whether Neanderthals, early humans or both had lived in each cave for some period of time between about 128,000 and 11,500 years ago.
As they scanned the tools, the major clue the researchers looked for was how worn down the stones were. That, in turn, hinted at how much moving around their owners did.
If people stayed put, for example, they tended to stockpile rocks and just chip off new flakes whenever they needed one. If they were moving around a lot, on the other hand, they carried tools with them, and they ended up sharpening and re-sharpening those tools over and over – ultimately leading to duller, more worn tools. In the fossil record, the difference between the two tool-making strategies is clear.
When the researchers analyzed the tools, they found that climate determined the migration patterns of both early humans and Neanderthals. When it was warm and resources were abundant but patchy, well-worn tools suggested that everyone moved around from place to place. They didn't roam very far, so they wouldn't have had a lot of contact with each other, but they didn’t stay in one place for a long time, either.
But when temperatures plummeted as a new glacial period approached, both Neanderthals and our human ancestors set up home bases where they could keep flaking off sharp new tools. From there, foraging parties covered much longer distances. As they roamed, the two groups intermingled more than they had ever had before
Because humans outnumbered Neanderthals, computer modeling over 1,500 generations showed that intermingling would have led to the eventual disappearance of a distinct Neanderthal group, whereas the smaller-scale movements seen in warmer times wouldn’t have had much of an effect at all.
Neanderthals didn't exactly go extinct, the researchers reported in the journal Human Ecology. Instead, Barton said, their genes were essentially swallowed up into the human genome.
Studies have shown similar patterns of extinction in endangered animal species, Barton said. And he pointed to recent evidence that remnants of Neanderthal DNA still persist in modern Europeans.
Neanderthals were certainly intelligent enough to adapt to climate change, said Richard Klein, an anthropologist at Stanford University in Palo Alto, Calif. But he is more skeptical about the conclusion that mating between Neanderthals and early humans was widespread enough to lead to Neanderthal extinction.
And studies like the new one, he said, are not sufficient to prove the theory true or false.
"There is a lot of argument about whether there was interbreeding," Klein said. "The ancient DNA turns out to be enormously incomplete. If there is an answer, that's where it is. If the Neanderthals became extinct without interbreeding, that’s where it is. If there was interbreeding, that's where it is."
In the meantime, Barton said, the new findings may serve as a warning call for what’s to come.
"Our hope is that this gives us some kind of idea of the kinds of things that happen when people respond to environmental change," he said. "One thing we’re seeing with large-scale globalization today is that there are local cultures that disappear. The same thing happened in the past."