Computer Simulation: Neanderthal replacement by migration & random species drift

NEW YORK (AP) — What killed off the Neanderthals? It's a big debate, and now a study says that no matter what the answer, they were doomed anyway.

Our close evolutionary cousins enjoyed a long run in Europe and Asia, but they disappeared about 40,000 years ago after modern humans showed up from Africa.

The search for an explanation has produced many theories including climate change, epidemics, or inability to compete with the modern humans, who may have had some mental or cultural edge.

The new study isn't intended to argue against those factors, but just to show that they're not needed to explain the extinction, says Oren Kolodny of Stanford University.

He and colleague Marcus Feldman present their approach in a paper released Tuesday by the journal Nature Communications.

They based their conclusion on a computer simulation that represented small bands of Neanderthals and modern humans in Europe and Asia. These local populations were randomly chosen to go extinct, and then be replaced by another randomly chosen population, with no regard for whether it represented the same species.

Neither species was assumed to have any inherent advantage, but there was one crucial difference: Unlike the Neanderthals, the modern humans were supplemented by reinforcements coming in from Africa. It wasn't a huge wave, but rather "a tiny, tiny trickle of small bands," Kolodny said.

Still, that was enough to tip the balance against the Neanderthals. They generally went extinct when the simulation was run more than a million times under a variety of assumptions.

If survival was a game of chance, "it was rigged by the fact that there's recurring migration," Kolodny said. "The game was doomed to end with the Neanderthals losing."

Kolodny said the evidence that such migrations actually occurred is suggestive rather than conclusive. Such migrations would not be expected to leave much of an archaeological trace, he said.

Experts in human origins said the paper could help scientists pin down the various factors that led to the Neanderthals' demise. It fits in with other recent attempts to explain the extinction without assuming behavioral differences between Neanderthals and our ancestors, said Wil Roebroeks of the University of Leiden in the Netherlands. The notion of such differences is largely disproven, he said.

Katerina Harvati of the University of Tuebingen in Germany said while the new work could be useful in solving the extinction mystery, it doesn't address the question of why modern humans dispersed from Africa into Europe and Asia. It's important to figure out what was behind that, she said in an email.

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https://www.nature.com/articles/s41467-017-01043-z

Most hypotheses in the heated debate about the Neanderthals’ replacement by modern humans highlight the role of environmental pressures or attribute the Neanderthals’ demise to competition with modern humans, who occupied the same ecological niche. The latter assume that modern humans benefited from some selective advantage over Neanderthals, which led to the their extinction. Here we show that a scenario of migration and selectively neutral species drift predicts the Neanderthals’ replacement. Our model offers a parsimonious alternative to those that invoke external factors or selective advantage, and represents a null hypothesis for assessing such alternatives. For a wide range of parameters, this hypothesis cannot be rejected. Moreover, we suggest that although selection and environmental factors may or may not have played a role in the inter-species dynamics of Neanderthals and modern humans, the eventual replacement of the Neanderthals was determined by the repeated migration of modern humans from Africa into Eurasia.

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