Neanderthal Extinction Down to Inbreeding and "a Stroke of Bad Luck," say Scientists

We may be overestimating the role our ancestors played in the demise of the Neanderthals, say researchers writing in Plos One. Instead, inbreeding and a "stroke of bad luck" could be to blame.

Computer simulations suggest Neanderthals were already on the brink of extinction and had been for hundreds of thousands of years when—due to one unfortunate event or another—they were pushed over the edge.

Small populations and inbreeding may have primed the species for extinction so that random demographic fluctuations was all that was needed to wipe them out for good. This means that while their extinction may have taken place 40,000 years ago, around the same time anatomically modern humans started their migrations to Europe and the Near East, the two events might not be all that interlinked.

"We want us to play a role in their extinction, because it helps us to answer the question as to what makes us unique or human," co-author Krist Vaesen, Associate Professor in the Philosophy of Innovation at Eindhoven University of Technology in the Netherlands, told Newsweek.

"So the standard story, according to which modern humans outsmarted Neanderthals, is attractive to us because it implies that our species can be distinguished from our sister species by reference to our superior intelligence."

Neanderthal Man
A moulding of a Neanderthal man. New research suggests inbreeding and demographic fluctuations were responsible for Neanderthals' demise. STEPHANE DE SAKUTIN/AFP/Getty

Vaesen and colleagues came to the conclusion that no external factors were needed after running population models of different sizes, ranging from 50 to 5,000 Neanderthals. The researchers applied three factors to the models to find out how they affected population rates over a 10,000-year period.

These included inbreeding and Allee effects, a phenomenon whereby the size of a population has an effect on the health of individuals in said population. In this case, a smaller population would have a negative impact on individual health.

The third factor was stochasticity, or random demographic fluctuations related to birth rates, death rate and sex ratio.

While inbreeding in and of itself was not enough to cause annihilation, the researchers found reproduction-related Allee effects could have caused extinctions in populations of 1,000 individuals if fewer than one in four females gave birth in any given year. When all three factors combined, extinctions could have taken place in populations of all sizes studied.

"If Neanderthals lived in small populations since 400,000 years, why did it take so long for them to become extinct?" wrote the study's authors. "Our results are consistent with a scenario in which a small population of Neanderthals persists for several thousands of years, and then, due to a stroke of bad luck, disappears."

"Did Neanderthals disappear because of us? No, this study suggests," they explained.

However, Homo sapiens don't get away scot-free. The study's authors say they may have played a role in the extinction of Neanderthals—but this was less likely to be due to competition between species and more likely to be the way incoming populations of modern humans reshaped existing populations of Neanderthals.

They hypothesize that the intrusion of Homo sapiens may have made it harder for Neanderthals to migrate and breed with other populations, thus exacerbating existing problems such as inbreeding and Allee effects.

Vaesen said: "Our models suggest that the Neanderthal population was always vulnerable to extinction—and the same is true of the population of modern humans during much of its existence."

Chris Stinger, an expert on human origins at the Natural History Museum in London, U.K., concurs, saying that Neanderthals were already an "endangered" species before the arrival of modern humans.

However, he emphasizes that the need to take a wider view of the extinction question—for example by looking at the climate, which was "extremely unstable" at that time and likely played a role in removing a level playing field.

Part of the problem that researchers face is the fact that we don't know where the last Neanderthals lived—or even the precise moment in time they died out.

"We really need to know the date of their disappearance across their whole range to examine all the factors that may be involved," Stringer told Newsweek.

"And there's an even wider question to explore," he added. "It wasn't just the Neanderthals who went extinct in the last 60,000 years, as the same fate befell the Denisovans in eastern Asia, Homo floresiensis on Flores, Homo luzonensis in the Philippines and perhaps also Homo erectus in Indonesia."

"Once we factor those events in as well, it's difficult to exclude economic competition from the coincident spread of Homo sapiens as a significant driver in these other human extinctions."