Since their discovery 163 years ago, palaeontologists have been bickering about what killed off the Neanderthals. In 1920, Marcellin Boule – a French palaeontologist who helped popularise the now-debunked idea that Neanderthals were brutish and dim-witted – posited a theory that Homo sapiens violently replaced Neanderthals, and later theories expanded on this theme that, whatever happened, it was probably us humans who were responsible.
Although we know that Neanderthals died out 40,000 years, until now no one really knew for sure why it happened. Some say they were killed by pathogens carried by their neighbouring Homo sapiens. Others argue that our ancestors had a competitive advantage, so took all of their food and shelter, or that the Homo sapiens slaughtered them all. Some theories even say that Neanderthals died out because they interbred with Homo sapiens – after all, if your ancestors are from Europe or Asia then it’s likely that you could have up to 2.5 per cent Neanderthal DNA.
But it turns out that we might not be totally responsible for this ancient ecocide. According to new research in the journal PLOS One small populations, inbreeding and random demographic shifts likely spelled the end for these hominids.
Krist Vaesen from Eindhoven University of Technology and his colleagues discovered that rather than competition or disease being the cause of the Neanderthals’ demise, the answer might be much simpler. Perhaps because the population was already fairly small and they lived in isolated groups, that would have been sufficient for them to go extinct.
“We saw that they were really vulnerable to extinction and our model suggests that’s just by living in a smaller population,” says Vaesen. Since 2014, scientists have been developing a strong case that Neanderthals tended to conceive with their close relatives. Researchers from the Max Planck Institute for Evolutionary Anthropology analysed a genome sequence of a Neanderthal woman from Siberia and showed that her parents were half-siblings and mating among close relatives was common among her recent ancestors. Mating between people family members with very close DNA increases the chance of recessive genetic disorders and decreases evolutionary fitness, leaving the Neanderthals open to becoming extinct.
Vaesen’s team used a simulation to see whether the Neanderthals could have died out on their own. They took data from existing hunter-gatherer populations, and developed population models for Neanderthal groups of different sizes: 50, 100, 500, 1,000, or 5,000 individuals.
They then simulated the effects of inbreeding, finding that this alone caused extinction in the smallest model population. While inbreeding wasn’t enough to kill the bigger groups, they tried combining it with Allee effects – where a smaller population negatively impacts a species’ ability to survive. It has been shown in extant hunter-gatherer populations that it is common for Allee effects to impact reproduction. If only 25 per cent or fewer Neanderthal females gave birth per year, in populations of up to 1,000 individuals this would bring extinction within a 10,000-year period. When combined with inbreeding, it caused an extinction event for all model sizes.
But this study is unlikely to settle the debate on the Neanderthal’s demise forever. Vaesen believes that one of the reasons nobody can agree on an answer is because they become attached to the ones they believe first. There were other subspecies of hominids that also disappeared in the last 70,000 years – the Denisovans in the far east, the Homo floresiensis from Flores in Indonesia and Homo erectus. Because of this, Chris Stringer, research leader at London’s Natural History Museum agrees that inbreeding and Allee effects may have been a contributing factor, but that humans aren’t off the hook altogether.
“It certainly points a finger at modern humans as the common factor that can apply in all of those parts of the world where those different humans were living,” says Stringer. What this study does suggest is that even though Homo sapiens could have had an impact on their Neanderthal cousins, inbreeding and demographic shifts would have killed them off anyway.
“Many people think that if we understand Neanderthals, we also get a better sense of ourselves, who we are and what makes us special or not,” says Vaesen. His study suggest that it wasn’t necessarily Homo sapien’s superiority that meant we survived the evolutionary lottery and make it through to the present day.
Even today animals such as Indian tigers – which live in extremely isolated populations – run the risk of inbreeding themselves out of existence. Vaesen says humanity as a whole should be protected by its large population and growth rate, but we’re not entirely in the clear. Perhaps if there was an environmental event which reduces the current numbers, it might happen at this point,” he adds.
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