It’s about 40,000 years since the Neanderthals disappeared – and it’s also about 40,000 years since modern humans began expanding out of Africa and into Europe and the Middle East.
Because those dates match up so well, it’s always been assumed that the latter caused the former – but a new study suggests the Neanderthals may have become extinct for reasons that were nothing to do with Homo sapiens’ arrival.
A team at Eindhoven University of Technology in the Netherlands produced computer models to simulate the effects of various factors on Neanderthal populations of different sizes (50, 100, 500, 1,000 or 5,000), based on data gleaned from studies of modern hunter-gatherer populations worldwide.
Factors modelled included inbreeding, fluctuations in birth and death rates, changes in the ratio of the sexes and ‘Allee effects’ – a phenomenon first identified in the 1950s whereby, in a shrinking population, the average health and fitness of each individual tends to decline over time.
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The researchers found that Allee effects alone could explain the extinction of any population numbering less than 1,000 individuals, while inbreeding plus Allee effects could easily account for the entire Neanderthal species’ decline over a 10,000-year period, without modern humans’ arrival having any effect at all.
“Did Neanderthals disappear because of us? No, this study suggests. The species’ demise might have been due merely to a stroke of bad demographic luck,” said the paper’s lead author Krist Vaesen.
The true picture, say the researchers, is probably a more complex amalgam of the two mechanisms. For instance, conflict with incoming humans may have caused an acceleration of Allee effects within the population, as well as simply reducing its size.
Reader Q&A: Did humans and Neanderthals interbreed?
Asked by: Benjamin Hatch, Leeds
Yes, and more than once!
DNA analysis suggests that the earliest encounter between the two species was 100,000 years ago, just as the earliest wave of Homo sapiens was migrating out of Africa. They met Neanderthals moving eastwards from Europe to Asia and swapped genes.
Later interbreeding periods happened 55,000 and 40,000 years ago, and each time we acquired some Neanderthal genes. Unless you are of sub-Saharan descent, your genome contains 1-4 per cent Neanderthal DNA.