Did bad luck kill the Neanderthals?

The last Neanderthals were snuffed out around 40,000 years ago. But what led to their demise – and whether our own ancestors had a hand in their downfall – is an enduring mystery.

A new study, published in the journal PLoS ONE, suggests that simple fluctuations in the make-up of the population – and a dose of bad luck – were probably enough to push Neanderthals over the edge.

Over the years, scientists have invoked everything from climate change to disease epidemics to explain Neanderthals’ extinction.

One leading theory, that the arrival of modern humans in Europe was responsible, remains popular and is fuelled by an outdated view of Neanderthals as hulking oafs.

“What we used to know or think we knew was that we were superior,” says Krist Vaesen from Eindhoven University of Technology in the Netherlands.

But evidence is stacking up showing that Neanderthals were every bit as sophisticated as our own forebears.

Another theory suggests that although our ancestors weren’t superior, they were more numerous, arriving in a constant trickle from Africa. 

In any case, none of these external explanations are necessary to explain the Neanderthals’ demise, according to Vaesen and his colleagues.

“One thing we know about the Neanderthal population is that it was very small,” he says. Estimates suggest there were only ever 5000 to 70,000 Neanderthals at any given time, spread across Eurasia in even smaller, isolated bands.  

That’s not unusual: most early human species had small population sizes. Modern humans are a notable outlier. But the population boom that saw Homo sapiens numbers skyrocket occurred around 15,000 years ago, long after Neanderthals met their end. 

Small populations can be particularly vulnerable to internal genetic and demographic factors. For instance, inbreeding – common among Neanderthals – imposes a fitness cost. Low population density, which can hinder attempts to find mates, or hunt and parent cooperatively, and random fluctuations in births, deaths and the balance of females to males can also have an outsized influence on small populations. 

Starting with populations of between 50 and 5000 individuals, Vaesen and colleagues ran computer simulations, adding in these effects to see how the hypothetical populations fared over a 10,000-year period.

Inbreeding, though harmful, was not enough on its own to result in extinction, except in the smallest population.

When low population density was factored in, leading to 25% or fewer females giving birth each year, populations starting with up to 1000 individuals were destined to fail.

Adding in changes to population make-up – factors that are down to pure chance – saw populations of all sizes go the way of the Dodo.

“What it strongly suggests is that we need to take into account demography,” says Vaesen, rather than looking just at external factors. “And many models don’t do that.”

Humans, Vaesen concedes, could have made things worse.

It’s possible, he says, that “the mere presence of Homo sapiens in Europe and the Near East just made it much more difficult for Neanderthals to migrate among subpopulations,” thwarting any attempts by subpopulations to occasionally meet and mate.

And luck – of the more favourable kind – could also have played a role in our own species prehistory, says Vaesen.