Inbreeding and small populations ‘could have led to Neanderthal extinction’

Inbreeding and small populations could have led to the extinction of Neanderthals, research suggests.

According to a new study, Neanderthal extinction could have occurred without environmental pressure or competition with modern humans.

Neanderthals disappeared around 40,000 years ago, around the same time anatomically modern humans started migrating to the Near East and Europe.

The authors said: “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.”

It has long been thought the replacement of Neanderthals by modern humans was due to environmental pressure or a superiority of modern humans with respect to competition for resources.

But a study published in the PLOS One journal suggests these factors may not be entirely responsible for the demise of the ancient human relatives.

Using data from extant hunter-gatherer populations as parameters, scientists developed population models for simulated Neanderthal populations of various initial sizes.

They then simulated for their model populations the effects of inbreeding, Allee effects – where reduced population size negatively impacts individuals’ fitness, and annual random demographic fluctuations in births, deaths, and the sex ratio.

They did this to see if these factors could bring about an extinction event over a 10,000-year period.

According to the findings, inbreeding alone was unlikely to have led to extinction.

However, reproduction-related Allee effects where 25% or fewer Neanderthal females gave birth within a given year, could have caused extinction in populations of up to 1,000 individuals.

Researchers suggest that together with demographic changes, Allee effects plus inbreeding could have caused extinction within the 10,000 years allotted in the population models.

But the scientists acknowledge their population models are limited by their parameters, which are based on modern human hunter-gatherers and exclude the impact of the Allee effect on survival rates.

They add it is possible modern humans could have impacted Neanderthal populations in ways that reinforced inbreeding and Allee effects – something which is not reflected in the models.

However, by showing demographic issues alone could have led to Neanderthal extinction, the researchers note these models may serve as a “null hypothesis” for future competing theories.

Krist Vaesen from Eindhoven University of Technology, the Netherlands, and colleagues, conclude: “Regardless of whether external factors (climate or epidemics) or factors related to resource competition played a role in the actual demise of Neanderthals, our study suggests that any plausible explanation of the demise also needs to incorporate demographic factors as key variables.”