Small populations, inbreeding, and random demographic changes could have been enough to cause Neanderthal extinction, according to a new study which suggests that humans alone are not to blame for wiping out our closest cousins.
Paleoanthropologists agree that Neanderthals disappeared around 40,000 years ago.
This is about the same time that anatomically modern humans began migrating into the Near East -- a transcontinental region roughly comprising Western Asia, Turkey and Egypt -- and Europe.
However, the role modern humans played in Neanderthal extinction is disputed, according to the researchers from Eindhoven University of Technology in the Netherlands.
The latest study, published in the journal PLOS ONE, used population modelling to explore whether Neanderthal populations could have vanished without external factors such as competition from modern humans.
Using data about existing hunter-gatherer populations as parameters, the researchers developed population models for simulated Neanderthal populations of various initial sizes -- 50,100, 500, 1,000, or 5,000 individuals.
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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.
The researchers analysed these factors to see if they could bring about an extinction event over a 10,000-year period.
The population models show that inbreeding alone was unlikely to have led to extinction, the researchers said.
However, Allee effects where 25 per cent or fewer Neanderthal females gave birth within a given year -- as is common in extant hunter-gatherers -- could have caused extinction in populations of up to 1,000 individuals.
In conjunction with demographic fluctuations, Allee effects plus inbreeding could have caused extinction across all population sizes modelled, the researchers said.
The 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 said.
The researchers noted that it is also possible that modern humans could have impacted Neanderthal populations in ways which reinforced inbreeding and Allee effects, but are not reflected in the models.
"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," the researchers explained.