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Did inbreeding drive the Neanderthals to extinction?

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Small populations, inbreeding and random fluctuations in birth and death patterns may have created a perfect storm that wiped out the Neanderthals, international researchers say. The role humans played in the demise of the ancient hunter-gatherers has been long disputed, but most agree they disappeared about 40,000 years ago. Testing three population sizes, Dutch researchers simulated how these groups could change over a 10,000 year-period and found inbreeding alone could wipe out populations of 1000 or less, and then additional fluctuations in births, deaths and sex ratios could have helped to wipe out larger groups.

Journal/conference: PLOS One

DOI: 10.1371/journal.pone.0225117

Organisation/s: Eindhoven University of Technology, Netherlands

Funder: Research by KV was supported by the Netherlands Organisation for Scientific Research (NWO).

Media Release

From: PLOS

Peer-reviewed                             Modelling study                                        People

 

Inbreeding, small populations, and demographic fluctuations alone could have led to Neanderthal extinction

 

Neanderthal extinction could have occurred without environmental pressure or competition with modern humans

Small populations, inbreeding, and random demographic fluctuations could have been enough to cause Neanderthal extinction, according to a study published November 27, 2019 in the open-access journal PLOS ONE by Krist Vaesen from Eindhoven University of Technology, the Netherlands, and colleagues.

Paleoanthropologists agree that Neanderthals disappeared around 40,000 years ago—about the same time that anatomically modern humans began migrating into the Near East and Europe. However, the role modern humans played in Neanderthal extinction is disputed. In this study, the authors used population modelling to explore whether Neanderthal populations could have vanished without external factors such as competition from modern humans.

Using data from extant hunter-gatherer populations as parameters, the authors developed population models for simulated Neanderthal populations of various initial sizes (50, 100, 500, 1,000, or 5,000 individuals). 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, to see if these factors 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 (this only occurred in the smallest model population). However, reproduction-related Allee effects where 25 percent 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 within the 10,000 years allotted.

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. It’s 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.

However, by showing demographic issues alone could have led to Neanderthal extinction, the authors note these models may serve as a “null hypothesis” for future competing theories—including the impact of modern humans on Neanderthals.

The authors add: “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.”

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