Neanderthal Extinction Caused by Inbreeding and Smallness of Their Populations, Study Suggests

Nov 28, 2019 by News Staff

A long-standing enigma in paleoanthropology is the demise of Neanderthals about 40,000 years ago. There is general agreement that their disappearance coincides with migration events starting 60,000 years ago by anatomically modern humans from Africa into the Near East and Europe. What is uncertain however, are the causes of Neanderthal extinction. New research suggests Neanderthals disappeared due to inbreeding, small populations and random demographic fluctuations.

Neanderthals are commonly thought to have relied on dangerous close range hunting techniques, using non-projectile weapons like the thrusting spears depicted here; this hunting approach is thought to have brought them into violent contact with large mammals. Image credit: Gleiver Prieto.

Neanderthals are commonly thought to have relied on dangerous close range hunting techniques, using non-projectile weapons like the thrusting spears depicted here; this hunting approach is thought to have brought them into violent contact with large mammals. Image credit: Gleiver Prieto.

In the study, Dr. Krist Vaesen from Eindhoven University of Technology and the University of Leiden and his colleagues from the Netherlands used population modeling to explore whether Neanderthal populations could have vanished without external factors such as competition from anatomically modern humans.

Using data from extant hunter-gatherer populations as parameters, the scientists 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% 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, these models may serve as a ‘null hypothesis’ for future competing theories, including the impact of modern humans on Neanderthals,” Dr. Vaesen and co-authors said.

“Did Neanderthals disappear because of us? No, our study suggests. The species’ demise might have been due merely to a stroke of bad, demographic luck.”

A paper describing the research was published in the journal PLoS ONE.

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K. Vaesen et al. 2019. Inbreeding, Allee effects and stochasticity might be sufficient to account for Neanderthal extinction. PLoS ONE 14 (11): e0225117; doi: 10.1371/journal.pone.0225117

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