Inbreeding, Allee effects and stochasticity might be sufficient to account for Neanderthal extinction

Abstract

The replacement of Neanderthals by Anatomically Modern Humans has typically been attributed to environmental pressure or a superiority of modern humans with respect to competition for resources. Here we present two independent models that suggest that no such heatedly debated factors might be needed to account for the demise of Neanderthals. Starting from the observation that Neanderthal populations already were small before the arrival of modern humans, the models implement three factors that conservation biology identifies as critical for a small population’s persistence, namely inbreeding, Allee effects and stochasticity. Our results indicate that the disappearance of Neanderthals might have resided in the smallness of their population(s) alone: even if they had been identical to modern humans in their cognitive, social and cultural traits, and even in the absence of inter-specific competition, Neanderthals faced a considerable risk of extinction. Furthermore, we suggest that if modern humans contributed to the demise of Neanderthals, that contribution might have had nothing to do with resource competition, but rather with how the incoming populations geographically restructured the resident populations, in a way that reinforced Allee effects, and the effects of inbreeding and stochasticity.

Skin colour and vitamin D: An update

“…At that time, still ancestral Neanderthal hominins lived in Europe, whom had arrived from Africa at least some 400 000 years earlier. [ 50‐52 ] Through interbreeding with modern humans , the Neanderthalers were outnumbered [ 53 ] and disappeared a few thousand years after the arrival of the modern humans. [ 52 ] In net effect, the genome of today’s Europeans contains in average 2.3% Neanderthal DNA.…”

An emerging consensus in palaeoanthropology: demography was the main factor responsible for the disappearance of Neanderthals

“…A second category pertains to hypotheses that refer to the internal, demographic dynamics of Neanderthal populations. Even in the absence of competition with modern humans, Neanderthal populations might, generally, have been too small to persist in the long run 25 – 28 . More specifically, their small size and limited interconnectedness would have made them highly susceptible to inbreeding (viz., reduction in fitness of individuals that arise from matings between genetic relatives), Allee effects (reduction in population growth rates due to problems in mate-finding), and stochastic fluctuations (sudden drops in population size due to random fluctuations in births, deaths and sex ratio) 28 , 29 .…”

Triangulating Neanderthal cognition: A tale of not seeing the forest for the trees

“…A first point to be made lies in the assessment that anatomically modern humans' population size was possibly up to an order of magnitude greater than the Neanderthals' and also featured a greater population density in at least some geographic areas, as supported by a number of studies (Bocquet‐Appel & Degioanni, 2013; Castellano et al, 2014; Fabre et al, 2009; Mafessoni & Prüfer, 2017; Mellars & French, 2011; Prüfer et al, 2014; Vaesen, Scherjon, Hemerik, & Verpoorte, 2019). This situation can be made sense of for ecological reasons alone as climatic conditions in Europe were comparably harsh and included climate cycles that regularly thinned out and pushed back Neanderthal subpopulations into Eurasian refuges, before climate conditions allowed them to spread back into a larger Eurasian territory (e.g., Bradtmöller, Pastoors, Weninger, & Weniger, 2012; also reflected in changing lithic strategies as in for example, Shipton et al, 2013).…”

“…From such a fundamental demographic imbalance between Neanderthals and anatomically modern humans, further worsened by reoccurring demographic weaknesses within the Neanderthal population and a resulting decrease in Neanderthals' genetic fitness, completely neutral interpretations of demographic pressure, and migratory behavior on the side of anatomically modern humans are potentially sufficient to explain the Upper Paleolithic replacement (Degioanni, Bonenfant, Cabut, & Condemi, 2019; Goldfield, Booton, & Marston, 2018; Kolodny & Feldman, 2017; Mellars & French, 2011; Vaesen et al, 2019). Indeed, there is evidence that the dispersal, a.k.a.…”