Solving the Neanderthal extinction

The replacement of Neanderthal by modern man is typically attributed to environmental pressure or superiority of modern humans with respect to competition for resources. According to a new study small populations, inbreeding, and random demographic fluctuations could have been enough to cause Neanderthal extinction.
​The study was conducted by Krist Vaesen et. al from the Eindhoven University of Technology, which is published in the open-access journal PLOS ONE. One of the biggest conundrums of palaeoanthropology is the demise of Neanderthals approximately 40000 years ago.
According to a general agreement the extinction event occurred after a long period of largely separated coexistence coinciding with migration events starting around ~60 000 years ago by Anatomically Modern Humans (AMHs) from Africa into the Near East and Europe following which AMHs took over the territories previously occupied by our sister species. However, the causes of Neanderthal extinction has been attributed to a wide variety of intensely debated factors, including climatic change, epidemics, a disputed theory is of supposed "superiority" of AMHs over Neanderthals in competing for the same resources. 
The investigators argue that no such contested factors might be needed to account for the demise of Neanderthals. They present two independent models that capture the internal dynamics of Neanderthal populations―the models thus ignore, among other things, competitive interactions with AMHs―and that suggest that the disappearance of Neanderthals might have resided in the small size of their population(s) alone. Accordingly, the study substantiates the suggestion, that it may simply be the case that Neanderthal populations declined below their minimum viable population threshold.
The study models of the investigation presented three basic factors that, according to conservation biology, would put such small populations at risk of extinction: inbreeding, Allee effects, and stochasticity.
Inbreeding depression refers to the reduction in fitness of individuals that arise from matings between genetic relatives, matings thus that are more likely to occur in small populations. Inbreeding, which seems to have been common in Neanderthals might lead to a lower fitness because it increases the chances of the expression of recessive, deleterious traits and because homozygotes often have a general disadvantage relative to heterozygotes. Harris and Nielsen estimate that, due to inbreeding, Neanderthals had at least 40% lower fitness than modern humans on average. Allee effects refer to the effects that population density has on reproduction and, thus, on population growth. At lower densities, growth rates might drop due to problems in mate-finding, and to several problems that highly cooperative species, such as Neanderthals, are particularly susceptible to, including low availability of helpers in cooperative hunting, defending kills from kleptoparasites, and allo-parenting. Finally, stochastic, annual fluctuations in births, deaths, and sex ratios are more likely to place smaller populations on a trajectory towards extinction than bigger ones. The models indicate that these factors alone could have resulted in Neanderthal extinction, even if Neanderthals and AMHs were identical in terms of individual-level traits that are deemed relevant to persistence or extinction (e.g., cognitive and technological ability, sociality).
The results of the study support the hypothesis that the disappearance of Neanderthals might have been the result of demographic factors alone, that is, the result merely of the internal dynamics that operate in small populations. 
The authors said, "The present study which provides an explanation solely in terms of the internal dynamics of the Neanderthal population, as the one presented here, serves as a null hypothesis against which competing, and less parsimonious, hypotheses are to be assessed. 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."
Thus, the study finally indicates that the arrival of AMHs would have been a contributory factor rather than the cause of the extinction. Importantly, population-level characteristics―e.g., many of the characteristics that conservation biology has shown to be critical for a species’ persistence, including population size, species distribution, intraspecific variability, and patterns of dispersal―might also account for the successful range expansion of AMHs. In other words, our species’ success need not be the result of superiority in its individual-level traits.​

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Source:
Materials provided by PLOS One, Content  edited for style and length.
Journal Reference:

1. Krist Vaesen, Fulco Scherjon, Lia Hemerik, Alexander Verpoorte. Inbreeding, Allee effects and stochasticity might be sufficient to account for Neanderthal extinction. PLOS ONE, 2019; 14 (11): e0225117 DOI: 10.1371/journal.pone.0225117