Sperm competition: an additional cause for the rapid demise of Neanderthals
Neanderthals rapidly perished after modern humans settled in their habitat in Europe and Western Asia. Differences in sperm competition between Neanderthals and modern humans have now been proposed as an additional cause.
Various hypotheses were suggested to explain the extinction of Neanderthals: differences in metabolism, in demography, in the use of fire, as well as hunting with domesticated dogs. Competitive exclusion would make it difficult for the two species to permanently populate the same area, but why did the Neanderthals disappear? Pathogens, climate and vegetation changes and volcanic eruptions were discussed as possible causes. Genetic studies showed that Neanderthals interbred with modern humans. Hence, Neandertals assimilated into in-migrating populations of modern humans. As a consequence, recent Europeans inherit a few percent of their genomes from Neanderthal ancestors.
The rapidity of the demise of the Neanderthals is striking. Neuhäuser and Ruxton (2024) argue that advantages to modern humans in sperm competition might be important in explaining the speed of genetic assimilation. This hypothesis is based on two findings: Both Neanderthals and modern humans were likely more promiscuous during the Pleistocene than most contemporary populations. Moreover, modern human social group size was larger than that of Neanderthals. This offers approaches for statistical modeling.
When groups of the two species met, each female (Neanderthal or modern human) might have copulated, just as a matter of group size, with a larger probability with a modern man, and the more pronounced the promiscuity was, on average with more modern human than Neanderthal males. In addition, sperm competition is, when mating systems are similar, more pronounced in a species living in larger groups. As a consequence, adaptations to intensive sperm competition such as larger testis sizes are more likely. Any adaptation to higher sexual competition within modern humans could further strengthen their benefit in sperm competition with Neanderthals.
The larger the advantages to modern humans in sperm competition were, the less in-migrating modern humans are needed to explain the rapid assimilation. Of course, advantages in sperm competition are not mutually exclusive to other explanations.
Participating institutions:
Koblenz University of Applied Sciences, Germany
University of St. Andrews, United Kingdom
Wissenschaftlicher Ansprechpartner:
Prof. Dr. Markus Neuhäuser
Koblenz University of Applied Sciences, RheinAhrCampus, Dept. of Mathematics, Informatics and Technology
neuhaeuser@rheinahrcampus.de
Originalpublikation:
Neuhäuser M, Ruxton GD (2024) Sperm competition: an additional cause for the rapid demise of Neanderthals. Science (eLetter) https://www.science.org/doi/10.1126/science.adi1768#elettersSection
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