Coexistence between Neanderthals and modern humans is similar to the competition-exclusion principle. Modern humans assert their way of life more quickly. Obviously, Neanderthals disappeared quite quickly. So far, there have been some arguments in favour of this rapid disappearance. Now, however, with sperm competition, another, perhaps even the most important reason becomes clear.

Koblenz, September, 05th 2024 The most common hypotheses for the extinction of the Neanderthals include differences in metabolism, demography and the use of fire. The hunt for domesticated dogs is also one of them. The reasons why modern humans were able to prevail have so far been seen in the causes of pathogens, climate and vegetation changes, including volcanic eruptions. Even if the proportion in the genetic make-up of modern humans is only a few per cent and therefore relatively small, studies show that Neanderthals and modern humans have assimilated and mixed.

Neuhäuser and Ruxton (2024) now argue that advantages of modern humans in sperm competition may have been important and may have contributed to the speed of genetic assimilation. This hypothesis is based on two findings: Neanderthals, like modern humans, were probably more promiscuous and less monogamous in the Pleistocene than modern populations. In addition, the groups of Neanderthals were smaller than those of modern humans. This provides a basis for statistical modelling.

If two groups of the two species met, the probability of mating with a modern male could have been greater for each female (whether Neanderthal or modern) simply because of the group size. And the more pronounced the promiscuity was, the more sexual partners of each female were modern males and not Neanderthals. Furthermore, sperm competition, at least with the same mating system, is stronger in species living in larger groups and adaptations to intense sperm competition, such as larger testes, are more likely. Any adaptation to greater sexual competition in modern humans would increase the advantages in terms of sperm competition.

The greater the advantages in sperm competition were, the fewer immigrating modern humans are needed to explain the rapid assimilation. Advantages in sperm competition and other explanations are of course not mutually exclusive.

Participating institutions:

Koblenz University of Applied Sciences, Germany
University of St Andrews, United Kingdom

Originalpublication:

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