Loser of the sperm competition
When modern humans arrived in Europe around 45,000 years ago, the Neanderthals' days were numbered. One reason for their rapid disappearance may have been sperm competition.
Around 40,000 years ago, the Neanderthals disappeared from Europe. Experts disagree as to why. Was it climate change? Or did modern humans introduce dangerous pathogens when they arrived around 45,000 years ago? Did they even come in such large groups that the Neanderthals simply had no chance? Experts led by Joshua Akey from Princeton University have sought an answer to this question in the Neanderthal genome. As they recently showed in "Science", Neanderthals were genetically assimilated by Homo sapiens relatively quickly when the two human forms produced common descendants. Two researchers who were not involved in the study by Akey and his colleagues have now built on their findings and also explained in "Science": Sperm competition had played a decisive role in this. Not only were there more anatomically modern males than male Neanderthals, but they were also genetically better equipped to compete with other males for their sexual partners. In terms of reproduction, the Homo-sapiens males were therefore obviously superior.
Akey and his team analysed the genetic material of 2,000 modern humans as well as the genome data of several Neanderthals and one Denisovan for their study. With the help of statistical algorithms, they calculated that the two human forms had mixed repeatedly between 250,000 and 50,000 years ago.
The researchers focussed in particular on the genetic material of the Neanderthals. They discovered that their genome contained more gene sequences that originally came from anatomically modern humans than previously assumed. This result has implications for the reconstruction of population sizes at that time. This is because the diversity of DNA segments in a gene pool provides an indication of the approximate number of individuals. As a result, the Neanderthal groups in the period from which genetic data is available, from 122,000 to 52,000 years ago, had fewer and fewer individuals capable of reproducing. Moreover, instead of the presumed 3400 individuals, there were only 2400 on average.
In terms of sperm competition, Homo sapiens was superior
In the case of Neanderthals, we should therefore not be talking about extinction, but about assimilation, say Akey and his team. The Neanderthal population gradually dwindled and was absorbed into the groups of modern humans. Markus Neuhäuser from the University of Koblenz and Graeme Ruxton from the University of St Andrews have now considered which biological mechanisms played a role in this process. In their opinion, sperm competition was decisive. This refers to the competition between a man's individual sperm trying to fertilise an egg and the competition between different men who want to have offspring with the same woman. Whichever of them produces more sperm is likely to have a greater chance of success.
Neuhäuser and Ruxton assume that Palaeolithic humans were less strictly monogamous than couples in modern societies. In addition, the Neanderthals were numerically inferior to the Homo-sapiens groups. This makes it more likely that a woman, whether Neanderthal or Homo sapiens, was more likely to have offspring with an anatomically modern man than with a Neanderthal man.
In addition, according to Neuhäuser and Ruxton, the Homo sapiens males were also exposed to greater sperm competition in their own communities than the Neanderthals - because their groups were much larger. Physically, this could have meant that they had developed larger testicles on average than Neanderthals and were therefore able to produce more sperm. In the end, the Neanderthals were outnumbered without a chance.
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