Homo sapiens and Neanderthals not only cohabited with each other in Europe and Asia for a period of time, but also had a fruitful sex life. Evidence of this are remains of the Neanderthal genome in our inherited genotype. In reality, however, only the combination of Homo sapien man and Homo neanderthalensis woman could be seen as fruitful. The opposite combination was much more unlikely to produce offspring.
Fernando Mendes and his colleagues from the University of Stanford came to this conclusion following a thorough analysis of the Y chromosomes of DNA found in a 50 thousand year-old Neanderthal man, in El Sidron, Spain. Modern humans carry anything between 1% – 3% Neanderthal genes in their genome but none are present in the Y chromosome, as this study evidently showed. The Y chromosome is passed down from father to son.
The evaluation was based on the assumption that Neanderthal men and Homo sapien women were not fully compatible with each other and therefore were not able to produce offspring. At the end of their existence Neanderthal males likely had sperm with poor quality and did not produce sufficient quantities to be able to reproduce and preserve their kind.
The remains of the man from El Sidron might provide an answer to this question. They give evidence of the mutation of three immune genes – among them one responsible for the production of an antigen that induces an immune response in pregnant woman. This would result in pregnancies that are interrupted by spontaneous abortions. Therefore male heirs in this type of union were sparse.
It is entirely possible that these mutations accelerated the extinction of the Neanderthal kind according to the researchers. As a testament to this hypothesis, in modern times we often observe that during organ transplants, where a male organ is transplanted in a female body, the latter rejects it.