The latest genetic study indicates that initial interactions between Neanderthals and modern humans were uneven, with some indication that most of the Neanderthals were men who slept with female modern humans, which could be the reason behind long term gaps in human DNA.

The experiment conducted by researcher, Alexander Platt and other researchers, investigates the distribution of Neanderthal genetic material in current human beings. Although the majority of those not in Africa have some traces of Neanderthal ancestry, these have been distributed unevenly throughout the human genome.

A particularly interesting characteristic is the existence of so-called Neanderthal deserts – large areas of the DNA, in which the genetic material of Neanderthals is virtually nonexistent. These deletions are more pronounced in the X chromosome and this poses a question on how the ancient interbreeding process occurred.

There has been long speculation among scientists as to whether these deletions were due to natural selection (whereby the deleterious Neanderthal genes are becoming more and more extinct) or that the interbreeding itself is the cause.

The researchers reversed the question to investigate. They did not simply study the Neanderthal DNA of the contemporary human beings, but rather the remnants of the early modern human DNA in the Neanderthal genomes. The comparison of these with genetic data of the sub-Saharan African populations, most of which do not have Neanderthal ancestry, helped the team recreate ancient gene flow patterns between the two groups.

Great Imbalances in DNA

Their results showed a great imbalance: the proportion of the modern human DNA in the Neanderthal X chromosomes was much higher than anticipated- approximately 62 percent higher. Researchers believe that such an asymmetry can best be attributed to the possibility that the vast majority of the interbreeding took place between male Neanderthals and female modern humans.

This would limit the survival of Neanderthal X-linked DNA into the subsequent generations of human population because males can only transmit their X chromosome to the females. This would over time lead to the low concentration of Neanderthal genetic material on the human X chromosome today.

The paper also indicates that social or behavioural influences, including mate preferences, could have contributed to the development of such patterns, but demographic influence, such as the variation in number or migration cannot be disqualified.

Natural Selection Behind Imbalance?

Moreover, this imbalance was probably supported by natural selection. Dangerous or incompatible genes of Neanderthals especially those associated with significant biological functions might have been gradually eliminated in human gene pool across generations.

The results provide a new understanding of the complicated relationships between the early human groups and the ones closest to their evolutionary lineage not only regarding genetic inheritance but also on social process that might have influenced the evolution of humans.

Using the combination of genomic evidence and evolutionary modelling, researchers indicate that the study is leading scientists nearer to the realization of how ancient interbreeding events still impact the genetic landscape of modern humans.

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