New trace of early contact: In crossing events possibly more than 350,000 years ago, early forms of Homo sapiens transferred their Y chromosome to the Neanderthals. This emerges from the first comprehensive analysis of the male sex chromosome in our two closest archaic relatives. In contrast to the Neanderthals, the Denisovans did not have a Y chromosome from our ancestors, the scientists report.
It was a bang in the history of anthropology: From genetic analyzes of the fossil genome of Neanderthals, it emerged that there was once a mixture of modern humans and their archaic cousins. These crosses, dating from around 40,000 to 60,000 years ago, resulted in 1.5 to 2 percent Neanderthals in the genomes of people outside of Africa. The genetic tools of anthropology then also revealed the existence of another archaic representative of the genus Homo. Analyzes have shown that this so-called Denisova human has also contributed to the genome of today’s people. Based on the genetic comparisons, it is believed that the Neanderthals and Denisovans were sister groups that split off from the lineage of modern humans more than 550,000 years ago.
Initially, the focus was on the gene flow from archaic forms to modern humans. But then researchers also found the opposite effect: In genetic data it became apparent that some Neanderthals – but not the Denisovans – carried traces of Homo sapiens genetic material. The interesting thing about it was: Dating showed that these crossings apparently took place very early – about 150,000 to 350,000 years ago. In addition to influencing the nuclear genome, it was apparently primarily a change in the mitochondrial DNA. This genetic material from the power stations of the cells is only passed on from the mothers to their offspring. The same is also the case with the Y chromosomes: They are only passed on from father to son. These two genome carriers are therefore particularly interesting for anthropology.
The “male chromosomes” in sight
But so far anthropologists have not been able to investigate the characteristics of the male sex chromosomes: Since the remains of Neanderthals and Denisovans, from which well-preserved DNA could be isolated, came from female individuals, extensive investigations of the male Y chromosomes have not yet been possible. But with the study by the researchers led by Martin Petr from the Max Planck Institute for Evolutionary Anthropology in Leipzig, that has now changed: They have succeeded in assigning usable Y chromosome sequences to the poorly preserved remains of three male Neanderthals and two Denisovans elicit. To do this, they developed a method with which they could fish out fragments of these genetic material carriers from the contamination by microbial DNA fragments. So it was finally possible to reconstruct and analyze the Y chromosome sequences.
The comparisons of the archaic human Y-chromosomes with each other and with the Y-chromosomes of people living today showed: The versions of the Neanderthals are more similar to those of modern humans than the Y-chromosomes of the Denisovans. “That was surprising, because we know from studies of their autosomal DNA that Neanderthals and Denisovans were closely related and that humans living today are their more distant evolutionary cousins. We had therefore expected that the Y chromosomes would show a similar picture, ”says Petr. The results now suggest that transmission of the Y chromosome occurred in early crossbreeding events between the two human forms. Estimates based on the genetic mutation rates show that the transmissions took place 370,000 years ago, but no later than 100,000 years ago.
Trace of early mixing
This means that modern humans – or forms closely related to them – have already reached Eurasia by this time, met and mingled with the Neanderthals. The H. sapiens Y chromosome was transferred to archaic humans and then established itself in their population. This result fits in with the pattern that has already emerged in the case of the mitochondrial DNA of the Neanderthals, the scientists emphasize.
But why did the H. sapiens Y chromosome prevail in Neanderthals? Presumably it had features that were cheap compared to the Neanderthal version, the scientists explain. Using computer simulations, they were able to show that the small size of the Neanderthal populations could have led to an accumulation of harmful mutations in their Y chromosomes. “In view of the important role the Y chromosome plays in fertility and reproduction, we suspect that the lower evolutionary fitness of the Y chromosomes in Neanderthals led to natural selection. Individuals with the variant adopted from modern humans had an advantage, which ultimately led to the exchange observed, ”explains Petr.
The scientists now hope that their exchange hypothesis could soon be confirmed by new data. The study’s senior author, Janet Kelso, says: “If we can obtain Y chromosome sequences from Neanderthals who lived before this hypothetical early exchange event, for example from the 430,000-year-old Neanderthals from the Sima de los Huesos cave in According to our prediction, Spain will still have the original Neanderthal Y chromosomes and will therefore be more similar to the Denisovans than to modern humans, ”says the anthropologist.
Source: Max Planck Institute for Evolutionary Anthropology, specialist article: Science, doi: 10.1126 / science.abb6460