Neanderthal males selected sapiens girls: genetics reveals what the intercourse of our ancestors was like | Science | EUROtoday

About 50,000 years in the past, someplace in Eurasia, two completely different human species had intercourse. They additionally had offspring. The results of these encounters between Neanderthals and Sapiens remains to be seen at the moment within the DNA of any individual of non-African ancestry, who carries between 1% and 4% Neanderthal heritage. But this inheritance shouldn’t be distributed equally: there are massive areas of the human genome the place the Neanderthal hint is totally absent. Scientists name them “Neanderthal deserts” and they’re particularly hanging on the X chromosome. A examine revealed this Thursday within the journal Science provides the strongest rationalization up to now for that hole: the interbreeding between the 2 species was, above all, between Neanderthal males and trendy human females. What science nonetheless can’t make clear is the explanation for these encounters.

The work, signed by Alexander Platt, Daniel Harris and Sarah Tishkoff, from the University of Pennsylvania, begins from a seemingly easy query: why is there so little Neanderthal DNA on the X chromosome of recent people? The consultants began from two hypotheses. The first is that the Neanderthal genes on that chromosome have been dangerous to our species and pure choice eradicated them. The second journey is that the Neanderthal

Since Svante Pääbo demonstrated greater than 15 years in the past that we’ve got Neanderthal DNA—a piece that earned him the Nobel Prize in 2022—scientists have studied this interbreeding by our personal genome. This examine does the alternative: it seems to be for the traces of sapiens in Neanderthal DNA to deduce, for the primary time, what these encounters have been like. “Since the Neanderthal genome was published, what has been studied preferably is the influence of Neanderthal genes on us. The approach of this work is the opposite, and that is very novel and interesting,” highlights Antonio Rosas, researcher on the National Museum of Natural Sciences of the CSIC, who has directed the research on the Neanderthals within the El Sidrón cave, in Asturias.

Scientists studied how a lot DNA A sensible man There is within the genome of three Neanderthals, these present in Altai and Chagyrskaya (Siberia) and in Vindija (Croatia). There was an earlier interbreeding episode, about 250,000 years in the past, wherein early trendy people left their mark on the Neanderthal genome. To refine the evaluation, they used as reference genetic information from three teams of hunter-gatherers from sub-Saharan Africa: the Xoo, the Ju/’hoansi and the Chabu, who shouldn’t have any Neanderthal ancestry.

The consequence was overwhelming. The X chromosome of Neanderthals accommodates 62% extra DNA of recent human origin than can be anticipated if the crossings had been random. That extra is just too massive to be defined by easy demographic fashions alone. The most obvious rationalization is that there was a mating choice, that’s, it was not unintentional, that Neanderthal males interbred extra with trendy human females than the opposite means round, and that sample was maintained for a number of generations. But did Neanderthal males want Sapiens over their very own females? Were the Sapiens those who selected them? And have been these conferences voluntary or pressured?

“It’s difficult or impossible to know why it was happening,” Platt acknowledges. “Yes, it is possible that social dynamics alone explain it. But the reason we prefer an answer that involves what we call pairing preference is that, to achieve this ratio without it, a fairly specific and complicated set of circumstances would be needed. Partner preference is a simple answer,” he adds.

María Martinón, director of the National Center for Research on Human Evolution, agrees: “This opens up different scenarios, from mating preferences to migration dynamics in which sapien women integrated into Neanderthal groups, which in anthropological terms could be described as a patrilocal tendency.”

The most uncomfortable question is whether those encounters were forced, that is, whether those females were raped, and it is also another question that genetics cannot answer either. “It’s not something we can address with this data,” Platt admits. What the models do suggest is that the process continued for several generations beyond the initial encounter, which implies that the mating patterns were consolidated within the populations and were not isolated episodes,” he adds.

Carles Lalueza-Fox, researcher at the Institute of Evolutionary Biology of the CSIC and expert in genomics of ancient populations, positively values ​​the approach of this study, but calls for caution. “There were undoubtedly complex cultural mechanisms operating in these processes, perhaps facilitated by the scarcity of Neanderthal women in a population that was in decline,” he points out. The researcher also points out that the proposed mechanism is not incompatible with other biological factors: “For instance, the low viability of hybrid boys and never ladies would additionally reinforce the low presence of Neanderthal genes on the X chromosome.” And he concludes: “It is an attention-grabbing speculation, nevertheless it doesn’t appear incompatible with different extra organic mechanisms that could be superimposed. It can be essential to have extra genomes near the blending occasion.”

Antonio Rosas points out a possibility that the study does not explicitly consider: that the hybrids of a Sapiens male with a Neanderthal female were simply not viable, or were so much less frequently. “In that case we’ve got no document, as a result of they haven’t left offspring. We are solely seeing the offspring which have left traces,” he points out. Rosas proposes a specific scenario to visualize how these crossings could have occurred: young sapien women incorporated into Neanderthal groups, “by hook or by crook,” whose daughters or granddaughters were later able to reintegrate into sapien groups carrying that genetic legacy. “That scenario fits better with the little we know when it comes to visualizing these movements,” he concludes.

“When we talk about attraction in prehistoric contexts we should not reduce it to the purely biological: physical robustness or health could have influenced it, but also status, the capacity for cooperation, alliances between groups or even the perception of what is different,” says Martinón. “Human relationships are complicated at the moment, inside our personal species; there isn’t any motive to suppose that they have been less complicated prior to now, and even much less so after they occurred between completely different human teams,” he adds.

The study also clarifies another related puzzle: why Neanderthal mitochondrial DNA, which is transmitted exclusively through the maternal line, has not reached us. If the interbreeding was predominantly between Neanderthal males and human females, their children would have always inherited the mitochondria from the sapiens mother, not from the Neanderthal father. The pattern observed on the Y chromosome also fits with this direction of crossing, although the authors recall that the Y chromosome of Neanderthals does indicate that there was some crossing in the reverse direction.

Platt points out that the next step will have to be to go “step by step, gene by gene.” Mate preference has a broad effect, spanning entire chromosomes, if not entire genomes, he explains. “Natural choice is dependent upon the particular capabilities of every fragment of the genome. That makes it a gradual and troublesome course of, however it may be enormously rewarding, as a result of every gene has its personal story to inform,” concludes Platt.

“It is exciting because science, through genetic data, leads us to a deeply human dimension: that of personal relationships and preferences,” says Martinón. What started as a statistical anomaly on the X chromosome of Neanderthals is popping out to be, ultimately, the genetic echo of a historical past of encounters between two human species that has but to be written.