Are you sure Neanderthals had sex with modern humans?

One of the most provocative recent scientific studies has attempted to determine How did the crossbreeding between modern humans and Neanderthals occur?The sensationalized news in many media outlets claims that ancestral pairings primarily occurred between Neanderthal men and modern human women. However, in science, press headlines are not always a reliable reflection of scientific data, because they often seek social impact or clickbait

Because, in reality, this study doesn't exactly say this, but rather states that hybridization relationships between these two hominin groups were relatively unidirectional, with preferential matings over many generations between males with a high percentage of Neanderthal DNA and females with a high percentage of modern human DNA.

It arrives at these conclusions from the analysis of the X chromosome sequence of only three female Neanderthal fossils, which they compare with the X chromosomes of 73 present-day African women, who are modern humans with no introduction of Neanderthal DNA. So, is this result, which is an inference from computer simulations with limited genetic data, an absolute certainty, or does it have other interpretations?

A very complex story

The evolution of the human species has always been complex. Different migrations of individuals of the genus Homo They spread across the Old Continent: the Neanderthals—in two different waves—and the Denisovans. Then came the migrations of modern humans, in several waves. It must be emphasized that we are not, nor have we ever been, a pure species: individuals of the genus Homo They have interbred successively, generating hybrids with varying degrees of success. Thus, when modern humans arrived in Eurasia, they interbred with Neanderthals at least twice: once between 200,000 and 250,000 years ago, and again, more recently, between 60,000 and 50,000 years ago.

As a result, all present-day non-African modern humans have fragments of the Neanderthal genome in their genome, between 1% and 6%. It is poetic that, although Neanderthals became extinct about 45,000 years ago, but patches of their genome still survive in us.

Natural selection acts very rapidly on hybrids, and within a few generations, the regions of the genome that were incompatible or that reduced the survival capacity of individuals were eliminated from the genome of their descendants. Thus, today we find large regions of our chromosomes that are "deserts" of Neanderthal sequences. Among these regions is the X chromosome.

It is important to remember that in mammals, the sex chromosomes X and Y are inherited differentially: women have two X chromosomes, and men have only one X and one Y chromosome. Daughters inherit one X chromosome from their mother. In contrast, sons inherit one X chromosome from their mother and one Y chromosome from their father. Therefore, evolutionary forces impact the X and Y chromosomes differentially because they are not distributed equally among offspring and because women have twice as many X chromosomes as men.

The question many try to answer is why the current human X chromosome contains so few Neanderthal sequences, especially considering that the remnants in the rest of the chromosomes (1 to 22) are somewhat greater. There are two simple explanationsEither there was a lot of purifying selection against the Neanderthal X chromosome, or the hybridization pairings were very biased towards women – who have twice as many X chromosomes – from the group of modern humans.

Exploring other stories

The researchers in this article, instead of looking at modern humans to study their Neanderthal content, tried to determine how much of the modern human genome remained in Neanderthals during the first interbreeding (no Neanderthal remains have been found after the second interbreeding). They hypothesized that if they figured out what happened during this first hybridization, the evolutionary forces would be the same for the second period of Neanderthal-human interbreeding. The surprise is that in these three fossils, they find an unexpected excess of modern human sequences on the Neanderthal X chromosome compared to the rest of the genome. They then run computer simulations to try to explain this bias and conclude that only one very continuous biased matching over time The relationship between women with a higher proportion of modern human X genes and men with a higher proportion of Neanderthal genes could explain this. This would suggest that Neanderthal parents may have passed on part of their genome to us.

However, there are other very likely alternative explanations, such as the possibility that the survival or fertility of Neanderthal-modern human hybrids was not the same for children of Neanderthal mothers as for children of modern human mothers. This phenomenon of differential survival or biased sterility of hybrid males The difference in parentage between different mammal species is common in hybridization. For example, the cross between a lion mother and a tiger father (tigon), or of a lion father and a tiger mother (liger), produces sterile male offspring and fertile female offspring.

If hybrid boys born to Neanderthal mothers could not survive or reproduce like reciprocal male hybrids, there would be a very rapid decline in the contribution of the Neanderthal X chromosome. In fact, other studies have shown that interbreeding occurred in both directions, since The modern human Y chromosome has been identified in the remains of male Neanderthals after the first period of interbreeding.These data have not even been considered in this latest work.

Ultimately, there is still no clear answer as to why the X chromosome is a desert of Neanderthal sequences today compared to the other chromosomes. Many aspects remain to be discovered about the relationships of our ancestors and how natural selection acted upon hybrids. It's best not to settle for the simplistic idea that Neanderthal-modern human relationships were unidirectional. What is certain is that these hybrid offspring were cared for, accepted, and loved. And they survived and interbred. And we are living proof of that.