At various points in our tangled history, modern humans have mated with each other Neanderthals.
This left a telltale signature on the genomes of today’s modern humans, and this Neanderthal DNA impacts our health in countless ways.
However, there is one part of our genome where no Neanderthal DNA is present: the Y chromosome. But why?
Experts told LiveScience that some of that could be a coincidence. But it’s also possible that genes from Neanderthal males were incompatible with those from the female Homo sapiens they wear. That would mean that only female hybrids could reproduce.
Related: The face of a Neanderthal woman brought to life in a stunning reconstruction
The disappearing “Y”
The Y chromosome is one of two types sex chromosomes in people. Females carry two copies of the X chromosome, while males carry one X and one Y chromosome. The Y chromosome can only be passed on from father to son.
Initially, scientists analyzed DNA from the fossilized remains of female Neanderthals. However, in 2016, a study was published in the American Journal of Genetics examined a Neanderthal Y chromosome from a 49,000-year-old man from Spain.
‘We have never observed Neanderthal Y-chromosome DNA in any human sample ever tested’ Carlos Bustamanteco-senior study author and a population geneticist at Stanford University, said in a rack at the time.
So why would this Neanderthal DNA seemingly disappear without a trace?
The simplest answer might be that it randomly disappeared from the human gene pool over thousands of years.
“The amount of Neanderthal DNA in modern humans is relatively low today, so it could have been lost through drift,” Fernando Mendezlead author of the study, then a postdoctoral researcher at Stanford, told ABC News. In other words, it is not that Neanderthal DNA was less “fit” from an evolutionary perspective than modern human DNA, but rather that it was simply lost over time.
Another possibility is that the Neanderthal Y chromosome was incompatible with our own DNA. For example, the team found that three of the Neanderthal genes on the Y chromosome, which are different from those in humans, function as part of the immune system. These genes allows the immune system to distinguish friend – or the body’s own cells – from enemy. The inability to properly distinguish ‘own’ from ‘intruder’ is the reason why tissue transplants from men to women take place can be rejected or why a mother’s immune system can attack a male fetus during the pregnancycausing a miscarriage.
Related: ‘It just didn’t work’: Mating between Neanderthals and modern humans may have been a product of failed alliances, says archaeologist Ludovic Slimak
It is therefore conceivable that modern women’s immune systems consistently attacked male babies carrying Neanderthal DNA on their Y chromosome, leading to repeated miscarriages and ultimately the loss of Neanderthal Y genes.
If modern women who interbred with Neanderthals had fewer offspring than other pairs, then those still alive will systematically have fewer offspring, Mendez said. told ABC News. This hypothesis is consistent with an old theory called “Haldane’s rule,This suggests that if breeding between genetically different populations leads to infertility, it is likely in the sex – in this case the male – that carries two different sex chromosomes.
Far-reaching loss
This isn’t the first time in our evolutionary history that the Neanderthal Y chromosome has failed to compete with its modern human counterpart.
Between 550,000 and 765,000 years agothe population that would give rise to Homo sapiens diverged from Neanderthals and another group of now extinct human relatives Denisovans.
However, a study published in the journal Science in 2020 revealed that sometime between 370,000 and 100,000 years ago, Neanderthals and early modern humans interbred. 100,000 years ago, the Neanderthal Y was completely replaced by that of Homo sapiens.
Scientists don’t know why this happened, Martin Petera co-senior study author and researcher and programmer at the University of Copenhagen in Denmark told LiveScience.
But a likely explanation, Petr hypothesized, is that Neanderthals had a very small population size compared to early modern humans for hundreds of thousands of years. The low population size allows harmful mutations to accumulate in the gene pool more quickly than within a larger population, he said.
If you then introduced ‘healthier’ early modern human DNA into the mix, natural selection would favor this modern human DNA, which would then have spread throughout the Neanderthal population.
However, without much more genomic data from Neanderthals that would allow scientists to study the functional impact of inheriting this early modern human DNA, this is just a hypothesis, Petr said.
Related: Scientists finally solve the mystery of why Europeans have less Neanderthal DNA than East Asians
Many unknowns
Unfortunately, it is still too early to say definitively why the Neanderthal Y-DNA was lost in both cases. Adam Siepela computational biologist from Cold Spring Harbor Laboratory in New York, told LiveScience in an email.
It’s still possible that the replacement occurred due to random genetic drift, he added.
Neanderthal DNA lives within us
In general, the loss of a Y chromosome lineage due to crossing over is not a rare phenomenon. Carles Lalueza-Foxco-author of the 2020 Science study and a paleogenomics researcher at the Institut de Biologia Evolutiva in Spain, told LiveScience in an email.
This is because the Y chromosome is only inherited paternally, he said. This is in contrast to other chromosomes in the body that are passed on to the next generation by both parents. This means the Y chromosome is more prone to being “lost” over time, he said.
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