Neanderthals, modern humans’ closest relatives, lived in parts of Europe and Asia until they became extinct about 30,000 years ago.
Genetic studies are increasingly revealing links between modern humans and these long-extinct relatives, and recently revealed a relatively short period of rapid interbreeding between humans. About 47,000 years agoBut there is still one mystery remaining.
of Homo sapiens Our genomes today contain bits of Neanderthal DNA, and these genetic traces come from almost every part of the Neanderthal genome except the Y chromosome, which makes us male.
So what happened to the Neanderthal Y chromosome? Perhaps it was lost by chance or because of inferior mating patterns or function. But the answer may lie in a century-old theory about the health of interspecies hybrids.
Neanderthal sex, genes, and chromosomes
Neanderthals and modern humans went their separate ways somewhere between 550,000 and 765,000 years ago in Africa, with the Neanderthals heading off to Europe and our ancestors staying put.
They never met again Homo sapiens They migrated into Europe and Asia between 40,000 and 50,000 years ago.
Thanks to well-preserved DNA, scientists have reconstructed complete copies of the genomes of a Neanderthal male and female. Bones and teeth Neanderthal populations in Europe and Asia.
Not surprisingly, the Neanderthal genome was very similar to ours, containing about 20,000 genes bundled across 23 chromosomes.
Just like us humans, women have two copies of the 22 chromosomes (one from each parent) and also have a pair of sex chromosomes: women have two X chromosomes, and men have one X and one Y chromosome.
The Y chromosome is difficult to sequence because it contains so many genes. Repetitive “junk” DNAThe Neanderthal Y genome Partially orderedBut the larger fragments that were sequenced contain some versions of the same genes found on the modern human Y chromosome.
In modern humans, a Y chromosome gene called SRY initiates the process by which an XY embryo develops into a male. The SRY gene plays this role in all great apes, so it may have played a similar role in Neanderthals, although the Neanderthal SRY gene itself has never been found.
Interbreeding left Neanderthal genes behind
DNA sequence is Neanderthal Homo sapiensThis means we can look for parts of Neanderthal DNA sequences in the genomes of modern humans.
All human lineages originating from Europe contain approximately 2% Neanderthal DNA sequences in their genomes. Asian and Indian lineages moreoverOn the other hand, it is absent in lineages restricted to Africa.
ancient Homo sapiens The genome contained many more genes (around 6%), so it appears that Neanderthal genes are gradually disappearing.
Most of this Neanderthal DNA was brought over a period of 7,000 years, from when modern humans left Africa for Europe to when the Neanderthals became extinct, about 30,000 years ago, to about 47,000 years ago. Interbreeding between Neanderthals and humans.
At least half of the Neanderthal genome Stitched together From fragments found in the genome of modern humans, thanks to our Neanderthal ancestors Features These include red hair, arthritis, and resistance to some diseases.
There is one glaring exception. There are no modern people It was found to be hiding part of the Neanderthal Y chromosome.
What happened to the Neanderthal Y chromosome?
Was the loss of the Neanderthal Y chromosome just bad luck? Or were they just not very good at their job of making men? Or did Neanderthal women interbreed but men didn’t? Or Was there something toxic? Regarding the Neanderthal Y chromosome, does it have no function in human genes?
The AY chromosome may simply have been lost over thousands of generations, as it comes at the end of a lineage if its carrier has no sons.
Or maybe the Neanderthal Y gene was not present in the interbreeding. Was it always modern men who fell in love with (or sold, robbed, or raped) Neanderthal women? All of the sons born to these women were Homo sapiens Morphology of the Y chromosome.
But this idea, Neanderthal mitochondrial DNA (This is limited to the female lineage) in modern humans.
Or the Neanderthal Y chromosome, Homo sapiens Rivals. Neanderthal populations were always small, which would have made it more likely that harmful mutations would have accumulated.
Y chromosomes that carry particularly useful genes (e.g. sperm quantity, quality, speed, etc.) have been found to rapidly replace other Y chromosomes in a population (known as the hitchhiker effect).
The Y chromosome The deterioration of humanity as a wholeIt’s even possible that SRY was lost from the Neanderthal Y gene, leading to a disruptive process that allowed Neanderthals to evolve new sex-determining genes. Some rodents.
Was the Neanderthal Y chromosome toxic to hybrid boys?
Another possibility is that the Neanderthal Y chromosome doesn’t coordinate with genes on other chromosomes in modern humans.
The loss of the Neanderthal Y gene may be explained as follows.Haldane’s LawIn the 1920s, British biologist J.B.S. Haldane pointed out that in interspecies crosses, if one sex is infertile, rare or unhealthy, it is always the sex with the different sex chromosomes.
In mammals and other animals in which females have XX chromosomes and males have XY chromosomes, it is the male hybrids that are disproportionately unsuitable or infertile. In birds, butterflies and other animals in which males have ZZ chromosomes and females have ZW chromosomes, the females are unsuitable.
This pattern is seen in many matings between different mouse species, and is also seen in feline matings: for example, in a mating between a lion and a tiger (liger and tigon), the females are fertile but the males are infertile.
There is still no satisfactory explanation for Haldane’s rule, and it is one of the enduring mysteries of classical genetics.
But it seems reasonable to think that the Y chromosome of a species might evolve to cooperate with genes on other chromosomes in that species and not with genes in related species. Even small changes.
Genes on the Y chromosome evolve much faster than genes on other chromosomes, and some of them are known to play a role in sperm production, which may explain why male hybrids are infertile.
This could therefore explain why the Neanderthal Y chromosome was lost, and raises the possibility that the Y chromosome imposed a reproductive barrier that caused Neanderthals and humans to become separate species in the first place.
Jenny GravesDistinguished Professor of Genetics and Vice Chancellor’s Fellow, La Trobe University
This article is reprinted from conversation Published under a Creative Commons license. Original Article.