By Jenny Graves /The conversation
Neanderthals, the closest cousins of modern humans, lived in parts of Europe and Asia until their extinction about 30,000 years ago.
Genetic studies are revealing more and more about the connections between modern humans and these long-extinct relatives. The latest is that a wave of interbreeding between our species occurred in a relatively short period of time, about 47,000 years ago. But one mystery remains.
THE A wise man The current genome contains some Neanderthal DNA. These genetic traces come from almost all parts of the Neanderthal genome, except the Y sex chromosome, which is responsible for forming males.
So what happened to the Neanderthal Y chromosome? It could have been lost by accident, or due to reproductive patterns or inferior function. However, the answer may lie in a centuries-old theory about the health of interspecies hybrids.
Sex, genes and chromosomes of Neanderthals
Neanderthals and modern humans split up between 550,000 and 765,000 years ago in Africa, when the Neanderthals wandered into Europe, but our ancestors stayed behind. They didn’t see each other again until Homo sapiens migrated to Europe and Asia between 40,000 and 50,000 years ago.
Scientists have found complete copies of the male and female Neanderthal genomes, thanks to DNA from well-preserved bones and teeth of Neanderthals in Europe and Asia. Not surprisingly, the Neanderthal genome was very similar to ours, containing about 20,000 genes grouped across 23 chromosomes.
Like us, they had two copies of 22 of these chromosomes (one from each parent), as well as a pair of sex chromosomes. Females had two X chromosomes, while males had one X and one Y.
Y chromosomes are difficult to sequence because they contain a lot of repetitive “junk” DNA, so the Neanderthal Y genome has only been partially sequenced. However, the large portion that has been sequenced contains versions of many of the same genes found in the modern human Y chromosome.
In modern humans, a gene on the Y chromosome called SRY triggers the process of developing an XY embryo into a male. The SRY gene plays this role in all apes, so we assume it did so in Neanderthals as well, although we have yet to find the Neanderthal SRY gene itself.
Neanderthals coexisted with modern humans in the Negev Desert.Kovalenko I (/ Adobe Stock)
Interspecies crossbreeding left us with Neanderthal genes
There are many small clues that indicate that a DNA sequence comes from a Neanderthal or a Homo sapiensSo we can look for fragments of Neanderthal DNA sequences in the genomes of modern humans.
The genomes of all human lineages originating in Europe contain about 2% Neanderthal DNA sequences. Lineages from Asia and India contain even more, while lineages restricted to Africa have none. A wise man The genomes contained even more – about 6% – so it seems that Neanderthal genes are gradually disappearing.
Most of this Neanderthal DNA arrived about 47,000 years ago, after modern humans moved out of Africa into Europe, and before the Neanderthals went extinct about 30,000 years ago. During this time, many Neanderthal-human pairings must have occurred.
At least half of the Neanderthal genome can be reconstructed from fragments found in the genomes of various contemporary humans. We owe traits such as red hair, arthritis, and resistance to certain diseases to our Neanderthal ancestors.
There is one glaring exception: no contemporary human has been identified as carrying any part of the Neanderthal Y chromosome.
What happened to the Neanderthal Y chromosome?
Was the Neanderthal Y chromosome lost by bad luck? Was it not very efficient at creating males? Did Neanderthal women, but not men, engage in interspecies mating? Or was there something toxic about the Neanderthal Y chromosome that couldn’t work with human genes?
The AY chromosome occurs at the end of the line if its carriers have no sons, so it may simply have been lost over thousands of generations.
Or maybe the Neanderthal Y was never present in interspecies matings. Maybe it was always modern men who fell in love with (or traded, seized, or raped) Neanderthal women? The sons born to these women would all have the Homo sapiens form of the Y chromosome. However, it is difficult to reconcile this idea with the finding that there is no trace of Neanderthal mitochondrial DNA (which is restricted to the female line) in modern humans.
Or maybe the Neanderthal Y chromosome just wasn’t as efficient at its job as its counterpart. Homo sapiens Neanderthal populations were always small, so harmful mutations would have been more likely to accumulate.
We know that Y chromosomes with a particularly useful gene (for example for more, better, or faster sperm) quickly replace other Y chromosomes in a population (called the hitchhiker effect).
We also know that the Y chromosome is generally degraded in humans. It is even possible that the Y chromosome disappeared from the Neanderthal Y chromosome and that Neanderthals were disrupting the evolution of a new sex-determining gene, as is the case in some rodents.
Was the Neanderthal Y chromosome toxic in hybrid boys?
Another possibility is that the Neanderthal Y chromosome does not work with genes on other chromosomes of modern humans.
The absence of the Neanderthal Y chromosome could then be explained by the “Haldane rule”. In the 1920s, the British biologist JBS Haldane noted that, in hybrids between species, if one of the sexes is sterile, rare or sick, it is always the sex whose sex chromosomes are different.
In mammals and other animals where females have XX chromosomes and males have XY chromosomes, it is disproportionately the male hybrids that are unfit or sterile. In birds, butterflies and other animals where males have ZZ chromosomes and females have ZW chromosomes, it is the females.
Many crosses between different mouse species exhibit this pattern, as do feline crosses. For example, in lion-tiger crosses (ligers and tigons), females are fertile, but males are sterile.
We still lack a good explanation of Haldane’s rule. It is one of the enduring mysteries of classical genetics.
But it seems reasonable that the Y chromosome of one species has evolved to work with genes on other chromosomes of its own species, and may not work with genes from a related species that contain even small changes.
We know that genes on the Y chromosome evolve much more rapidly than those on other chromosomes, and that several of them have functions in sperm production, which may explain the infertility of male hybrids.
This could therefore explain why the Neanderthal Y chromosome was lost. It also raises the possibility that it was the fault of the Y chromosome, by imposing a reproductive barrier, that led Neanderthals and humans to become separate species.
Top image: Artist’s reconstruction of a Neanderthal, featured in the exhibition ‘Britain: One Million Years of the Human Story’. Source: The Trustees of the Natural History Museum, London
This article was originally published under the title “Modern human DNA contains fragments of the entire Neanderthal genome, except for the Y chromosome. What happened?‘ by Jenny Graves on The conversationand has been republished under a Creative Commons license.