Neanderthals, the closest relatives of modern humans, lived in parts of Europe and Asia until their extinction about 30,000 years ago.
Genetic studies are increasingly revealing more about the links between modern humans and these long-gone relatives – most recently that a wave of interbreeding between our species occurred over a relatively short period of time about 47,000 years ago. But one mystery still remains.
The Homo sapiens the current genome contains a small amount of Neanderthal DNA. These genetic traces come from almost every part of the Neanderthal genome – except the Y sex chromosome, which is responsible for making men.
So what happened to the Neanderthal Y chromosome? It may be lost accidentally, or due to mating patterns or inferior function. The answer, however, may lie in an age-old theory about the health of interspecies hybrids.
Neanderthal gender, genes and chromosomes
Neanderthals and modern humans went their separate ways in Africa sometime between 550,000 and 765,000 years ago, when Neanderthals strayed into Europe, but our ancestors remained there.
They wouldn’t meet again until H. sapiens migrated to Europe and Asia between 40,000 and 50,000 years ago.
Scientists have recovered copies of the complete male and female Neanderthal genomes, thanks to DNA from well-preserved bones and teeth of Neanderthal individuals in Europe and Asia.
Unsurprisingly, the Neanderthal genome was very similar to ours, with about 20,000 genes bundled into 23 chromosomes.
Like us, they had two copies of 22 of those 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 several of the same genes found in 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 apes, so we assume this was also the case in Neanderthals – even though we did not find the Neanderthal SRY gene itself.
Interspecies mating left us Neanderthal genes
There are many little giveaways that mark a DNA sequence as coming from a Neanderthal or a H. sapiens. So we can look for bits of Neanderthal DNA sequence in the genomes of modern humans.
The genomes of all human lineages originating in Europe contain approximately 2% Neanderthal DNA sequences. Lineages from Asia and India contain even more, while lineages restricted to Africa have none.
Some old Homo sapiens genomes contain even more – about 6% – so it appears that Neanderthal genes are gradually dying out.
Most of this Neanderthal DNA arrived over a period of 7,000 years, about 47,000 years ago, after modern humans came to Europe from Africa, and before the Neanderthals became extinct about 30,000 years ago. During this time there must have been many matings between Neanderthals and humans.
At least half of the entire Neanderthal genome can be pieced together from fragments found in the genomes of several modern-day humans. We have our Neanderthal ancestors to thank for traits like red hair, arthritis and resistance to certain diseases.
There is one notable exception. No modern-day humans have been found to harbor any part of the Neanderthal Y chromosome.
What happened to the Neanderthal Y chromosome?
Was it just bad luck that the Neanderthal Y chromosome was lost? Wasn’t it very good at its job of making men? Did Neanderthal women, but not men, indulge in interspecies mating? Or was there something toxic about the Neanderthal Y so that it wouldn’t work with human genes?
The AY chromosome comes to the end of the line if its carriers have no sons, so it may simply have been lost over thousands of generations.
Or perhaps the Neanderthal Y was never present at interspecies matings. Maybe it was always modern human men who fell in love with Neanderthals (or traded, confiscated, or raped them)? Sons of these women would all have the H. sapiens shape 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 good at the job as it did H. sapiens rival. Neanderthal populations were always small, so it was more likely that harmful mutations had accumulated.
We know that Y chromosomes with a particularly useful gene (for more, better or faster sperm, for example) quickly replace other Y chromosomes in a population (called the hitchhiker effect).
We also know that the Y chromosome generally breaks down in humans. It is even possible that SRY was lost by Neanderthal Y, and that Neanderthals engaged in the disruptive process of evolving a new sex-determining gene, as some rodents have done.
Was the Neanderthal Y chromosome toxic in hybrid boys?
Another possibility is that the Neanderthal Y chromosome will not work with genes on other chromosomes of modern humans.
The missing Neanderthal Y can then be explained by “Haldane’s rule”. In the 1920s, British biologist JBS Haldane noted that in hybrids between species, if one sex is sterile, rare, or unhealthy, it is always the sex with dissimilar sex chromosomes.
In mammals and other animals where females have XX chromosomes and males have XY, it is disproportionately male hybrids that are unfit or infertile. In birds, butterflies and other animals where males have ZZ chromosomes and females have ZW, they are the females.
Many crosses between different species of mice show this pattern, as do crosses with cats. 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 evolved to work with genes from the other chromosomes of its own species, and might not work with genes from a related species that contain even minor changes.
We know that genes on the Y evolve much faster than genes on other chromosomes, and several have a function in sperm making, which could explain the infertility of male hybrids.
So this could explain why the Neanderthal Y was lost. It also raises the possibility that it was the fault of the Y chromosome, which erected a reproductive barrier, that Neanderthals and humans became separate species in the first place.
Jenny Graves, Professor of Genetics and Vice-Chancellor, La Trobe University
This article is republished from The Conversation under a Creative Commons license. Read the original article.