Neanderthals’ original Y chromosomes were gradually wiped out in the wake of interbreeding with modern humans more than 100,000 years ago, a study found.
Once injected into the Neanderthal gene pool, the ‘superior’ Y chromosomes from the humans spread throughout the ancient hominid population by natural selection.
This saw the Neanderthals’ own Y chromosomes — which had acquired bad mutations as a result of their small population size — ultimately get totally replaced.
The process would have taken in the order of tens of thousands of years, German experts predicted based on models of the Neanderthals’ shifting genetic makeup.
The findings support the results of previous studies, which found genetic transfer occurred from humans to Neanderthals prior to the major out-of-Africa migration.
The presence or absence of a Y chromosome in mammals typically determines an individuals’ sex — with the Y chromosome only passed down from father to son.
Neanderthals’ original Y chromosomes were gradually wiped out in the wake of interbreeding with modern humans more than 100,000 years ago, a study found (stock image)
Once injected into the Neanderthal gene pool, the ‘superior’ Y chromosomes from the humans (blue) spread throughout the ancient hominid population by natural selection. This saw the Neanderthals’ own Y chromosomes (light green)— which had acquired bad mutations as a result of their small population size — ultimately get totally replaced
HOW NEANDERTHAL Y CHROMOSOMES ENDED UP REPLACED
Neanderthals and modern humans met and interbred some time between 370,000 and 100,000 years ago.
Some modern human genetic material was passed into the the Neanderthal gene pool — perhaps, say 5 per cent.
The original Neanderthal Y chromosome had become evolutionarily ‘less fit’ as a result of their small population size.
This had allowed detrimental mutations to build up in their original Y chromosomes.
As Neanderthals went on reproducing with each other, the superior Y chromosomes from the humans were ‘chosen’ by natural selection over the Neanderthals’ own Y chromosomes.
Over tens of thousands of years, this eventually saw the Neanderthals original Y chromosomes completely wiped out from their gene pool.
According to the researchers, this would have happened even if there had only been one interbreeding-based ‘injection’ of modern human genes into the Neanderthal gene pool.
However, the replacement could potentially have been hastened by further interbreeding episodes.
Until now, studies of the Y chromosomes of early hominids like Neanderthals had not been possible — given, as fate would have it, that the best-preserved specimens that could yield enough DNA for analysis have all been female.
In the study, however, the researchers were able to identify three male Neanderthals and two Denisovans — another species of archaic human — that were suitable subjects for DNA analysis.
The team developed a method to filter out the target Y chromosome molecules from the microbial DNA that typically contaminates ancient remains — after using which they were able to reconstruct each specimen’s Y chromosome sequence.
When they compared these sequences from the archaic specimens with both each other and those of modern humans, the researchers found that Neanderthal Y chromosomes are more similar to those of people living today than the Denisovans’.
‘This was quite a surprise to us,’ said paper author and evolutionary geneticist Martin Petr of the Max Planck Institute for Evolutionary Anthropology in Leipzig.
‘We know from studying their autosomal DNA that Neanderthals and Denisovans were closely related and that humans living today are their more distant evolutionary cousins,’ he explained.
‘Before we first looked at the data, we expected that their Y chromosomes would show a similar picture.’
Furthermore, the team calculated that the most recent common ancestor of the Neanderthal and modern human Y chromosome would have existed around 370,000 years ago — a date much more recent than had been previously thought.
The researchers said that they were initially confused as to how the Y chromosomes from modern humans came to completely replace the Neanderthals’ native ones.
However, their modelling revealed that the relatively small size of the Neanderthal population allowed detrimental mutations to build up in their original Y chromosomes — which made those genes less evolutionarily fit.
(This would have worked in a similar way to how breeding within small groups can increase the risk of certain diseases — such as how haemophilia ended up being common in the European royal families of the 19th and 20th Centuries.)
When interbreeding gave the Neanderthal gene pool a small influx of genetic material from modern humans, this placed the original Neanderthal Y chromosomes in competition with those from modern humans — a war they were destined to lose.
‘Given the important role of the Y chromosome in reproduction and fertility, the lower evolutionary fitness of Neanderthal Y chromosomes might have caused natural selection to favour the Y chromosomes from early modern humans,’ Mr Petr said.
This, he added, ‘eventually lead to their replacement.’
Models suggests that if only 5 per cent of the Neanderthal gene pool was altered by interbreeding with modern humans — and Neanderthal Y chromosomes were 1–2 per cent less fit — there would be 25–50 per cent chance of total Y chromosome replacement within 50,000 years.
The process of replacing the Neanderthal’s original Y chromosomes would have taken in the order of tens of thousands of years, German experts predicted
In the study, the researchers were able to identify three male Neanderthals and two Denisovans — another species of archaic human — that were suitable subjects for DNA analysis. Previously work has typically involved female specimens. Pictured, the upper molar of a male Neanderthal unearthed in Spy, Belgium, as seen from multiple angles
The team developed a method to filter out the human Y chromosome molecules from the microbial DNA that typically contaminates ancient remains — after using which they were able to reconstruct each specimen’s Y chromosome sequence. Pictured, researcher Matthias Meyer works on samples at the Max Planck Institute for Evolutionary Anthropology
The researchers are optimistic that it will be possible in the near future to put the replacement hypothesis to the test.
‘If we can retrieve Y chromosome sequences from Neanderthals that lived prior to this hypothesized early introgression event,’ began paper author and computational biologist Janet Kelso, also of the Max Planck Institute for Evolutionary Anthropology.
‘…we predict that they would still have the original Neanderthal Y chromosome and will therefore be more similar to Denisovans than to modern humans,’ she finished.
One potential target for such analysis, she noted, would be the Neanderthals from the Sima de los Huesos cave site in Spain — whose remains have been dated back to around 430,000 years ago.
The full findings of the study were published in the journal Science.
A close relative of modern humans, Neanderthals went extinct 40,000 years ago
The Neanderthals were a close human ancestor that mysteriously died out around 40,000 years ago.
The species lived in Africa with early humans for millennia before moving across to Europe around 300,000 years ago.
They were later joined by humans, who entered Eurasia around 48,000 ears ago.
The Neanderthals were a cousin species of humans but not a direct ancestor – the two species split from a common ancestor – that perished around 50,000 years ago. Pictured is a Neanderthal museum exhibit
These were the original ‘cavemen’, historically thought to be dim-witted and brutish compared to modern humans.
In recent years though, and especially over the last decade, it has become increasingly apparent we’ve been selling Neanderthals short.
A growing body of evidence points to a more sophisticated and multi-talented kind of ‘caveman’ than anyone thought possible.
It now seems likely that Neanderthals had told, buried their dead, painted and even interbred with humans.
They used body art such as pigments and beads, and they were the very first artists, with Neanderthal cave art (and symbolism) in Spain apparently predating the earliest modern human art by some 20,000 years.
They are thought to have hunted on land and done some fishing. However, they went extinct around 40,000 years ago following the success of Homo sapiens in Europe.