The end of men? Generations of war are to blame for the collapse of diversity of the Y chromosome
From around 7,000 years ago, something strange seems to have happened to men.
Genetic records show that their diversity collapsed almost entirely, and stayed that way for the next 2,000 years.
So extreme was the collapse in the Y chromosome that it was as if there were only one man left to mate for every 17 women.
Now scientists think they know why.
According to researchers at Stanford University, the collapse was the result of generations of war between clans, whose membership is determined by male ancestors.
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Generations of war are to blame for the collapse in diversity of the Y chromosome starting around 7,000 years ago, scientists say. Pictured is a skeleton from 7,500 BC
The 'Neolithic Y-chromosome bottleneck' marks a time when human genetic diversity took a dramatic nosedive.
The slaughter was so severe the male population was decimated to one-twentieth of its original level.
However, by looking at genetic patterns in modern humans, researchers found that the decline was observed only in men - specifically on the Y chromosome, which fathers pass on to their sons.
'An abrupt population bottleneck specific to human males has been inferred across several Old World (Africa, Europe, Asia) populations 5000–7000 BP', researchers, led by Tian Chen Zeng, a Stanford undergraduate in sociology, wrote in their paper published in Nature.
This suggested large numbers of men were dying before having sex and producing children which led researchers to suggest it could a result of generations of warfare.
The team also thought that it might have been compounded by changing social structures - specifically the creation of patrilineal clans.
After the onset of farming and herding around 12,000 years ago, societies grew increasingly organised around extended kinship groups, which were largely patrilineal.
This cultural fact has potentially significant biological consequences as it is key how clan members are related to each other.
The Y chromosome is one of the sex chromosomes in animals and is responsible for the sex determination of unborn offspring. Previous research has suggested the Y chromosome has just 4.6 million years before it completely disappears
WHAT IS THE Y CHROMOSOME AND WHY IS IT DISAPPEARING? The Y chromosome is one of two sex chromosomes found in humans - the other is the X chromosome. It is the only chromosome in an organism that isn't essential for life - women survive just fine without one, after all. In humans, the 22 other pairs of chromosomes - the autosomes - are identical. The Y chromosome spans more than 59 million building blocks of DNA and represents almost 2 percent of the total DNA in cells. But the human Y-chromosome is still one of the smallest in the genome. It is carried by roughly half of a man's sperm, and dictates whether a child will be male or female. Despite this, it carries very little other important information. And researchers think it is quickly disappearing. The number of genes on the Y has dropped from over 1,000 to roughly 50, a loss of more than 95 per cent. If the same rate of degeneration continues, the Y chromosome has just 4.6 million years left before it disappears completely. But the Y chromosome hasn't always been so small. It was once the size of the X chromosome and contained all the same genes. The problem, however, is that Y chromosomes are only ever found as a single copy, passed from fathers to their sons, rather than a pair This means that genes on the Y chromosome cannot undergo something known as 'genetic recombination'. This is switch of genes that takes place in each generation which helps to eliminate damaging gene mutations.
While women may have married into a clan, men in such clans are all related through male ancestors.
This meant they tended to have the same Y chromosomes.
Researchers point out that if there are wars between the clans and wars repeatedly wiped out entire clans over time, they could also wipe out a good many male lineages and their unique Y chromosomes in the process.
This would therefore greatly reduce overall diversity.
To test their ideas about the 'Neolithic Y-chromosome bottleneck', the Stanford researchers used mathematical models and computer simulations in which men fought and died.
As predicted, wars between patrilineal clans drastically reduced Y chromosome diversity over time.
However, conflict between non-patrilineal clans - groups where both men and women could move between clans - did not.
Now the researchers are looking at applying the framework in other areas - anywhere 'historical and geographical patterns of cultural interactions could explain the patterns you see in genetics,' said one of the researchers Professor Marcus Feldman a professor of biology in Stanford's School of Humanities and Sciences.
The outlines of the idea came to Mr Zeng who shared his idea with Alan Aw, a fellow Stanford undergraduate studient in mathematical and computational science.
They then went on to get a paper published about their findings.
'Working with these talented guys is a lot of fun', said Professor Feldman.
This latest study follows on from previous research which suggested the Y chromosome has just 4.6 million years left.
This may sound like a long time, but it isn't when you consider that life has existed on Earth for 3.5 billion years.
The early 'proto-Y' chromosome was originally the same size as the X chromosome and contained all the same genes.
However, Y chromosomes have a fundamental flaw, according to Dr Peter Ellis and Professor Darren Griffin from the University of Kent.
Unlike all other chromosomes, which we have two copies of in each of our cells, Y chromosomes are only ever present as a single copy, passed from fathers to their sons.
This means that genes on the Y chromosome cannot undergo genetic recombination, the 'shuffling' of genes that occurs in each generation which helps to eliminate damaging gene mutations.
Deprived of the benefits of recombination, Y chromosomal genes degenerate over time and are eventually lost from the genome.