Humans
PUBLISHED
If you look closely at the Y chromosome – the male sex chromosome passed down from biological fathers to their sons – it looks as if something very strange happened around 7,000 years ago.
In a relatively sudden crash, the diversity of the Y chromosome nosedived, which could suggest the male population across Europe, Africa, and Asia had been reduced to a strikingly small number. So small, in fact, that it would appear there was only one reproductive man for every 17 women.
So, what on Earth happened?
If they say Y, Y... Tell 'em that it's human nature
In a typical human, DNA is tightly packed into 46 chromosomes in (almost) every cell organized into 23 pairs, half inherited from the mother and half from the father. One of those chromosome pairs determines the chromosomal sex of a child: females have an XX pair and males have an XY pair.
While a small stretch of the X and Y chromosomes is identical, sequencing shows that over 95 percent of the Y chromosome – known as the male-specific region, or MSY – exists in males alone.
On a slight side note, this region diverges so sharply from the X chromosome that the often-repeated claim that "humans are 99.9 percent genetically identical" only truly applies when comparing two people of the same sex.
But by looking at the arrangement of genes in the Y chromosome and its MSY, scientists can trace back paternal lineages for thousands of years and potentially see glimmers of historical episodes that were otherwise left unwritten.
The strange “post-Neolithic Y-chromosome bottleneck”
In 2015, an international team of scientists studied hundreds of Y-chromosome sequences from around the world and noticed an “intense bottleneck” around 5,000 to 7,000 years ago.
It was so severe that it suggests the diversity of the Y-chromosome suddenly shrunk to one-twentieth – or around 5 percent – of its original level.
Crucially, though, the sudden drop in diversity only showed up across Afro-Eurasia, not in the Americas. Additionally, other research found that a similar bottleneck doesn't appear in mitochondrial DNA, which is inherited from the mother via the egg cell. For some reason, females were not affected.
The paper hypothesized that the genetic bottleneck was caused by “cultural changes affecting variance of reproductive success among males” without digging much deeper into what the upset might have actually looked like.
Was bloodshed and war the problem?
Then in 2018, a team of scientists at Stanford University weighed in on what might be the cause, drawing on a bunch of anthropological theory, population genomic data, and mathematical models.
They argued it’s no coincidence that the timeframe in question – approximately 7,000 years ago – lines up with a pivotal shift in how human societies were organized across a large part of the planet.
Agriculture had emerged in West Asia around 12,000 years ago, then gradually dispersed across much of Eurasia and Africa.
Societies, in turn, became increasingly complex. Life was no longer just about keeping a small, close-knit band of hunter-gatherers alive, but maintaining and managing a network of extended kinship groups spread across the region, all vying for resources with other tribes.
It has been argued that most of these were patrilineal clans, in which social status and power passed almost exclusively down the male bloodline. More often than not, violence was the most effective way to mediate differences between the testosterone-fueled groups of intensely competing humans — wipe out the rival men and your competitors won't be a problem for much longer.
Under this system, women would typically leave their birth families upon "marriage" to join the male's family. Alternatively, they could be taken as "spoils of war" or passed between groups like pawns. As you might have guessed, feminism wasn't much of a force in 5,000 BCE.
According to this theory, this social order would have sparked ruinous conflicts, with clans regularly having their male lineages extinguished, while women survived and continued to pass on their genes, regardless of which group came out on top.
Using mathematical models and computer simulations, the team showed that it would be possible for the Y-chromosome to suffer this drastic decline in diversity through generations upon generations of patrilineal clan conflict, leading to the “post-Neolithic Y-chromosome bottleneck."
However, it doesn't necessarily mean that there was "only one man for every 17 women," as some have suggested. Instead, it could mean that the rampant violence bred genetically homogeneous clans, in which all the males had extremely similar Y chromosomes derived from a single top patriarch.
"You may just have 100 people with the same Y chromosome, because they're all descended from one man." Marcus Feldman, a population geneticist at Stanford and author of the 2018 study, told Scientific American at the time.
"They all had this great-great-great-great-great-great-great-grandfather. And every male relative of that individual who's a first-degree relative has the same Y chromosome," he added.
A peaceful end
It's a compelling idea, but some scientists have other theories. One report, published in 2024, concludes instead that the same pattern could arise within patrilineal clan groups through far more peaceful means, rather than bloodshed.
In short, their model suggests some patrilineal groups would simply out-reproduce others over time, through chance and socioeconomic advantage, without any need for bloodline-ending massacres.
Nevertheless, the cause of the post-Neolithic Y-chromosome bottleneck must have been extreme. Over the past few hundred years alone, hundreds of millions of people have died in some of the biggest known wars in history, but geneticists have yet to find any evidence of a significant genetic bottleneck.
7