Tiny compared to the X and with barely fifty functional genes, the Y chromosome has been grabbing headlines for decades, often with apocalyptic overtones. This attention has focused, above all, on two issues: its degradation throughout evolution – and speculation about an eventual “end of men” – and its role in male health, especially at advanced ages, when its loss in some cells is associated with various pathologies.

Both debates advance in parallel and are sometimes confused, but the reality is more nuanced: there is broad consensus that the Y chromosome has suffered considerable historical degradation, discussion persists about its very long-term fate and, at the same time, data is accumulating that links its cellular loss with possible implications for health. Everything at once.

So, to understand this genetic controversy, it is best to start at the beginning.

The evolutionary origin of the Y chromosome

Approximately 170 to 180 million years ago, the sex chromosomes of mammals formed a virtually identical pair. Over time, that ancestral pair differentiated to give rise to the current X and Y chromosomes.

In humans and other mammals, sex is determined chromosomally: women usually have two X chromosomes and men, one X and one Y. Although this pair represents only about 4% of the total DNA, according to reports BBC Science Focusits biological role goes far beyond determining the development of testicles or ovaries.

The Y chromosome, in particular, stands out for its reduced gene content. While the

One of them, the SRY gene, acts as a trigger for male development. Some others are involved in sperm production. The functions of the rest continue to be the subject of study and debate, which has contributed to the perception of the Y as an atypical chromosome from an evolutionary point of view.

Australian evolutionary biologist Jenny Graves has been studying this degradation process for decades. In an article published in The Conversation in 2014, estimated that if gene loss continued at the observed rate – almost ten genes per million years since the divergence of humans and platypuses 166 million years ago – the Y chromosome could disappear in about 4.5 million years.

The statement, included almost in passing in a 2004 white paper, provoked a disproportionate reaction. “I’m very surprised that anyone would worry about the extinction of men in five or six million years,” Graves later told Science Alert, remembering that the human species has not even existed for 100,000 years.

Why does the Y chromosome degrade?

According to Graves, there are two main reasons. As stated to BBC Science Focus in 2024, the first is its biological context. The Y chromosome is transmitted exclusively through the male line and, in each generation, it passes through the testes, which constitute a genetically demanding environment. Sperm production requires numerous cell divisions, each of which provides a new opportunity for mutations to accumulate.

The second, and perhaps more decisive, is its isolation. Unlike the rest of the chromosomes, the Y does not have a homologue with which to exchange DNA segments to correct errors. In most cases, chromosomes “lean on” their partner during recombination to compensate for damage. The Y lacks that structural support, so accumulated alterations are much more difficult to eliminate.

Is he really doomed?

Not the entire scientific community shares the “pessimistic” diagnosis. As collected Vice At the end of last year, evolutionary biologist Jenn Hughes, from the Whitehead Institute at MIT, has held the opposite position for years: the essential genes on the human Y chromosome have remained stable. for approximately 25 million years, a conclusion reinforced by later studies in primates.

Their argument is that genes that have survived perform crucial functions throughout the organism, creating strong evolutionary pressure to conserve them. From this perspective, the Y chromosome would not be disappearing, but rather stabilizing.

Jenny Graves does not deny that stability, but she introduces a nuance: just because something remains stable today does not mean that it will last indefinitely. The Y chromosome contains abundant repeated sequences that are susceptible to degradation generation after generation, and genes that seem stable today could be replaced if evolutionary conditions change.

She summarizes, according to Vicethe time horizon of the And as “anything from now to never.” When both scientists debated publicly in 2011, The audience was divided equally. The discussion remains open.

What we do know is that alternative systems of sexual determination have emerged in other animals. According to BBC Science Focussome Eastern European mole rats and Japanese spiny rats have completely lost the Y chromosome and have redistributed or replaced its functions on other chromosomes. And they continue to be viable populations. Fruit flies, for their part, have lost almost all of their genes.

Graves argues that if a more efficient sex-determining gene emerged in a small, isolated human population – where genetic accidents are more likely – it could spread without needing to eliminate visible male traits. “Maybe it has already happened in a human population somewhere,” he told ScienceAlert.

Loss of the Y chromosome and diseases in older men

The evolutionary debate millions of years in the future coexists with a much more immediate concern: the loss of the Y chromosome in body cells as men age.

In a recent article in The Conversation, Graves explains that new genomic detection techniques show that this phenomenon is common in tissues from older men. Over time, the Y chromosome disappears from some cells and their descendants no longer recover it.

The tissue ends up becoming a heterogeneous mixture: cells that maintain the Y chromosome coexist with others that have lost it. According to data collected by Graves, 40% of 60-year-old men already have this loss, a figure that rises to 57% in 90-year-old men. Environmental factors such as smoking or exposure to carcinogens also play a role.

For years it was assumed that this loss was irrelevant. If the Y chromosome contains few genes and cells can survive without it, it seemed logical to think that its absence would not have major consequences. However, according to Graves, recent data points in another direction. Various studies have found associations between the loss of the Y chromosome and cardiovascular, neurodegenerative diseases and different types of cancer.

A large-scale German study observed that men over 60 years of age with a high frequency of Y loss were at greater risk of heart attacks. It has also been detected This loss is ten times more frequent in Alzheimer’s patients, and has been linked to higher mortality from COVID-19, which could help explain part of the mortality gap between men and women during the pandemic.

The challenge of causality

As Graves explains, establishing a causal relationship is complex: diseases could cause the loss of Y, or a third factor could be behind both phenomena. However, an experiment with mice suggests a possible direct effect. By transplanting blood cells without a Y chromosome into irradiated mice, the animals developed more pathologies associated with aging, including alterations in cardiac function and cases of heart failure.

How can a chromosome with so few genes have such a broad impact? Part of the answer lies in the fact that several of its genes are expressed throughout the body, not just in the testicles, and some act as tumor suppressors. In addition, the Y chromosome harbors non-coding genes that appear to regulate the activity of other genes on different chromosomes. Its loss could, therefore, alter mechanisms that go far beyond sex determination.

Will men disappear then?

Not overnight. Humans cannot reproduce by parthenogenesis and there are at least 30 imprinted genes that must come from sperm. If the Y chromosome were to disappear, the alternative would not be immediate extinction, but rather the evolution of a new system of sexual determination. In theory, that process could even lead, in the very long term, to a differentiation of species, as Graves suggests.

At the moment, there is no evidence of an imminent evolutionary emergency. The Y chromosome could persist for millions more years or transform into something different. Its future trajectory remains uncertain, but it is far from an immediate outcome.

Furthermore, complete DNA sequencing of the human Y chromosome was achieved just a couple of years ago, so our detailed knowledge about how it works is still relatively recent. More than the obituary of a chromosome in decline, this new stage could allow a more detailed understanding of its role in human biology and evolution.