For millions of years, the Y chromosome has been the key determinant of male sex in humans and other mammals.
But now, scientists have uncovered a surprising fact: the Y chromosome is gradually shrinking, having lost most of its genes, and could completely disappear within the next few million years.
Does this mean that men are on the brink of extinction?
The Y Chromosome: A Miniature Hero of Sex Determination
In the human body, women carry two X chromosomes (XX), while men carry one X and one Y (XY). Though small in size and containing only around 55 active genes, the Y chromosome plays a crucial role: it houses the SRY gene, the key gene that initiates male sex development in the early embryo.
The SRY gene signals other genes, such as SOX9, to form the testes and produce male hormones like testosterone. This process takes place around 12 weeks after fertilization. Without the SRY gene, the embryo's development tends to follow the default female pathway.
However, evolutionary history shows that the Y chromosome was not always this small. It once contained over 900 genes, nearly as many as the X chromosome.
Over the past 166 million years, the Y chromosome has lost almost all of its genes. If this rate of gene loss continues, scientists estimate that the human Y chromosome could disappear entirely in about 11 million years.
Shrinking Y Chromosome: Threat or Evolution?
When news of the Y chromosome's potential extinction emerged, many quickly imagined a dramatic scenario: a future with no men. But science is never that simple.
Several animal species, such as mole voles in Eastern Europe and spiny rats in Japan, have already shown that life without a Y chromosome is still possible.
These two rodent species have completely lost their Y chromosomes, and yet they continue to reproduce. In the case of the spiny rat, researchers at Hokkaido University led by Asato Kuroiwa uncovered their secret.
By studying the DNA of both male and female rats, the team discovered a small section of DNA, only about 17,000 base pairs long, located near the SOX9 gene on chromosome 3. This DNA segment exists only in males and appears to activate SOX9 without needing the SRY gene.
When this DNA fragment was tested in laboratory mice, it significantly increased SOX9 activity, much like the SRY gene does. In other words, the spiny rat has evolved its own "replacement" for the SRY gene, rendering the Y chromosome unnecessary.
What Does This Mean for the Future of Humanity?
For humans, this situation presents two possibilities. The first: if the Y chromosome truly disappears without a replacement, we would lose the ability to produce males. Gradually, our species could face extinction due to the inability to reproduce sexually.
Unlike some reptiles that can reproduce without males (through parthenogenesis), humans cannot. We require sperm, which means we need males.
However, the second, and more optimistic, scenario suggests that, much like spiny rats, humans might eventually evolve and develop a new sex-determination system. This means that a new gene capable of activating SOX9, or another relevant pathway, could naturally emerge in the future and take over the role of the SRY gene.
But this scenario comes with an intriguing consequence: if human populations in different parts of the world evolve distinct sex-determination systems, it's possible that entirely new human species could arise, each with reproductive systems that are incompatible with the others. We may not go extinct, but we could split into several types of humans, each with its own unique biology.
Could This Be the Start of a New Era?
So, is this the end for the Y chromosome and for men? Perhaps yes, perhaps no. The Y chromosome is indeed on a path toward extinction, but nature has a remarkable way of adapting and surviving. The discovery in spiny rats offers a glimmer of hope that we, as a species, might also find a new evolutionary path forward.
Though the future remains uncertain, one thing is clear: the Y chromosome is racing against time, and science is racing to understand where we're headed next.
