The sex of human and other mammal babies hinges on a gene found on the Y chromosome, which plays a key role in developing males. However, the Y chromosome is slowly deteriorating and could disappear within a few million years, raising fears about the future of humans unless a new sex-determining gene emerges. Interestingly, some rodents have already lost their Y chromosome and managed to adapt, providing a glimpse of hope for our species.

How the Y Chromosome Determines Sex

In humans, females have two X chromosomes, while males have one X and one Y chromosome. The Y chromosome is small, containing only about 55 genes, compared to the X chromosome's 900 genes. Despite its size, the Y chromosome carries a crucial gene known as SRY (sex-determining region on the Y), which triggers male development in the embryo. Around 12 weeks after conception, this gene activates others that lead to the formation of the testes, which produce male hormones, ensuring the baby develops as male.

The Disappearing Y Chromosome

Most mammals share a similar X and Y chromosome system, but this comes with challenges due to the unequal dosage of X genes between males and females. Surprisingly, Australia's platypus has sex chromosomes more akin to those of birds, suggesting that the mammalian X and Y chromosomes were once an ordinary pair of chromosomes. Over millions of years, the Y chromosome has lost most of its active genes, leading to predictions that it might disappear within 11 million years.

Rodents That Lost Their Y Chromosome

Despite the alarming possibility of the Y chromosome's disappearance, some rodents offer a glimmer of hope. The mole voles of Eastern Europe and the spiny rats of Japan have lost their Y chromosome and the SRY gene but continue to survive. Research on the spiny rats, conducted by Hokkaido University biologist Asato Kuroiwa, revealed that most Y-linked genes had relocated to other chromosomes. However, the gene that substitutes for SRY was not initially found.

In 2022, Kuroiwa's team identified a small duplication near the SOX9 gene on chromosome 3 in spiny rats, which may serve as a new sex-determining switch. This discovery suggests that while the Y chromosome may disappear, a new mechanism for determining sex could evolve.

What This Means for the Future of Humanity

The potential disappearance of the Y chromosome has sparked discussions about the future of the human race. While some lizards and snakes can reproduce without males through parthenogenesis, humans and other mammals require sperm and men for reproduction. This means that the loss of the Y chromosome could lead to human extinction. However, the evolution of a new sex-determining gene could prevent this, albeit with the risk of different systems evolving in different regions, potentially leading to the emergence of new human species.

In 11 million years, Earth might be home to no humans at all or several distinct human species, each with its own sex determination system. The future of our Y chromosome remains uncertain, but the adaptability seen in rodents suggests that evolution might offer a solution.