The sex of human and other mammal babies is determined by a gene on the Y chromosome, which is essential for male development. However, this vital chromosome is slowly deteriorating and could vanish in a few million years, possibly threatening our survival unless a new sex-determining gene emerges. On a positive note, two rodent lineages have already lost their Y chromosome and continued to thrive. A 2022 study published in the *Proceedings of the National Academy of Sciences* shows that the spiny rat has developed a new gene to determine maleness, providing hope for the future of humanity.
**Role of the Y Chromosome**
In humans, females have two X chromosomes, while males have one X and one Y chromosome. Although the Y chromosome is significantly smaller, containing about 55 genes compared to the X chromosome's 900, it is crucial for determining male sex. It does this by initiating the development of the testes in an embryo.
Approximately 12 weeks after conception, the SRY (sex-determining region Y) gene on the Y chromosome activates a genetic pathway that leads to the formation of male reproductive organs. This gene triggers the SOX9 gene, which is essential for male development in vertebrates.
**Decline of the Y Chromosome**
While most mammals have a similar structure for X and Y chromosomes, this system faces challenges due to the uneven gene dosage between males and females. Notably, the platypus from Australia has sex chromosomes that differ greatly from those in other mammals, resembling the sex chromosomes found in birds. This suggests that the mammalian X and Y chromosomes may have originally been ordinary chromosomes.
Over the past 166 million years since the divergence of humans and platypuses, the Y chromosome has lost many of its active genes, shrinking from 900 to just 55. If this trend continues, the Y chromosome could disappear entirely within the next 11 million years.
**Rodents Without a Y Chromosome**
Fortunately, two rodent lineages—the mole voles of Eastern Europe and the spiny rats of Japan—have successfully lost their Y chromosome and continue to thrive. In these species, while the X chromosome is present in both males and females, the Y chromosome and its key gene, SRY, have disappeared.
A research team led by Asato Kuroiwa from Hokkaido University found that in spiny rats, most genes originally on the Y chromosome have been relocated to other chromosomes. However, the SRY gene is absent. The team discovered a small duplication near the SOX9 gene on chromosome 3 in males, which appears to serve as a substitute for SRY. When introduced into mice, this duplication enhanced SOX9 activity, indicating that spiny rats have evolved a new mechanism for determining male sex without the Y chromosome.
**Implications for the Future of Humanity**
The potential loss of the human Y chromosome raises significant concerns about the future of our species. Unlike some reptiles that can reproduce asexually, mammals, including humans, need sperm for reproduction, making men essential for our species' survival.
Nevertheless, there is hope in the form of new sex-determining genes, as demonstrated by spiny rats. However, this evolution carries risks. If multiple new sex-determination systems arise independently, it could lead to the development of new human species with distinct sex chromosomes.
In an imaginative future scenario, if someone were to visit Earth in 11 million years, they might not find humans as we know them today. Instead, they could encounter several different human species, each separated by their own unique sex-determination systems.