The sex of human and other mammal babies is determined by a male-determining gene on the Y chromosome. However, this crucial chromosome is gradually degenerating and may disappear in a few million years, potentially leading to our extinction unless a new sex-determining gene evolves.
The good news is that two branches of rodents have already lost their Y chromosome and survived. A 2022 study published in peer review journal ‘Proceedings of the National Academy of Science’ reveals that the spiny rat has successfully evolved a new male-determining gene, offering hope for humanity’s future.
Role of the Y chromosome
In humans, females have two X chromosomes, while males possess one X and one Y chromosome. The Y chromosome, though much smaller with only about 55 genes compared to the X chromosome’s 900, plays a vital role in determining male sex by triggering the development of the testis in an embryo.
At around 12 weeks after conception, the master gene on the Y chromosome, known as SRY (sex-determining region Y), activates a genetic pathway that leads to the formation of male reproductive organs. This gene works by stimulating another key gene, SOX9, which is crucial for male development across vertebrates.
Decline of the Y chromosome
Most mammals share a similar X and Y chromosome structure, but this system presents challenges due to the unequal gene dosage between males and females. Interestingly, Australia’s platypus possesses entirely different sex chromosomes, resembling those of birds, suggesting that the mammal X and Y chromosomes were once ordinary chromosomes.
Over the 166 million years since humans and platypuses diverged, the Y chromosome has lost a significant number of active genes, shrinking from 900 to just 55. If this trend continues, the Y chromosome could vanish entirely within the next 11 million years.
This potential disappearance has sparked debate among scientists, with predictions ranging from the Y chromosome lasting indefinitely to vanishing within a few thousand years.
Rodents without a Y chromosome
Fortunately, two rodent lineages—the mole voles of eastern Europe and the spiny rats of Japan—have already lost their Y chromosome and continue to thrive. In these species, the X chromosome remains in both males and females, but the Y chromosome and SRY gene have disappeared.
A research team led by Asato Kuroiwa from Hokkaido University discovered that in spiny rats, most genes from the Y chromosome had been relocated to other chromosomes. However, the SRY gene was missing, and they found a small duplication near the SOX9 gene on chromosome 3 in males, which could substitute for SRY.
When introduced into mice, this duplication increased SOX9 activity, suggesting that spiny rats have evolved a new mechanism for male sex determination without the Y chromosome.
Implications for the future of humanity
The possible disappearance of the human Y chromosome raises concerns about the future of our species. Unlike some reptiles that can reproduce asexually, mammals, including humans, require sperm to reproduce, making men indispensable for the continuation of our species.
However, the evolution of a new sex-determining gene, as seen in spiny rats, offers a glimmer of hope. Yet, this process comes with risks — if multiple new sex-determination systems evolve in different regions, it could lead to the emergence of new human species, each with distinct sex chromosomes.
In a scenario where someone visits Earth in 11 million years, they might not find humans as we know them today. Instead, they might encounter several different human species, each kept apart by their unique sex-determination systems.
First Published: Aug 27 2024 | 12:01 PM IST
