Human biological sex is determined by the sex chromosomes X and Y. In most cases, females possess two X chromosomes, while males have one X and one Y. However, there is some evidence that the Y chromosome is being phased out of the evolutionary pipeline, and could one day disappear altogether — potentially giving rise to a new sex gene in the process.
Why is the Y chromosome disappearing?
The Y chromosome has seen significant changes over the course of human evolution. "In comparison to the X, the Y is tiny. There are only 45 genes on the Y chromosome, and it is just one of these that makes you male," Jenny Graves, a geneticist and professor at the La Trobe Institute for Molecular Science, said to BBC Science Focus. "A few others help make sperm, but for the others, we don't quite know why they are there. We just can't really get rid of them. That's compared to between 900 and 1400 in the X." The human Y did once contain as many genes as the X chromosome, but has lost them in the past 166 million years. As a result, most of the Y today is made up of repetitive "junk DNA." With such an unstable composition, the Y chromosome is at risk of disappearing over multiple generations.
Males tend to only have one copy of the Y chromosome, so it does not have the opportunity to go through genetic recombination, or the "shuffling of genes that occurs in each generation which helps to eliminate damaging gene mutations," Darren Kent, a professor at the University of Kent and Peter Ellis, a lecturer at the University of Kent, said at The Conversation. Without recombination, Y chromosomal genes degenerate over time. "This significant gene loss over the centuries has led scientists to predict that the Y chromosome might vanish entirely in about 11 million years," said Earth.com.
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What are the implications?
The most important part of the Y chromosome is the master sex gene called SRY, which triggers a fetus' development as a biological male. "The X and Y were once upon a time just an ordinary pair of chromosomes. Then one partner acquired a variant gene (SRY) that determines maleness," Graves said to Newsweek. So what happens if the Y disappears? "When humans run out of Y chromosome, they might become extinct (if we haven't already extincted ourselves long since), or they might evolve a new sex gene that defines new sex chromosomes." Rest assured, this type of evolution will take millions of years.
Some clues may be found in rats. "We could simply evolve a new sex-determining gene. That's what we've already seen happen in some types of rodents, and it seems to have worked successfully," said Graves to Science Focus. A 2022 study published in the journal PNAS found that the important Y chromosome genes could be capable of relocating to other genes, as observed in mice. There is also the possibility that humans develop an entirely new sex gene. "This evolutionary change could have profound implications for human evolution," said the Economic Times. "Scientists caution that the development of multiple sex-determining systems in different human populations could lead to the emergence of new human species."
Still, human males are not going away any time soon. Some experts firmly believe that the Y chromosome will not disappear at all, claiming that it has several defense mechanisms in place. A 2017 study published in the journal PLOS Genetics said the chromosome's evolution of palindromes — DNA sequences that read the same forward as backward — and repetitive copies of genes could keep it from degrading faster by allowing "damaged genes to be repaired using an undamaged back-up copy as a template." On a positive note, "by embracing ongoing research and understanding the complexities of genetics, we can find reassurance in the resilience of our species," said Earth.com.
