Could a New Sex Gene Replace the Y Chromosome as the Future of Men?

In 2002, evolutionary biologist Jenny Graves proposed a striking idea: the human Y chromosome might be on a path toward disappearance. She later elaborated that this male-specific chromosome has lost roughly 97% of its original genes over the past 300 million years. If the trend continues, she suggested, it could vanish in a few million years. The claim sparked widespread media attention, often without the careful nuance Graves intended.

Graves emphasized that her calculations were not a prediction of humanitys extinction but rather a rough scientific estimate. "It amazes me that people worry about men disappearing in 5 or 6 million years," she told ScienceAlert. "Humans have existed for only 0.1 million years. Surviving the next century is the bigger challenge."

Despite the dramatic headlines, the situation is complex. Similar Y chromosomes in other mammals, as well as in fish and amphibians, have lost their sex-determining role over evolutionary time without threatening the species survival. Certain rodents, for instance, now exist without a Y chromosome. Three mole vole speciesEllobius talpinus, Ellobius tancrei, and Ellobius alaicushave only X chromosomes, with their sex-determining genes relocated elsewhere. Spiny rats (Tokudaia osimensis) replaced the Y chromosome with a new chromosome that assumes the male-determining role.

"If a more efficient variant arose, it could quickly replace our aging Y chromosome," Graves suggested. "It might even have happened in some human populations alreadyhow would we know?" Such variants are rarely screened in genomic studies, so any transfer of sex-determining function could go unnoticed.

The debate over the Y chromosome's fate has been ongoing. Two competing perspectives dominate the discussion: one sees the Y as a deteriorating chromosome at risk of being replaced, which Graves supports; the other views it as resilient and evolutionarily stable, supported by researchers like Jenn Hughes from MITs Whitehead Institute. Hughes work shows that few essential Y genes have been lost in the last 25 million years, indicating long-term stability in primates, unlike in fish and amphibians where Y degradation is more pronounced.

"Our comparisons across mammals reveal that gene loss on the Y slowed after an initial rapid decline," Hughes explained. "The remaining genes are crucial for bodily functions, so natural selection strongly preserves them."

Graves, however, disagrees with the notion of long-term stability. She notes that some recently identified genes on the human Y are repetitive copies, which may be nonfunctional. She has described the Y chromosome as a "DNA junkyard," where duplicated genes can survive or malfunction, akin to a game of telephone where messages persist but distort over time.

The evolutionary history of the Y chromosome offers context. In early placental mammals, the X and Y were nearly identical, each containing around 800 genes. When the Y became specialized for male determination roughly 200 million years ago, it stopped recombining with the X in males, leading to gene loss. The X, still recombining in females, remained largely intact. Today, humans retain only 3% of the original shared genes.

Both Graves and Hughes agree that gene loss has not been uniform. Graves projection of total Y disappearance in 6 million years assumes a constant rate, which she herself calls highly uncertain. "It could be anytime from now to never," she remarked. Even periods of apparent stability may be temporary, as degradation can occur intermittently over evolutionary timescales.

The debate continues. At a 2011 public discussion between Hughes and Graves, scientists attending the 18th International Chromosome Conference were evenly split on whether the Y chromosome is stable or doomed, showing how deeply divided opinions remain.

While headlines often dramatize the story, the fate of the Y chromosome remains an active, unresolved question in genetics, with potential implications for the future of male biology and evolution.

Author: Harper Simmons