The Y chromosome, at the bottom right of this set of human
chromosomes, is dwarfed by the X.
Over the last few decades, scientists and journalists have speculated that the end of man—men, that is—was nigh. The biological reason for this possibility is the ever-shrinking Y chromosome: 300-200 million years ago, the Y, like females’ X chromosome, had hundreds of genes, but it now contains less than 80, 19 of which code for specifically male traits such as sperm production. This remarkable contraction set people’s imaginations spinning, especially after an opinion piece said in Nature 10 years ago that the Y chromosome might disappear, as it already has in voles, in 10 million years.
A Nature paper published this week, however, may indicate that the Y is sticking around. Biologists at the Whitehead Institute have compared the Y chromosome of rhesus monkeys with the human Y chromosome, and they’ve found that the two have the same number but one of key male-specific genes. This implies that the human Y chromosome’s shrinkage, at least when it comes to key genes, stopped at least around 25 million years ago, when the common ancestor of humans and rhesus monkeys was alive. The team says that this 25 million years of stasis indicates that the Y’s days of sloughing genes are over, that the genes it carries now are the essential ones and cannot be removed without seriously impacting reproductive function, while the genes lost in the past were expendable.
It’s hard to say that evolution of the Y chromosome has categorically ceased, though—evolution doesn’t necessarily follow a straight line. And it’s worth remembering that we had males before we had the Y chromosome: the male genes, at that time, were just spread across the genome. Even if more shrinking events eventually do send the Y the way of the leisure suit, it doesn’t mean that males will follow suit.
Image courtesy of National Human Genome Research Institute