The Y chromosome is the runt of our 46-chromosome litter. Despite its well-known role—determining whether a mammal will be male—it pales in comparison to the other chromosomes, especially its partner, X. Indeed, 200 million to 300 million years ago Y shared roughly 600 genes with X. Today they share only 19. Those losses, some geneticists noted in 2002, indicated Y was actually rotting away. Give it another 10 million years, they said, and Y would be extinct. Others then wondered whether males would go with it.

But Y has stopped shedding those genes, according to recent research, and, in fact, has been stable for the past 25 million years, says David Page, a biologist at the Massachusetts Institute of Technology and an author of the study, which appeared in Nature. (Scientific American is part of Nature Publishing Group.) Page and his colleagues found that although Y was much skimpier in more recently evolved species, the attrition had stopped millions of years ago.

That stability may come from a core of about 12 genes that have nothing to do with sex and are instead responsible for vital cellular functions in the heart, blood, lungs and other tissues. “These are powerful players in the central command room of cells,” and natural selection would favor their survival, Page says.

One proponent of the rotting Y idea is not convinced. The past several million years may simply be a lull, says Jennifer Graves, a geneticist at the Australian National University. She notes that at least two rodent groups have managed to dispense with it altogether. The new research suggests, however, that Y will remain at its current, if slight, size.