Another reason for the intensity of selective pressures on the Y chromosome in both chimps and humans may be that natural selection sees it as a single unit, so a change in any one of its genes affects the survival of all the rest. On the other chromosomes, selection is more focused on individual genes because chunks of DNA are swapped between the members of each pair of chromosomes before the generation of eggs and sperm.

This DNA swapping process is forbidden between the X and the Y pair, keeping the male-determining gene from being transferred into the X chromosome, creating gender chaos.

But this prohibition has caused most of the genes on the Y chromosome to decay for lack of fitness. In the rest of the genome, a gene damaged by a mutation can be swapped out for the good copy on the other chromosome.

In the Y, which originally had the same set of genes as the X, most of the X-related genes have disappeared over the last 200 million years. Until now, many biologists have assumed either that the Y chromosome was headed for eventual extinction, or that its evolutionary downslide was largely over and it has sunk into stagnation.

Dr. Page’s new finding is surprising because it shows that the Y chromosome has achieved an unexpected salvation. The hallmark of the Y chromosome now turns out to be renewal and reinvigoration, once the unnecessary burden of X-related genes has been shed.

“Natural selection is shaping the Y and keeping it vital to a degree that is really at odds with the idea of the last 50 years of a rotting Y chromosome,” Dr. Page said. “It is now clear that the Y chromosome is by far the most rapidly evolving part of the human and chimp genomes.”

This does not mean that men are evolving faster than women, given that the two belong to the same species, but it could be that the Y’s rate of change drives or influences the evolution of the rest of the human genome in ways that now need to be assessed. It would be “hard to imagine that these dramatic changes in the Y don’t have broader consequences,” Dr. Page said.