Y chromosome study stirs a debate over men's evolution

A new study comparing the Y chromosomes from humans and chimpanzees, our nearest living relatives, show that they are about 30 percent different. That is far greater than the 2 percent difference between the rest of the human genetic code and that of the chimp's, according to a study appearing online Wednesday in the journal Nature.

These changes occurred in the last 6 million years or so, relatively recently when it comes to evolution.

“The Y chromosome appears to be the most rapidly evolving of the human chromosomes,” said study co-author Dr. David Page, director of the prestigious Whitehead Institute in Cambridge and a professor of biology at MIT. “It's an almost ongoing churning of gene reconstruction. It's like a house that's constantly being rebuilt.”

Before men get too impressed with themselves, lead author Jennifer Hughes offers some words of caution: Just because the Y chromosome, which determines gender, is evolving at a speedy rate it doesn't necessarily mean men themselves are more evolved.

Researchers took the most detailed examination of the Y chromosome, which females do not have, of both humans and chimps and found entire sections dramatically different. There were even entire genes on the human Y chromosome that weren't on the chimp, said Hughes, also of the Whitehead Institute.

The two-year research took twice as long as expected because of the evolutionary changes found, Hughes said.

There is a bit of a proviso to the comparison to other chromosomes. While all human and chimp chromosomes have been mapped, only two chimp chromosomes have been examined in great detail: Y and chromosome 21. Yet, there's still enough known to make the claim that the Y is the speediest evolver, Hughes and Page said.

Until recently the Y chromosome was considered the unassuming figure of genetics, especially because it had fewer genes than other chromosomes. A few years ago some researchers even suggested that the Y chromosome was shrinking so that in 50,000 years it would just disappear — and so would men.

There are a couple of reasons Page and Hughes cite for Y being such an evolutionary powerhouse. One is that it stands alone and isn't part of a pair like 44 other chromosomes. So when there are mutations there's no matching chromosome to recombine and essentially cover up the change, Hughes said. Because women have two X chromosomes, the X chromosome doesn't have this situation.

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