Image: X and Y chromosome
NHGRI
A comparison of human X and Y chromosomes shows how small the Y chromosome is.
updated 1/13/2010 1:38:41 PM ET 2010-01-13T18:38:41

Women may think of men as primitive, but new research indicates that the Y chromosome — the thing that makes a man male — is evolving far faster than the rest of the human genetic code.

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."

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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.

Finally, some respect
Until recently the Y chromosome was considered the Rodney Dangerfield 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.

"The story is not as cut and dried as many would have liked to predict," Hughes said. "It's kind of fun to say that men are going to die out, but the science is proving — now that we've got data — that that's not true at all."

Page agreed. "The Y chromosome has many more tricks up its sleeve than it was given credit for," he said.

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.

Another reason has to do with the nature of mating. When female chimps are in heat, they mate frequently and with many partners, so there is an evolutionary pressure on the male to produce the most and best sperm to propagate his genes, Page said.

To test this out, Hughes said she hopes to soon examine the Y chromosomes of a rhesus macaque, which is fairly promiscuous, and the marmoset, which is more monogamous than early humans probably were.

Outside scientists praised the study.

"Wow," said R. Scott Hawley, a genetics researcher at the Stowers Institute in Kansas City. "That result is astounding."

"The Y chromosome clearly has the strength and tenacity to fight back," said Hawley, who wasn't part of the research. "I certainly think the Y chromosome has taken a bad rap for a long time with people doing maps showing areas for channel surfing."

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