OTTAWA — The one chunk of DNA that defines all men is evolving far faster than any other human genetic material, says a new study.

Men have a sex chromosome called the Y chromosome. Women don’t. And the male Y, once scoffed at for being small, short of genes, and full of “junk DNA,” is turning out to be a powerhouse of evolution.

Most of our DNA has stayed more or less the same since our ancestors split from those of chimps, scientists say. Chimp genes look like human ones.

But not the Y. It has changed as many as half of its genes since that split, some six million years ago, in an evolutionary race to produce better sperm cells.

Scientists say having a Y is like living in a house that’s constantly under renovation, with work in progress at high speed.

It’s a guy thing, says the Whitehead Institute near Boston, publishing Thursday in the research journal Nature.

“The Y is actually reinventing itself through continuous, wholesale renovation,” says the team led by Jennifer Hughes, a post-doctoral fellow at Whitehead.

All the chromosomes except the Y occur in pairs. Over generations, they evolve by swapping pieces of themselves back and forth, like children trading hockey cards to improve their collection.

But change in the Y always occurs alone. It has no partner to swap with, and was therefore believed to be unable to evolve.

It seemed to be a genetic dead end, a 97-pound weakling doomed to shrivel away to nothing in time.

Instead it now looks like a manly brute, “prone to rearrangements,” Hughes says. It rewrites its own code internally, without asking other chromosomes for directions.

“There’s more going on than just degeneration,” she says. “There’s remodelling and renovation at the same time.”

The institute’s director, David Page, has been studying the human Y for years. He says the Y’s changes are like a renovation.

“People are living in the house, but there’s always some room that’s being demolished and reconstructed,” says Page. “And this is not the norm for the genome (set of all genes) as a whole.”

There’s one main point to fast evolution: The Y is constantly trying to produce as many high-quality sperm cells as possible. For the Y, it’s all about making more babies.

“That’s why this selective pressure on the Y is so intense: It’s a chromosome that’s devoted to sperm production,” Hughes says.

“But they also have a downside,” she adds. Reshuffled Y genes sometimes produce infertility or diseases.

Y chromosome research has developed into its own offshoot of genomics. The one method used to decipher the entire human genome, using high-speed automation, doesn’t work with the Y. Its structure has to be teased apart in a much slower way.

Only two Y chromosomes — human and chimp — have ever been decoded. Hughes hopes to decode Y’s from more species.

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