The furry and venemous platypus, it turns out, is helping scientists pinpoint the birth of the Y chromosome that male humans inherited. In nearly all mammals, a Y chromosome carries a gene that determines maleness, while in birds, reptiles and other animals, different systems separate the boys from the girls.

The platypus is officially a mammal but it's part of a tiny family that branched off from other mammals very early - 166 million years ago, said Graves, who works at the Australian National University in Canberra. Among its many weird traits, the platypus has 10 sex chromosomes - and it isn't clear whether any are related to the X and Y carried by humans.

Over the last year, Graves, along with Australian and U.S. colleagues, sequenced the entire genome of the platypus, publishing their results last week. They found that the 10 sex chromosomes of the platypus carry genes similar to those believed to determine sex in birds. There was nothing resembling the human Y chromosome, which must have evolved after 166 million years ago.

"Now, we know our sex chromosomes are quite young," she said. "They've been degenerating much faster than we thought." She gives the human Y about five million years. But don't fear, she said. Several other mammals appear to have lost their Y chromosome, and somehow they still have males. Nature just found some other way to make those bad boys.

- Faye Flam