http://www.wired.com/news/technology/0,1282,32156,00.html

The History of Sex
by Lindsey Arent

3:00 a.m. 28.Oct.99.PDT

They say men are from Mars and women are from Venus. But new research indicates that the two were at one time genetically identical.

Scientists studying genes on the X and Y chromosomes have concluded that the biological element that determines sex in humans evolved from a pair of identical chromosomes hundreds of millions of years ago.

"We're reporting a timeline by which this perfectly ordinary matched pair of chromosomes evolved into today's X and Y," said researcher David Page of the Howard Hughes Medical Institute at the Whitehead Institute for Biomedical Research. "The X is now bigger than the Y and it carries more genes. But 300 million years ago, they were essentially identical."

Scientists have long believed that the events that led to the creation of sex chromosomes occurred about 170 million years ago. But Page and co-author Bruce Lahn of the University of Chicago report in the 29 October issue of Science that it may have occurred 240 million to 320 million years ago.

For more than a decade, scientists have been on a quest to understand how sex is determined during fetal development - that is, why an embryo that carries two X chromosomes is female and one that carries an X and a Y is a male.

But Page and his team wanted to broaden the scope of the inquiry. "We wanted to know how did this system come to be during evolution," Page said. "How was this system put in place originally?"

By studying a series of XY gene pairs in much the same way that geologists study fossils, Page was able to craft a timeline of the evolution of X and Y chromosomes. "After a while, we realized that the XY genes were sorting themselves out according to their evolutionary age, and when we thought about this question we realized that the genes were shouting at us about the history of the sex chromosome."

Millions of years ago, in addition to XX and XY, all humans carried other non-sex chromosomes in matched pairs called autosomes, Page explained.

The X and Y evolved from what was a perfectly ordinary matched pair of chromosomes, but today's X and Y look different from one another.

"The X chromosome retained all of the genes of the ancestral chromosome, but the Y has lost virtually all of the genes that it once shared with X," Page said. "We know
of 19 genes that they both still share, and we think they are remnants of the ancestral gene."

By studying the few shared genes on the Y chromosome that remain today, and by comparing the genes that are common to the X and Y, Page and his team were able to measure the amount of time that has passed since the gene pairs were identical.

"We found all of the XY gene pairs and looked at them as a group and found that the pattern and flow of the sex chromosome evolution became obvious when we had them lined up."

"We now recognize that these shared genes are a kind of living fossil," Page said. "It's through the study of today's human X and Y that we can reconstruct their past,"
he said.

"It's a kind of molecular archaeology...We're not looking at bones or fossils or even other species, we're just looking within ourselves."