The two chromosomes that determine an animal's sex evolved from an identical pair of ordinary chromosomes about 240 to 320 million years ago.

It is an event, called a genetic hijacking by some scientists, that has profound ramifications today.

Of the 46 human chromosomes, 44 are identical pairs. But two - the X and the Y - are different because they have no perfect match. Embryos with two X chromosomes develop into females, while embryos with an X and a Y chromosome develop into males.

All due to the Y chromosome

David Page, a member of the Whitehead Institute in the United States, wondered how these unique chromosomes first came about.

In a study published in Science, Dr Page and Bruce Lahn report that they have discovered four stages of sex chromosome evolution.

Hundreds of millions of years ago sex was probably determined not by chromosomes, but by some environmental factor, like the temperature at which the egg was incubated. It still happens that way in some animals like crocodiles and sea turtles.

Today the X is home to thousands of genes, but the Y has only a few dozen. Of those, only 19 are shared between the X and Y.

...and to two X chromosomes

"These 19 genes are essentially living fossils. They are able to provide scientists with information about the history of sex chromosomes," says Dr Page.

The researchers compared the locations of all 19 pairs of genes on the human X and Y chromosomes. They found that all of these genes are concentrated on the tip of the short arm of the X, whereas they are scattered across the length of the Y.

To their amazement, the scientists found that the genes were clustered into four groups, each group with a different level of sequence similarity.

"The most striking observation was that on the X chromosome, the four groups of genes are physically arranged as four consecutive blocks, essentially like the layers of rock are arranged in geological strata," explains Dr. Page. In contrast, the groups appear to be scrambled on the Y chromosome.

By comparing the genes in each stratum with similarities in other mammals, Page and Lahn were able to determine the minimum and maximum ages for all of the strata.

• The first strata differentiated 240 to 320 million years ago, shortly after the ancestors of mammals parted company with the ancestors of birds.
• The second strata differentiated 130 to 170 million years ago, shortly after our ancestors parted company with the ancestors of the duck bill platypus.
• The third strata differentiated 80 to 130 million years ago, shortly after our ancestors parted company with the ancestors of kangaroos.
• Finally the fourth and most recent strata differentiated 30 to 50 million years ago, shortly after our ancestors parted company with the ancestors of lemurs.

Each of these events caused an inversion and shuffling of regions of DNA on the Y chromosome so that they could no longer line up with analogous regions of DNA on the X chromosome partner.

This prevented DNA exchange between the similar regions of the two sex chromosomes, and made it possible for portions of the X and Y chromosomes to differentiate from each other.

Genes that are not needed by the male may gradually accumulate causing problematic mutations. "In humans," said Page, "the ramifications of the hijacking are still being played out."