With the X's complete code in hand, Ross and his colleagues were able to make detailed comparisons with the corresponding chromosomes of other animals, including chickens, fish and rats. The similarities and differences confirmed previous hints about how the X and Y -- and with them, sex as we know it -- arose.

It happened about 300 million years ago, long before the first mammals. A conventional chromosome in a forebear of humans -- probably a reptile of some sort -- apparently underwent a mutation that allowed it to direct the development of sperm-producing testes.