In the world of genes, the sexes are terribly mismatched: The Y chromosome carries one hundredth the genetic information of the X and is just one third the size. But this odd couple was once a pair of identical twins, according to biologist David Page of the Howard Hughes Medical Institute and geneticist Bruce Lahn of the University of Chicago. Page and Lahn compared DNA sequences and traced the chromosomes back to the time when they were exactly the same. "In our reptilian ancestors, gender was most likely determined by the temperature of incubating eggs, like in today's turtles or crocodiles," Page says. Then around 300 million years ago--about the time mammals parted company with birds--a mutation occurred on one of the pair, creating a gene that when present always produced a male. That put sex forever in our DNA.