Tracing The Genes That Made Adam And Eve So Different

The war between men and women evidently started about 300 million years ago. That's when the sex-determining X and Y chromosomes first evolved, according to a new report in Science. Before then, gender was likely determined by environmental factors, such as the temperature of the water in which eggs were laid (the method still used by crocodiles and sea turtles). By reconstructing the evolution of the X and Y, David Page at the Whitehead Institute for Biomedical Research in Cambridge, Mass., and Bruce T. Lahn of the University of Chicago have shed light on the origins of the many hereditary diseases that are passed on only to men, such as hemophilia. The X and Y chromosomes are the only two of the 24 human chromosomes that are not part of a matched pair. Women have two Xs, while males have an X and a Y. But the X and Y do share 19 genes, dating back to when their ancestral chromosomes were still matched. By measuring changes in the surrounding genetic code, the researchers were able to date each of these shared genes and found that the first variation arose about 300 million years ago. The Y chromosome mutated and acquired the master switch for creating a male about the same time mammals diverged from birds. Over time the X and Y shared fewer and fewer genes until, some 30 million to 50 million years ago, monkeys diverged from the lower primates and male and female chromosomes went their separate ways forever.

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