Archaeogenetics [creativecommons license] via Wikimedia CommonsSome time ago, the future of the human Y chromosome was called into question. After all, other species, such as mole voles and spiny rats, have lost their Y chromosomes, and sex-determination responsibilities have fallen to other chromosomes. In fact, the human Y did not start off as a sex-determining chromosome either. Human X and Y chromosomes were once a pair of autosomes, like the 22 others we carry, and sex was determined by environmental factors rather than genetics. The ancient X and Y swapped gene copies during crossing-over, just like the other autosomes, to maintain genetic diversity and eliminate potentially harmful mutations. About 300 million years ago, however, one part of the X stopped swapping with the Y, then another, a third, a fourth and—about 30 million years ago—a fifth. As a result, the corresponding portions of the Y chromosome decayed, and it eventually lost ~97% of the genes that it once shared with its partner, the X chromosome. It seemed that the Y was disappearing. Some said this was happening at an unsustainable rate, that it would be gone altogether within 10 million years. Prospects seemed dim for the withered Y.
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