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Jennifer Marshall Graves, an Australian biologist, is probably best known for a dire prediction: the human Y chromosome, which makes males male, could disappear in the next 5 million years. In the last 190 million years, the number of genes on the Y has plummeted from more than 1,000 to roughly 50, a loss of more than 95 percent. The X chromosome, in contrast, stands strong at roughly 1,000 genes.

Media reaction has been predictable, with overheated headlines proclaiming “Men on road to extinction.” But Graves, a biologist at La Trobe University in Melbourne, notes that 5 million years is a long time for a species such as ours, which is only 200,000 years old. More importantly, the loss of the Y chromosome might not spell disaster. If the Y does disappear, we may well develop a new mechanism for making men.

Scientists are discovering that the mechanisms that organisms use to determine sex are in a remarkable state of flux. When one system is destroyed, evolution seems to easily come up with a new one. Birds, fish and snakes have found myriad ways of making males and females. Sex chromosomes are frequently lost or swapped. Even closely related species can determine sex in quite different ways, suggesting that the system is highly flexible and evolving rapidly.

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