When it comes to genitalia, nature enjoys variety. Snakes and lizards have two. Birds and people have one. And while the former group's paired structures are located somewhat at the level of the limbs, ours, and the birds', appear a bit further down. In fact, snake and lizard genitalia are derived from tissue that gives rise to hind legs, while mammalian genitalia are derived from the tail bud. But despite such noteworthy contrasts, these structures are functionally analogous and express similar genes. Researchers have now discovered how functionally analogous genitalia can arise from divergent tissue.
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