Most guys probably don't sit around the locker room pondering the finer points of how the male penis evolved, so leave this one to Harvard researchers: The mystery is solved thanks to … lizard limbs. Yup, Harvard scientists investigating the origin of external genitalia have found a link relating to the development of limbs in a variety of animals. Scientists initially hoped to discover why snakes didn't grow limbs, the Boston Globe reports. But they instead found that embryonic cells—which produce hind limbs in lizards, limb buds in snakes, and "tail-bud" tissue in mammals—form genitalia with a signal from something called a cloaca. As Discovery reports, cloaca is "tissue that eventually develops into the urinary and gut tracts." When they transplanted a cloaca onto hind limb cells in a chicken embryo, eureka—genital-like buds formed.
"It demonstrates that there is a flexibility with what kind of cells can get recruited during development to form genitalia," the lead author tells the BBC of his research in Nature. He says the link may help explain why "babies that are born with malformations in their limbs often also have malformations in their genitalia." While snakes and lizards have two penises, "it appears that the way all amniotes, including reptiles, birds, and mammals, build their genitalia is very similar," a researcher says. The Globe adds that external genitalia developed when animals moved from sea to land, rendering water-borne fertilization obsolete. (Another study finds sex is 385 million years old.)