Male Genitalia: Why Evolution Gave Lizards Two Penises And Humans Just One

Scott Falkner

A study published yesterday in Nature by researchers at Harvard Medical School looked at the differences in male genitalia caused by evolution -- not only in humans, but in other animals as well, according to Live Science.

Did you know that lizards have two penises? The new study suggests that genitals developed from the same cells that created hind legs in lizards, or the same cells that form of the vestiges of limb buds in snakes. When these animals were first scrambling out of the water and onto land, limbs and lungs were required. The animals also needed a new way of reproducing as the aquatic environment they'd previously used to lay eggs was no longer available to them. Nature -- namely, evolution -- had to figure out a way to solve the problem. However, it didn't solve that problem in the same way for all species.

Crocodiles, birds and mammals all developed one penis, but lizards and snakes developed two.

The study revealed for the first time the common denominator between male genitalia and limbs. Researchers used what some might consider to be macabre measures to bio-medically examine the genes that change during embryonic development, even resorting to Frankenstein-like surgeries to see whether transplanting genital buds would allow them to form in unusual spots.

The researchers found that in both birds and mammals, a set of cells located close to the tail area of the animal (or the tail bone in humans) represent a place of primary male genital formation. This development, the study found, is similar to limb formation, leading the research team to conclude that the evolutionary origins of genitalia and limbs must be common.

Patrick Tschopp, an author from the Harvard research team, explained the findings and what they mean to BBC News.

"It has been known in the medical field that babies that are born with malformations in their limbs often also have malformations in their genitals. We knew there was some sort of genetic link between the two, and this could provide some information about where these genetic links are."

Tschopp and his team stumbled across the basis for this study when one of his colleagues was studying animal evolution. Scientists were researching whether or not snakes lacked a specific developmental mechanism needed to develop limbs. When they realized that the same cellular program that enable limbs to develop was developing the snake's unusual two-pronged penis, they decided to investigate further.

The difference between birds and mammals, and lizards and snakes, is that the male genitalia in the former creates itself out of the same cellular plan that would create a tail - hence there's only one. The cellular plan that creates the male genitalia in lizards and snakes is closer to the plan that forms the hind legs, hence, they luck out and get two.

[Image via Arianna Gillis]