How Boy Bits First Came To Be

Nov 6, 2014
Originally published on November 7, 2014 12:13 pm

Evolution has shaped every part of the body, and that includes our private parts. New research published this week sheds light on how the penis evolved and how it forms in different animals.

The research might also one day help illuminate a medical mystery: Birth defects of the penis have risen sharply in recent decades, and nobody is sure why.

Penises weren't necessary when our early ancestors lived in the ocean. A female could lay eggs, and a male could just swim by and excrete some sperm. It would all mix and fertilize in the water.

That all changed when our ancestors crawled up on land. "If you go to land, your eggs would dry out if you just lay them on the ground," says Patrick Tschopp, a researcher in genetics at Harvard Medical School.

So nature came up with a different way: the penis. It became an essential trick for getting sperm to eggs. But where did it come from? Tschopp and his colleagues thought a clue might lie in the way embryos of different animals develop.

"So we started out looking at lizards and snakes," Tschopp says.

Snakes were particularly interesting because of one theory about how the penis formed. Legs are another essential element of life on land, and some scientists thought the penis and limbs evolved around the same time.

Snakes have since lost their limbs, but they've kept the penis.

"Well, actually, they have two, which is the fascinating part," Tschopp says.

Each one is called a hemipenis; the plural is hemipenes. Tschopp and his colleagues looked closely at the embryos of snakes, and it got even more fascinating. Some kinds of snake embryos still have limb cells in them today.

"The limbs actually get hijacked to become the hemipenis," he says. "And because you have two limbs, that's most likely why you end up with two hemipenes."

The researchers also looked at mice. There they found the penis formed not around the legs, but near the tail. The results appear this week in the journal Nature.

So what causes two penises to grow from the region where a snake once had its legs, and one to grow from the tail region of a mouse? It turns out that the cells are getting orders from another part of the body: the anus.

This may surprise you, but the digestive tract is among the most ancient parts of any animal. Even the most primitive animals have mouths and bottoms, says Marty Cohn, who studies evolutionary and developmental biology at the University of Florida.

And when more complex animals are developing in the womb, it's actually the gut that spurs other organs to grow. Organs like the liver and pancreas, he says, "bud off of the gut."

And, apparently, so does the penis. Cohn also has a paper out this week in the journal Scientific Reports showing that the chicken's penis starts near its bottom. Wherever the gut happens to end, signals go out telling the penis to form.

It's not just the penis. In women, the clitoris is formed by these same signals. But interestingly, the rest of the female reproductive system seems to have followed a different evolutionary path, according to Denis Duboule at the University of Geneva in Switzerland. "It's useless to have a penis if you don't have a vagina or somewhere to put it" he says. "So how could they co-evolve?" Researchers still aren't sure.

The new work shows a common way in which penises form among many different animals. But exactly how orders from the gut shape the cells is still an open question. Cohn says it may be important to determine the details of this signaling system, especially for humans, as a way of figuring out how the system can go wrong in fetal development.

"In the past 30 to 40 years, the incidence of genital-urinary defects has risen, sharply," he says. "We don't really understand why."

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ROBERT SIEGEL, HOST:

Evolution has shaped our bodies, including our private parts. This week, new research shed light on how male genitals evolved and how they form in the womb. The research could have health implications. Birth defects in male children have been on the rise, and scientists are not sure why. And, of course, listeners should be aware this report by NPR's Geoff Brumfiel does include some frank language.

GEOFF BRUMFIEL, BYLINE: In the history of evolution, genitals are very, very important.

PATRICK TSCHOPP: So basically, external genitalia are considered to be a real essential adaptation when animals crawl onto land.

BRUMFIEL: Patrick Tschopp is a researcher at Harvard Medical School, and when he says, external genitalia...

TSCHOPP: ...We are talking about penises.

BRUMFIEL: Penises weren't necessary back in the ocean. A female could lay eggs, and a male could swim by and excrete some sperm. It would all mix and fertilize in the water.

TSCHOPP: Now, if you go to land, your eggs would dry out if you would just lay them on the ground.

BRUMFIEL: So nature came up with a different way - the penis. It became an essential trick for getting sperm to eggs, but where did it come from? Tschopp and his colleagues thought a clue might lie in the way embryos of different animals develop.

TSCHOPP: So we started out looking at lizards and snakes.

BRUMFIEL: So snakes do have penises, then.

TSCHOPP: Well, actually, they have two, which is the fascinating part.

BRUMFIEL: They are called hemipenises, and snakes have one on the right and another on the left. Tschopp and his colleagues looked at closely at the embryos of snakes, and it got even more fascinating. Early in evolution, snake ancestors had legs right where their penises are.

TSCHOPP: The limb actually gets hijacked to become the hemipenis. And because he has two limbs, that's most likely also why you end up with having two hemipenises.

BRUMFIEL: The group also looked at mice. There, they found the penis formed not around the legs, but near the tail. The results appeared this week in the journal "Nature." So what causes two penises to grow from the region where a snake once had its legs and one to grow from the tail region of a mouse? It turns out the cells are getting orders from another part of the body. And that part is...

TSCHOPP: ...The most terminal structure of your digestive tract.

BRUMFIEL: The most terminal structure of your digestive tract - I think we'll leave it at that.

TSCHOPP: It's the anus. (Laughter).

BRUMFIEL: This may sound surprising, but digestive tracts are among the most ancient parts of any animal. Marty Cohn studies evolutionary and developmental biology at the University of Florida. He says, even the most primitive animals have mouths and bottoms.

MARTY COHN: Food comes in. It's processed and digested and goes out the other end - single inlet and a single outlet.

BRUMFIEL: And when more complex animals develop in the womb, it's actually the gut that spurs other organs to grow.

COHN: Organs bud off of the gut, like the liver and the pancreas.

BRUMFIEL: ...And, it appears, the penis. Cohn also as a paper out this week showing the chicken's penis starts near its bottom. Wherever the gut happens to end, signals go out telling a penis to form. Exactly how all these orders shape the cells is still an open question. But Cohn says, it may be important to figure out the details, especially for humans.

COHN: In the past 30 to 40 years, the incidence of genitourinary defects has risen sharply, and we don't really understand why.

BRUMFIEL: Problems in penis formation are now among the most common birth defects. And studies like these might eventually help scientists to figure out what's been causing the signals to go awry. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.