When the first animals scrambled out of the water to live on land, they needed limbs and lungs. And something else: a new way to reproduce.
On terra firma, prospective parents no longer had the luxury of an aquatic environment where eggs could be laid and sperm released over them.
Evolution elegantly solved the problem, but the solution did not always take the same form. Crocodiles, birds, and mammals have one penis. Lizards and snakes have two.
The sex organs of animals are some of evolution’s most diverse handiwork. Yet until now, scientists have not understood exactly how they evolved and took shape at the earliest stages of development.
In a study published Wednesday in the journal Nature, researchers at Harvard Medical School have begun to unravel how genitals form. Their tools range from standard biomedical techniques, such as examining which genes are flipped on during embryonic development, to Frankenstein-like surgeries in which they performed surgical transplants to see whether they could trigger genitals to bud in unusual, out-of-the-way spots.
The researchers found that genitals develop from the same cells that give rise to hind legs in lizards or that form the remnants of limb buds in snakes. In mammals and birds, a different set of cells closer to the tail area give rise to genitalia. Despite the different source materials, they found the process of genitalia formation is deeply similar among those creatures, triggered by the same basic genetic programs and signals. That process is, in turn, similar to the way limbs form, suggesting that genitals and limbs share evolutionary origins.
“It’s clearly really basic research, mostly about evolution and development. But what is interesting is it has been known in the medical field that babies that are born with malformations in their limbs often also have malformations in their genitalia,” said Patrick Tschopp, a postdoctoral researcher at Harvard who led the research. “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 began working on the question in Clifford Tabin’s genetics laboratory at Harvard, where researchers study how animals evolved different forms. A colleague studying limb development had been trying to figure out whether snakes lacked the basic developmental mechanism that gave rise to limbs.
Tschopp got interested when researchers noticed the cellular program that gave rise to snakes’ unusual, two-pronged genitalia looked very similar to limb development.
The researchers began to study which genes were responsible for the development of genitals and to pinpoint the cells in the embryo from which they originated.
Once it was clear that two different sets of cells were the origins of genitals, they tried to figure out why. They noted that the cloaca, a precursor to the gut, varied in its position in reptiles and mammals, and theorized that the origin of the genitals might be due to signals sent by the cloaca to surrounding cells.
When they transplanted a cloaca onto the cells that typically give rise to hind limbs in a chicken embryo, they were able to cause genital-like buds to form there.
That shows genitalia were not passed from a common ancestor to reptiles, mammals, and birds, but they are biologically related in a deep way, formed in response to signals from the same ancient source: the cloaca.
Dr. Martin Cohn, a professor of developmental biology at the University of Florida, simultaneously published a paper in the journal Scientific Reports reporting that genitals start as two buds that eventually migrate toward the center line of the body. In some animals, such as the chick embryos Cohn studied most recently, they join up. In others they don’t, creating the bizarre double “hemipenes” of reptiles.
Despite the fact that scientists are interested in studying the origins of genitalia for purely scientific reasons, both scientists said it makes for better fodder when people ask about their work.
“In terms of the questions, I’m most interested in how changes in gene regulation can shape evolution,” Tschopp said. “But working on external genitalia makes for better cocktail conversations than before, when I [worked] on digits.”
A previous version of this story incorrectly described developmental biologist Martin Cohn’s research interests. He is interested in the evolution of genitalia and in birth defects that result in malfromed genitalia during development.