The way animals reproduce has long fascinated scientists, but a recent discovery by a team of researchers in the U.S. has unlocked a fundamental mystery in evolutionary biology—how the penis develops in different species.
This groundbreaking research reveals new insights not only into the evolution of mammals and reptiles, but also into the factors that can cause genital malformations in humans.
In what is being hailed as a major breakthrough, scientists have finally answered a pressing question: Why do some animals, like snakes and lizards, have two penises while others, such as mammals and birds, have just one?
The answer lies in the fascinating way each species’ anatomy evolves and develops during embryonic stages.
But what’s more interesting is what this research could mean for the future of human medicine, especially in the realm of genetic malformations.
So, let’s break down the discovery, the science behind it, and what it tells us about how animal sex organs evolved over millions of years.
The Role of the Cloaca in Penis Development
The secret to understanding why animals have different numbers of penises lies in a key feature of their embryonic development: the cloaca.
A relatively unknown anatomical structure, the cloaca is a cavity at the end of the digestive tract that serves as the bodily exit for digestive, urinary, and reproductive systems in many animals.
What makes the cloaca especially fascinating is its pivotal role in the development of sex organs.
During the early stages of development, the cloaca sends out molecular signals that guide the surrounding tissues and instruct them to form either male or female genitalia.
This is where things start to get really interesting, as the location of the cloaca within the animal’s body has a direct impact on how its genitalia will develop.
For example, in reptiles like lizards and snakes, the penis develops from tissues that will eventually become the hind legs (or, in the case of snakes, the leg structures that are no longer present).
This is why snakes and lizards have two penises—a result of the cloaca signaling the development of leg-bud tissues into two separate organs.
Meanwhile, in mammals and birds, the penis develops from tissues that are more closely related to the tail—leading to the formation of just one penis.
So, in short, the location of the cloaca during development determines whether an animal will have one or two penises.
This insight, first revealed by a study published in Nature, is revolutionizing how we understand the evolution of sex organs across the animal kingdom.
And while this discovery is groundbreaking, it also points to an even bigger question: What exactly caused these differences to evolve, and how can we apply this knowledge to understand human biology better?
A Curious Evolutionary Split
The next major question scientists wanted to tackle was: Why did reptiles like lizards and snakes evolve two penises, while mammals and birds developed only one?
This question has puzzled researchers for years. In the past, scientists had been unable to determine why some vertebrates, like squamates (the group that includes snakes and lizards), developed two penises while others, like amniotes (which include birds and mammals), ended up with just one.
To get to the bottom of this evolutionary mystery, a team of scientists from Harvard Medical School’s Department of Genetics conducted a detailed study of genital development across different species. The researchers studied mice, lizards, chickens, and snakes using genetic markers to track the cells that would eventually give rise to the penis.
What they discovered was unexpected: In reptiles like lizards and snakes, the penis originates from the same tissue that will form the animal’s hind legs. This is why lizards and snakes have two penises—the cloaca signals the leg-bud tissue to develop into dual sexual organs.
On the other hand, in mammals like humans and mice, the penis forms from what’s known as the tail-bud tissue. This results in a single penis, but it can still exhibit a variety of shapes and sizes, depending on the species. So, while the penis itself forms in different ways depending on the species, the key factor is how the cloaca’s signals are interpreted by the surrounding tissue.
This realization helps explain the distinct evolutionary paths taken by reptiles and mammals when it comes to sex organ development. Reptiles retained a vestige of their ancestral limbs—evolving two penises—while mammals and birds streamlined their sex organs into a single structure.
A Deeper Look at the Genetic Signals Behind Genital Development
What sets this study apart is not just the discovery of how reptiles and mammals differ in genital formation, but the underlying genetic signals that regulate this process. The scientists discovered that all species, whether they develop one penis or two, share a “deep homology”—a shared genetic program that governs genital development across vertebrates.
The lead researcher of the study, Patrick Tschopp, explained that the cloaca essentially acts as a “master switch” for genital development. Wherever the cloaca is positioned in the embryo, it will send molecular signals to the surrounding tissue, which determines whether the penis will come from leg-bud tissue or tail-bud tissue.
Interestingly, the team was able to prove this theory experimentally by grafting the cloaca from a chicken embryo—which typically resides near the tail-bud tissue—next to leg-bud tissue. As expected, the presence of the cloaca caused this tissue to transform into genitalia. This proved that the cloaca could influence the development of genitalia, regardless of whether the species normally had one or two penises.
What This Means for Understanding Human Genetic Malformations
While the findings about reptile and mammal genitalia are fascinating, the study’s implications reach much further. The researchers hope that by understanding how the cloaca influences sexual organ development in animals, they can better understand human genital malformations.
Genetic malformations in humans can lead to a variety of conditions, including hypospadias, a condition where the opening of the penis is not at the tip, and ambiguous genitalia, where the sex of the individual is unclear. By studying the mechanisms that govern how the cloaca controls genital development in other species, scientists can develop a clearer picture of what goes wrong during human development and how these conditions arise.
Furthermore, the study could also provide valuable insights into the development of the female genitalia, particularly the clitoris—a part of the anatomy that remains understudied despite its crucial role in sexual health and reproduction. The research could help shed light on how the clitoris develops in humans and how various factors may lead to abnormalities in its formation.
A Groundbreaking Study with Far-Reaching Implications
As Denis Duboule, chairman of the Department of Genetics and Evolution at the University of Geneva, noted, the research is more than just a deep dive into evolutionary biology—it also has practical implications for human health. He explained, “The truth is that when you start understanding these things, you also understand what can go wrong when they don’t work properly. But that’s a whole other story.”
The discovery that the cloaca plays such a crucial role in determining genital development across species is a major leap forward in evolutionary biology and reproductive science. And while much more research is needed, this study has the potential to reshape our understanding of everything from human genetic malformations to the evolution of sex organs across the animal kingdom.
This is just the beginning of what promises to be an exciting new chapter in both evolutionary research and the study of human development. As scientists continue to explore the relationship between the cloaca and genital development, the answers could have profound effects on both human health and the field of evolutionary biology.
Source: EurekAlert
