What do lizard penises and fish fins have in common? Well, for starters, they both provide a clue into the evolutionary origins of man. As hard as it may be to believe, it seems that man’s most prized possession actually isn’t getting all the credit it deserves. Not only is it responsible for the proliferation of the human race but, according to a recent study, the development of penises is intricately linked to the beginning of life on land.

Science has finally answered the question you probably never asked: Where do penises come from? Well, the short answer is they came from a necessity to reproduce while on land. the fascinating evolutionary history of mankind. You see, as reported by IFL Science, unlike humans and birds, male snakes and lizards actually have two penises called hemipenes. Now, before you get all excited, no these creatures are not able to use both during intercourse. Still, scientists have not been able to accurately explain why this divide in male sex organs exists — that is, until now.

The hemipenes of an adult Anolis lizard Photo courtesy of Patrick Tschopp/ Harvard Medical School

"While mammal and reptile genitalia are not homologous in that they are derived from different tissue, they do share a 'deep homology,'" explained Clifford Tabin, lead researcher of the Harvard study in a press release.

Tabin and his team found that the reptiles’ multiple penises, or hemipenes, are actually made from the same tissue that our legs originate from. Mammal penises, on the other hand, are made from tail-bud tissue.

This finding may not seem like much on its own, but trust me this is where things get interesting. The study found that the opening for the cloaca — the opening for the reproductive, gut, and urinary tract — determines where penis formation will occur. The cloaca in reptiles is located closer to the tissue that would be used to make a pair of legs. Since this tissue is programmed to produce pairs, the formation of two penises just comes naturally. The cloaca in mice is closer to the tail bud, and since tails don’t come in pairs, neither do mammalian penises.

In order to prove that the cloaca position was really the key to how many penises a creature will have, the scientists grafted cloaca tissue next to the limb buds in a group of chicken embryos and next to the tail buds in another group. The result: In both cases, the cells grafted to the cloaca were partially concerted toward a genitalia fate.

“By misplacing a molecular signal you can misguide these cells in their developmental trajectory,” Patrick Tschopp, the study author, told the BBC.

11 day old python just beginning to grow his reproductive organs. Photo courtesy of Patrick Tschopp/ Harvard Medical School

Now while it’s nice to know where man’s most prized organ has come from, what does this all mean? Well, it’s answered a question that has long puzzled scientists.

"This paper provides a new twist to a previous hypothesis that genitals and limbs share a deep homology. It provides formal evidence of how this co-evolution between the two structures can happen in an organism," Tschopp said. It also shows that all penises, ranging from the 7mm-long mouse penis to the 10-ft-long blue whale penis, all have the same biological roots.

Source: Tschopp P, Sherratt E, Sanger TJ, et al. A relative shift in cloacal location repositions external genitalia in amniote evolution. Nature. 2014.