It’s got a third eye complete with lens and retina, and despite its appearance isn’t actually a lizard. But these aren't the only odd things about the tuatara reptile: It also lacks a penis. Paradoxically, it’s this fact that might just hold the clue as to how the organ evolved for the rest of us. Understanding whether the tuatara originally had a penis and then lost it could answer a problem that has been bothering biologists for a while now. Do we see such a massive diversity in animal penises because they all evolved independently, or did the penis evolve only once and then subsequently diversify?

The tuatara is the last survivor of a group of reptiles that flourished around 200 million years ago, but is now restricted to a few islands off the coast of New Zealand. Among a myriad of weird traits, this group is notable because out of all animals that require internal fertilization, or amniotes, it is the only one in which all members lack penises. In comparison, the rest of the male amniotes show an impressive amount of diversity when it comes to their sex organ. From the four-headed penis of the echidna, the permanently erect ones of the alligator, or the improbably long corkscrew of the Argentine lake duck, there seems no end to the variety.

Diagram showing how the penis probably evolved in amniotes, with tuataras (Sphenodon) found next to the lizards and snakes (squamates). Sanger, Gredler and Cohn 2015

This has led some to speculate that the erectable phallus didn’t evolve once, but that actually each group within the amniotes – mammals, lizards and snakes, crocodiles, turtles and birds – all came up with their own solution to getting sperm to the eggs within the females. One way to solve this mystery would be to look at tuatara embryos, and see if in the very early stages of life the reptile develops the beginnings of a penis which it then gets rid of by reabsorption.

But there’s a catch: the reptiles breed incredibly slowly. In fact, females only become sexually receptive at around 10-15 years of age, and even then only lay eggs around every 5 years. This means that New Zealand is obviously very protective of all tuatara embryos, not letting any go spare. Things, however, were a little more lax at the start of the 20th century, and it turns out that four of the reptiles’ embryos were prepared back then in thin slices at Harvard. Getting their hands on these rare specimens, the researchers were able to photograph and then reconstruct a 3D model of the early embryos.

What they found seems to settle the debate. There was indeed a small nubbin exactly where the penis develops in all other amniotes, along with what would eventually go on to form the cloaca, a multi-purpose opening for urination, defecation, and reproduction. In fact, the genital swelling they observed looked a lot like what is seen in bird embryos, most of which also develop the beginning of penis then take it all back in. The results are published in Biology Letters.

This suggests that the penis did actually only develop once in amniotes, and some animals then went on to lose theirs over time, while others developed elaborate spines, swellings, and scrapers.