Our human body form has evolved over the past 500 million years, since the first fishes appeared, through the slow acquisition of various anatomical novelties The sequence of these changes is forever captured, frozen in time, by fossils. This is a true and humbling revelation about one’s self. And there is a legacy inside all of us that demonstrates our direct links to the first armored fishes, and to their extraordinary methods of mating.
Today when we want to discover something new about our evolutionary history, we can do one of two things. First, we can go into the field and find new fossils that reveal new data showing how the primitive pattern of bones might have transitioned, from one form to another. The second method is to look at our distant history entrapped in our own genes within our chromosomes. The really big breakthrough of modern-day evolutionary biology is the discovery of Hox genes that determine the sequence or organization of how body parts are built. In essence, they are a combined toolkit and blueprint for the way our bodies develop.
Certain Hox genes play an important role in limb development in fishes. Hoxd12 is active in clasper formation. Hoxd13 is particularly important because it activates the end of the clasper formation while also activating the cloacal urogenital area. In the mouse, the genital tubercles and limbs are both activated simultaneously by Hoxd13. Dr Marty Cohn, who published this work (reference below), concluded that “Limbs and external genitalia undergo many similar morphogenetic processes, and … the same molecular mechanisms may operate during development of the limb bud and the genital tubercle.”
This statement was a revelation for me. Whereas the actual substance of how genitals are made might differ throughout the evolution of vertebrates, the genes that trigger them appear to be the same and can be traced back to the very origins of limbs. Like building a house out of bricks, wood, or straw, as long as the blueprint is the same, the same kind of structures will be built. Both placoderm and shark claspers are formed from a part of the developing pelvic girdle that was predestined to one day become the legs of humans.
So initially, the reproductive structures of early jawed fishes were paired as part of the leg or hind-limb pattern of bones. After paired fins had transformed into legs, the claspers were lost, but in their place, other paired reproductive structures merged from the urogenital plate, such as the hemipenes of lizards and snakes. In subsequent evolutionary radiations, the paired units were unnecessary (because only one mating organ is needed to do the job properly), so the loss of one side meant that the single penis emerged, so to speak, as the dominant male reproductive organ in higher vertebrates.
Of course, losing the penis altogether happened several times in vertebrate evolution, as in some primitive reptiles (e.g., the Tuatara) and the vast majority of flying birds (passerines) when for one reason or another, there was an alternative reproductive strategy evolved. We humans often speak about “getting a leg over” when we refer to having sex, but for placoderms, it was more about “getting a leg in.” Our humble mammalian penis might not look like it has a long evolutionary history, but indeed it has a deep developmental pedigree that can now be traced back in time to the very origins of arms and legs in all backboned animals.
So when you are next making love to that special person, and enjoying all the physiological pleasures afforded by our anatomy, give a little cry of joy for the ancient armored placoderms for what they have given us. Be thankful that because of some strange twist of biological fate, we have kept the most interesting parts of our reproductive anatomy from our archaic evolutionary history when other lines of animals managed perfectly well to do without them.
Cohn,. M. 2004. Developmental genetics of the external genitalia. Advances in Experimental Medicine and Biology, 545: 149-157.
John A. Long is Vice President of Research and Collections at the Natural History Museum of Los Angeles County. His most recent book is Dawn of the Deed: The Prehistoric Origins of Sex, published by the University of Chicago Press.