Why We Have Sex

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That discovery led to you earning the unusual distinction of having made paleo pornography. Can you tell me a little bit about how that went down?

It was really fun. The clip took us about a month to make. We had to sort of model a fish in three dimensions and then scan it. Then we had to put the genitalia on the male and, you know, we changed the color schemes just a little bit, but we have no idea what the males and females were really like — we didn’t put red lipstick on the female, obviously.

Then we had to figure out the mating position. Male sharks have these  claspers [which are used to hold onto the female and transmit semen during mating] that are actually quite flexible and rubbery, whereas these placoderms had quite stiff bony claspers, so they didn’t have the same degree of flexibility. Our deduction was that it had to sort of mate in a 69 position, because the claspers were not flexible enough to come forward like in modern sharks. So that in itself is an interesting revelation — that an early form of copulation was kind of a reverse act.

I’m just imagining the conversations while you were going over the various permutations of how they might accomplish this act.

It was fueled greatly by a few beers in the pub, I can tell you.

To be a fly on the wall in that pub! Early on in the book, you write, “The idea that a male of some primitive archaic creature one day decided to put a part of his anatomy inside a rather delicate region of the female, then decided it felt funky enough to ejaculate his sperm, is pretty odd.” It is indeed. It’s easy enough to understand why sexual reproduction, as opposed to asexual reproduction, has thrived — but why the bizarre act of copulation as we know it?

Creationists would contemplate the evolution of the eye, and they couldn’t even contemplate what an intermediate form of eye might look like. In reality, evolution does have intermediate forms. When we think of copulation by internal fertilization such as in the sharks and rays, it’s hard to think of an intermediate stage, but there are examples in other fish groups that don’t have copulatory organs but do pass sperm into the females via the cloaca, guppies for examples. So I think with these placoderms, maybe the initial form was really just having the pelvic fins to grasp the female so that the cloacas were juxtaposed to pass sperm inside the female and then eventually part of the pelvic fin slipped inside the female and made that act easier to accomplish and more successful and you immediately have a precedent for an evolutionary advantage. Making that feel good by stimulating the nerves at the end would be the next logical evolutionary step. As a scientist, I can sort of envisage steps that could have happened, we just lack the fossil evidence.

Why is it that the human penis evolved as it did and not into a spikey or corkscrew-like member?

We’re so lucky, because many of our mammal cousins, like squirrels and rodents, do have these amazing swiss army knife type penises. It comes down to sperm competition. It’s also about the female, not just the male, and using their internal anatomy in such a way that the best sperm gets to the top to fertilize the egg. In primates did you know that only a short time ago primates, in the evolutionary line leading to human, had  spikes on their penis?