Blame it on the placoderms

Prior to 375 million years ago, fish reproduced through spawning, where the females deposited eggs into the water and the males fertilized them. The embryos that developed from the fertilized eggs were on their own and most ended up being eaten by other fish.

About 375 million years ago, creatures called placoderms appeared on the scene. The placoderms were very successful and ruled the planet for nearly 70 million years. One of the reasons behind their success was that they did not produce their young by spawning; they were the first animals known to fertilize the female’s eggs inside the female, where the young could grow and mature to a point where their chances of survival far exceeded those of young produced by external fertilization.

Babies produced by internal fertilization were larger and heartier.

But internal fertilization meant that males had to have some method by which to deposit the sperm in the female. This was done by means of two “claspers, flesh-covered extensions of the pelvic girdle” that were used to transfer sperm. Thus began the origins of sex among animal species on Earth.

Placoderms were also the first animals with a backbone to have jaws. Researchers have suggested that the jaw evolved to help the male grab hold of females and stabilize them during mating, only later taking on the role of food processing. As John A. Long, vice president of Research and Collections at the Natural History Museum in Los Angeles County, writes, “Sex, it seems, really did change everything.”

Internal fertilization was so successful that it became the means of reproduction of sharks, dinosaurs, birds, and mammals. Frogs and bony fish species reproduce through external fertilization.

Thus, the whole idea of prostitution among human beings may be seen as an outgrowth of a biological innovation by the placoderm more than one-third of a billion years ago.