Scientists believe they have discovered the origin of copulation, reports the BBC News.

The Microbrachius dicki, a primitive bony fish classified as a placoderm,  is the first known animal to start mating by having sex instead of reproducing by spawning, according to an international team of researchers.

The research, published in the journal Nature, reveals that the fish, about 8cm long,  lived about 385 million years ago, in ancient lakes in what is now Scotland.

“We have defined the very point in evolution where the origin of internal fertilisation in all animals began,” said lead author and project leader Prof. John Long,  a  palaeontologist from Flinders University in Australia. “That is a really big step.”

“This was totally unexpected,” said Long. “Biologists thought that there could not be a reversion back from internal fertilization to external fertilization, but we have shown it must have happened this way.”

The oldest bony fishes, which follow placoderms in the evolutionary tree, do not show evidence of internal fertilization, says Long. Therefore, at some point, early fishes must have lost the internal fertilization method seen in placoderms, before some of their descendants ‘re-invented’ organs with a similar function — ranging from similar claspers in sharks and rays, to the penises of modern humans, suggest the authors.

He was looking through a box of ancient fish fossils when he made the discovery, said Long.

Long noticed one of the M. dicki specimens had an odd L-shaped appendage: the genitals of the male fish.

On the other hand, the female fish had a small bony structure at its rear that locked the male organ into place.

“The male has large bony claspers. These are the grooves that they use to transfer sperm into the female,” explained Long.

Constrained by their anatomy, the fish probably had to mate side by side. “They couldn’t have done it in a ‘missionary position’,” said Long. “The very first act of copulation was done sideways, square-dance style.”

Their small arm-like fins helped the fish stay in position, added Long. “They act like Velcro, locking the male organ into position to transfer sperm.”