Scientists believe they have discovered the origin of copulation.
An international team of researchers says a fish calledMicrobrachius dickiis the first-known animal to stop reproducing by spawning and instead mate by having sex.
The primitive bony fish, which was about 8cm long, lived in ancient lakes about 385 million years ago in what is now Scotland.
The researchis published in the journal Nature.
Lead author Prof John Long, from Flinders University in Australia, said: “We have defined the very point in evolution where the origin of internal fertilisation in all animals began.
"That is a really big step."
Prof Long added that the discovery was made as he was looking through a box of ancient fish fossils.
He noticed that one of theM. dicki specimens had an odd L-shaped appendage. Further investigation revealed that this was the male fish’s genitals.
"The male has large bony claspers. These are the grooves that they use to transfer sperm into the female," explained Prof Long.
The female fish, on the other hand, had a small bony structure at their rear that locked the male organ into place.
Constrained by their anatomy, the fish probably had to mate side by side.
"They couldn’t have done it in a ‘missionary position’," said Prof Long. "The very first act of copulation was done sideways, square-dance style."