Scientists believe they have discovered the origin of copulation.
An international team of researchers believes that a fish called Microbrachius dicki was the first animal to us copulation to mate instead of spawning. The primitive bony fish, which was about 8cm long, lived in ancient lakes about 385 million years ago in what is now Scotland.
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 said that he was looking through a box of fossils when he realised that one of the M. dicki specimens had an odd L-shaped appendage which 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,"
The female fish, 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."The fish used their small arm-like fins to help stay in position. "The little arms are very useful to link the male and female together, so the male can get this large L-shaped sexual organ into position to dock with the female's genital plates, which are very rough like cheese graters. They act like Velcro, locking the male organ into position to transfer sperm."
Surprisingly, the researchers think this first attempt to reproduce internally was quickly abandoned.
As fish evolved, they reverted back to spawning. It was another few million years for copulation to make a come-back, first seen in ancient sharks and rays.