The act of sexual intercourse was invented by ancient armoured fish around 385 million years ago, in the lakes of Scotland, a new study has revealed.
Professor John Long of Flinders University in Australia has found that internal fertilisation and copulation was first used by a placoderm called Microbrachius dicki. Placoderms are the most primitive jawed vertebrates, and the earliest vertebrate ancestors of humans.
Male fossils of Microbrachius dicki developed bony L-shaped genital limbs called claspers to transfer sperm to females; and females developed small paired bones to lock the male organs in place for mating.
Measuring about 8cm long, Microbrachius lived in ancient lake habitats in Scotland, as well as parts of Estonia and China.
Professor Long discovered the fish's mating abilities when he stumbled across a single fossil bone in the collections of the University of Technology in Tallinn, Estonia, last year.
The fossils, he said, symbolise the most primitive known vertebrate sexual organ ever found, demonstrating the first use of internal fertilisation and copulation as a reproductive strategy known in the fossil record.
"Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for. We’ve solved this great mystery because they were there for mating, so that the male could position his claspers into the female genital area," Professor Long said.
"It was previously thought that reproduction spawned externally in water, and much later down the track in the history of vertebrate evolution. Our new discovery now pushes the origin of copulation back even further down the evolutionary ladder, to the most basal of all jawed animals.
Professor Long added that the fish probably copulated from a sideways position with their bony jointed arms locked together, enabling the male to manoeuvre his genital organ into the right position for mating.
"With their arms interlocked, these fish looked more like they are square dancing the do-se-do rather than mating," Professor Long said.
Flinders Postdoctoral Research Fellow Dr Brian Choo, a co-author on the paper, said the discovery signifies the first time in evolutionary history that males and females showed distinct differences in their physical appearance.
"Until this point in evolution, the skeletons of jawed vertebrates couldn’t be distinguished because males and females had the same skeletal structures," Dr Choo said.
"Recent studies show that our own evolution is deeply rooted in placoderms, and that many of the features we have, such as jaws, teeth and paired limbs, first originated with this group of fishes," Professsor Long said.
"Now, we reveal they gave us the intimate act of sexual intercourse as well."
The discovery is published in Nature.
Two videos below show an animation portraying the earliest known copulation and Professor Long's discovery of the origins of sex.