As it is for many people, so for evolution; the first sexual activity was...awkward. In the Devonian seas, fish solved the eternal problem of how to get it on by docking side by side and using L-shaped sexual organs to squeeze between genital plates.

Single-celled organisms have been swapping DNA for more than a billion years. However, when it comes to copulation, the oldest record we have is from placoderm fishes.

In 2008-9, Professor John Long stunned the world with a series of papers based on fossils found in Western Australia's Gogo Formation. These revealed that 380 million years ago fish were giving birth to live young, rather than laying eggs, and that males of the day held onto mates with pelvic claspers like some modern sharks.

Long wrote a book on the evolution of animal sex, but left open the question of exactly how the armored fishes of the era went at it, no Devonian Kama Sutra having survived the eons. Now, however, Long has solved even this problem with the discovery of a placoderm bone from a group of fishes from which all vertebrates may have come. 

The bone in question is the world's oldest known sex organ, and once belonged to an antiarch buried in what is now Estonia—which is five million years before the embryo-bearing ptyctodontids Long previously brought to light. 

Long started a search of antiarch fossils, with evidence coming from the genus Microbrachius from Orkney and China. He found not only more male claspers, but signs of bony genital plates in antiarch females. In the process, he solved a longstanding mystery. “Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for,” says Long. “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. With their arms interlocked, these fish looked more like they are square dancing the do-se-do rather than mating.” 

In Nature, Long brings these findings together, and also names the oldest species in which these organs can be seen: Microbrachius dicki.

The findings establish placoderms' place in animal evolution. “Placoderms were once thought to be a dead-end group with no live relatives but 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,” says Long. “Now, we reveal they gave us the intimate act of sexual intercourse as well.”

Puzzlingly, Long concludes fish subsequently abandoned internal fertilization before some species rediscovered its benefits.

^{John Long/David Choo/Flinders University. The evolutionary history of sexual organs.}

In the video below, animators at Flinders University—where Long is now based—reveal how they think these first sex acts worked. Although the male's push off when he's done might be an extrapolation from a subset of modern mating behavior. Probably Safe for Work, and do read the credits.