Scientists have thrown off the covers and revealed the origins of sex, discovering that it wasn't always performed as we do it today.
A team led by renowned Australian palaeontologist John Long has pinpointed the time in evolution when intercourse developed as a method of reproduction in our distant ancestors - ancient armoured fishes called antiarch placoderms.
Fossils of these ancient creatures, which ruled the earth around 430 million years ago, show they were the first animals to develop specific male and female genitalia, allowing them to have internal sex.
But their acts of intimacy were not conventional. In these primitive placoderms sex was performed side-on.
Professor Long said the sex organ of male antiarch placoderms was two big L-shaped claspers that were inserted into the female's tiny paired genital plates, which helped lock the claspers into position for mating.
"When you look at the shape of these structures they can't possibly do anything in a missionary position," said Professor Long, from Flinders University in Adelaide.
"We've printed out 3D models of these fish and I can play mating games with them and the only way possible they can do it is sideways, square dance style, with their little arms entangled," he said.
Professor Long said there was a growing body of evidence to suggest that many of the traits and behaviours that first appeared in placoderms had travelled through the rest of evolution leading up to humans.
While his team has previously shown these ancient armoured fish were having intercourse, a series of new and recently-interpreted fossils suggest the act evolved much earlier than first thought, at the point when vertebrate animals evolved jaws and paired hind-legs.
The first suggestion that sexual intercourse evolved early in jawed-vertebrates came from a 380 million year old fossil of an antiarch placoderm that Professor Long unearthed in Estonia last year. Further evidence came from a group of fossils that were part of a private collection in Scotland until recently.
"This is not just about bony structures on these fish, it's about the evolution of behaviour, of when sex first became fun," Professor Long said.
"Why would something develop these big bony clasper things and place them inside a female unless they enjoyed the act," he said.
Before fishes developed gender-specific sex organs both males and females shed their gametes in open water to fertilise.
The group's findings are published in the scientific journal Nature.
