Australian scientists have tracked down the origins of the intimate act of sexual intercourse.
The discovery, billed as one of the biggest in the evolutionary history of sexual reproduction, was announced in the international journal Nature today by palaeontologist John Long, a professor at Flinders University in Adelaide.
Professor Long has found that internal fertilisation and copulation was invented by ancient armoured fish, called placoderms, about 385 million years ago in Scotland.
Placoderms, the most primitive jawed vertebrates, are the earliest vertebrate ancestors of humans.
The discovery shows that male fossils of the Microbrachius dicki, which belong to the antiarch group of placoderms, 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 ancient fishes mating abilities when he stumbled across a single fossil bone in the collections of the University of Technology in Tallinn, Estonia, last year.
He says the fossils are the most primitive known vertebrate sexual organ found, demonstrating the first use of internal fertilisation and copulation as a reproductive strategy.
“Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for,” he says.
“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.”
It was previously thought that reproduction spawned externally in water, and much later down the track in the history of vertebrate evolution.
Earlier discoveries published in Nature in 2008 and 2009 of live birth and copulation in placoderms concerned more advanced placoderm groups.
The latest discovery pushes the origin of copulation back further down the evolutionary ladder to the most basic of all jawed animals.
“Basically it’s the first branch off the evolutionary tree where these reproductive strategies started,” Professor Long says.
He say the fishes probably copulated from a sideways position with their bony jointed arms locked together.
“This enabled the males to manoeuvre their genital organs into the right position for mating,” he says.
“With their arms interlocked, these fish looked more like they are square dancing the do-se-do rather than mating.”
Flinders Postdoctoral Research Fellow Dr Brian Choo, a co-author on the paper, says the discovery signifies the first time in evolutionary history that males and females showed distinct differences in their physical appearance.
“This is the first time in vertebrate evolution that males and females developed separate reproductive structures, with males developing claspers, and females developing fixed plates to lock the claspers in for mating,” he said.
Matt Friedman, a palaeobiologist from the University of Oxford, describes the discovery as remarkable.
“Claspers in these fishes demand one of two alternative, but equally provocative, scenarios: either an unprecedented loss of internal fertilisation in vertebrates, or the coherence of the armoured placoderms as a single branch in the tree of life,” Dr Friedman says.
“Both conclusions fly in the face of received wisdom, and suggest that there is still much to discover about this critical episode in our own extended evolutionary history.”
The research involved a team of collaborators from Australia, Estonia, the UK, Sweden and China, scrutinising a vast number of fossil specimens held in museum collections across the world.
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