Origins of Sex Discovered in Ancient Scottish Lake Between Our Distant Ancestors

Scientists may have just uncovered the origins of sex. Scientists have discovered ancient fossils of armored fishes, called placoderms, that reveal the creatures in the process of mating.

Placoderms are the most primitive jawed vertebrates and are actually the earliest vertebrate ancestors of humans. Measuring about 8 cm long, this particular placoderm, called Microbrachius, lived in ancient lake habitats in Scotland about 385 million years ago.

"Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for," said John Long, the lead author of the new study, in a news release. "We've solved this great mystery because they were there for mating, so that the male could position is 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."

The scientists believe that the creatures likely copulated from a sideways position with their bony, jointed arms looked together. This would have allowed the males to maneuver their genital organs into the right position for mating. What's even more interesting is the fact that this signifies the first time in evolutionary history that male and females showed distinct differences in 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," said Brian Choo, co-author of the new paper. "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."

The findings reveal the importance of placoderms in vertebrate evolution. Many features in placoderms, such as jaws, teeth and paired limbs, first originated with this group of fishes.

"Basically, it's the first branch off the evolutionary tree where these reproductive strategies started," said Long.

The findings are published in the journal Nature.