Scottish Fish to Thank for Awkward Origins of Sex

About 385 million years ago, a pair of ancient Scottish fish did a jig that was somewhat like square dancing, and now scientists are saying this was actually the awkward origin of sex.

The prehistoric armored fish, called "placoderms," were the first-known animal to stop reproducing by spawning and instead mate by having sex. A group of Australian scientists showed in their study, published in the journal Nature, that males 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.

"It was previously thought that reproduction spawned externally in water, and much later down the track in the history of vertebrate evolution," John Long, a professor at Flinders University in Adelaide, Australia, and the study's lead author, said in a statement. "Our new discovery now pushes the origin of copulation back even further down the evolutionary ladder, to the most basal of all jawed animals."

Measuring about eight centimeters long, M. dicki lived in ancient lake habitats in Scotland, as well as parts of Estonia and China. They are the earliest vertebrate ancestors of humans. But unlike humans, rather than partaking in the usual missionary position, these primitive bony fish interlocked their arms and positioned their bodies sideways in what looked like a square dance rather than sexual intercourse.

"This enabled the males to maneuver their genital organs into the right position for mating," Long explained. Females also developed small paired bones to lock the male organs in place for mating.

The new findings are also expected to make a significant contribution to an ongoing debate about placoderms' place in evolutionary history. According to scientists, other vertebrates likely shared a common ancestor with this ancient group of fish, which was considered to be an "evolutionary dead end."

"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... Now, we reveal they gave us the intimate act of sexual intercourse as well," Long said.

Below is a video portraying the copulation, described in the study.

[Credit: Flinders University]