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A Kink in the History of Sex

The first act of copulation in vertebrates

The first act of copulation in vertebrates was in these 385 million-year-old antiarch fishes from Scotland (Microbrachius dicki). The male (right) uses his bony L-shaped claspers to inseminate the female (left). Credit: Brian Choo

By John Long

The discovery of the first vertebrate to have copulated reveals not only the genesis of different male and female forms but also some surprising kinks in how sex has evolved.

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The evolutionary origin of the intimate act of copulation has been discovered in ancient armoured placoderm fishes called antiarchs. Fossils of the 385 million-year-old antiarch Microbrachius dicki show males with large bony L-shaped claspers for sperm transfer, while females bore small paired bones to help dock the male organs into position.

Our discoveries, published in Nature in October, represent the first appearance of sex involving copulation in vertebrates. It’s also the first time in vertebrate evolution that males and females appear with distinct differences in their physical appearance.

The new discoveries began late last year when I was working at the University of Technology in Estonia with my colleague Dr Elga Mark-Kurik. She handed me a box of isolated Microbrachius bones from Estonia to examine. I found a tiny plate, less than 2 cm long, which had a tube of bone attached to it. I couldn’t identify what it was for.

Eventually I realised that it was a bony clasper – a primitive vertebrate sexual organ. Claspers are found in male sharks and rays, and are used solely for copulation. It was one of those sublime eureka moments as the discovery was profound: it basically required a complete rethinking of the evolution of sexual strategies in jawed vertebrates.

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