How It Works
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Ancient fish reveal the origins of sexual intercourse

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Sexual intercourse among vertebrates – that’s animals with backbones, such as humans – is a lot older than first thought. 35 million years older, to be exact.

It was previously thought intercourse and internal fertilisation began in primitive fish known as placoderms that lived around 350 million years ago. However, vertebrate sex organs have now been discovered in placoderms that lived 385 million years ago.

L-shaped genital limbs called claspers have been discovered in the males of a species, rather aptly named Microbrachius dicki. The females were then found to have evolved a pair of small bones that would lock the male claspers during copulation.

Hypothetical mating of Microbrachius dicki, with the male on the right. Artwork by B. Choo.

Until recently, it was believed that our earliest vertebrate ancestors reproduced by external fertilisation, where eggs are fertilised by sperm outside of the body and the young develop out in the open. However, the discovery of a fossilised embryo of a placoderm fish inside its mother provided evidence that internal fertilisation took place 350 million years ago. This then lead to the research team, led by Professor John Long of Flinders University in Australia, to find evidence of sex and internal fertilisation that could be dated back to 385 million years ago. The findings have now been published in the Nature journal.

The ancient placoderms the team studied are known as the Antiarchi, a group of armoured fish that had bony plates covering the front portion of their bodies, and a scaly or naked body and tail. They were approximately eight centimetres (3.2 inches) long and lived in ancient lakes in Scotland, Estonia and China.

The two small bony structures of the males were known of before, but have only now been recognised as the most primitive sexual organs ever discovered. The bony structures are curved with distinct grooves to allow sperm to transfer to females.

The different bony structures of the male and female fishes are the first instance of physical differences between the sexes among vertebrates.

To find out how scientists date ancient fossils and discover oodles of fascinating facts and images, pick up the latest issue of How It Works magazine. Plus take a look at all of this:

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