Sexual intercourse was pioneered by a group of unsightly, long-extinct fish about 385 million years ago in Scotland, Australian scientists have reported.
After studying the fossils of these ancient fish called placoderms - armour-plated creatures, which gave rise to all current vertebrates with jaws - the researchers found that their descendants switched sexual practices from internal to external fertilisation, an event previously thought to be evolutionarily improbable.
"This was totally unexpected. Biologists thought that there could not be a reversion back from internal fertilisation to external fertilisation. We have shown it must have happened this way," said John Long, palaeontologist at the Flinders University in Adelaide, Australia.
Long's team studied placoderms and found structures in fossils that they interpret as bony 'claspers' - male organs that penetrate the female and deliver sperm.
The researchers had previously shown that one placoderm species was the earliest animal known to have engaged in penetrative sex.
But latest research reported by the journal Nature shows that an even earlier group of placoderms called antiarchs also used this method of fertilisation.
"The finding is significant because antiarchs are considered the most basal (meaning those closest to the roots of the animal family tree) jawed vertebrates, and so it suggests that all placoderms reproduced through internal fertilization using claspers," Long added.
But the implications of this finding are even more penetrating.
According to Long, the oldest bony fishes which followed placoderms in the evolutionary tree show no evidence for internal fertilisation.
"Thus, at some point, early fishes must have lost the internal fertilisation method seen in placoderms, before some of their descendants 're-invented' organs with a similar function - ranging from similar claspers in sharks and rays today to the penises of modern humans," the authors concluded.
