Sex has been a fact of life for at least 380m years, longer than previously thought.
Internal fertilisation was widespread among prehistoric fish living on ancient tropical coral reefs in the Devonian period, research published in the journal Nature showed today.
The discovery sheds new light on the reproductive history of all jawed vertebrates, including humans.
'It shifts how we think about how reproduction evolved. You're a jawed vertebrate and I'm a jawed vertebrate, so this is our own history,' said Zerina Johanson, a palaeontologist at the Natural History Museum in London.
Ms Johanson and colleagues in Australia, where the fossils were unearthed, deduced that copulation was common among armoured placoderms, extinct shark-like species, after finding embryos inside Materpiscis, Austroptyctodus and Incisoscutum placoderms.
Finding fossil evidence of reproduction is rare and experts initially missed the signs in the case of one specimen, where a tiny embryo was at first thought to be a last meal.
It was thought that such ancient fish would show a more primitive type of reproduction, with sperm and eggs combining externally in the water, as still happens with many modern fish.
Adding to the evidence is the discovery of a modification in the pelvic fin on the belly of adult fish. The scientists believe this was used by the male to grip the female during mating, as happens with modern sharks.
Placoderms, thought to be the oldest jawed vertebrates, were fearsome predators with bony armour covering their head and forming the biting surfaces of their jaws, which could act like self-sharpening scissors.
The biggest were as large as a great white shark.