Sex involving 'penetration' is part of life for at least 380 000 000 years. That's way longer than what we suspected. Internal fertilization was common with prehistoric fishes that lived at old tropical coral reefs during the Devonian, writes Nature, in a article that casts a new light on the history of sex with vertebrate animals, and thus also us humans.
The new discovery is that these ancient placoderms had developed this advanced form of mating well before the sharks. The prove that the fished did have internal fertilisation, was provided by the male fish fosiles. Those had an extra piece of bone at the height of the pelvis.