Any clue how old sex is? Scientists are putting its age at around 380 million years; that's a lot earlier than previously believed. The reason why they arrived at this conclusion is the discovery of a fossilised pregnant fish and her embryo.
According to ABC Online, remains of the primitive fish have provided the earliest known evidence of copulation and live births in the animal kingdom, the study published in Nature stated.
Until the evolution of the armoured fish, sex is thought to have been limited to external fertilisation techniques in which sperm and eggs were squirted into the water to mix.
In May 2008, Dr John Long and colleagues announced in Nature they had discovered a 380 million-year-old fossil of a pregnant female placoderm (Materpiscis attenboroughi), complete with an umbilical cord and embryo.
The fossil, which was found in 2005 in the Kimberley region of Western Australia, provided the oldest evidence of vertebrate sex. But without the father, no one knew how the fish did the act.
Long said that the 2005 discovery confirmed the earliest evidence of internal fertilisation.
According to him, the discovery of the M. attenboroughi embryo, lead to the latest finds.
An earlier study of Gogo fish fossils, which was collected by the Natural History Museum, London, in the 1960s, concluded the small skeletons inside the placoderms were the animal's last meal.
However, a closer inspection by Long and research team revealed they were the tiny bones of unborn fish from two adult species of arthrodires (Incisoscutum ritchiei) - the biggest and most diverse placoderm group.
The researchers then re-examined the fossils of male Incisoscutum fossils stored in Australia and the UK, which revealed how the ancient fish mated.
"Finding the first embryos last year was the Rosetta Stone to reinterpret existing material in Australian and overseas collections," says Long.
What his team found were structures on the fossilised pelvic girdle that allowed male fish to fertilise females, "the old fashioned way".