Fish sex illustration

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

The fishy origins of sex

Sexual intercourse is a lot older than we thought, ancient fish fossils have revealed.

Sex among vertebrates – animals with backbones, such as humans – evolved from a group of primitive fish called placoderms.

Scientists previously thought that intercourse and internal fertilisation began in placoderms that lived around 350 million years ago.

But the oldest vertebrate sex organs ever known have now been found in the group’s most primitive members, which lived 385 million years ago.

The first time

Specialised bones used for clasping during copulation have been found in placoderms, which are vertebraes’ primitive ancestors. From them we inherited jaws, teeth and pairs of limbs - and, as a new study has revealed - sex.

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

This is the first use of copulation and internal fertilisation as a reproductive strategy evolved in the fossil record.

Sexual awakening

Previously, it was believed that our earliest vertebrate ancestors reproduced by external fertilisation, where eggs and sperm meet outside the body and the young develop out in the open, as in the case of frog spawn.

But the fossilised embryo of a placoderm fish inside its mother, which provides evidence for internal fertilisation 350 million years ago, was recently found. The research team that made the find was led by Prof John Long of Flinders University in Australia and included Museum palaeontologist Dr Zerina Johanson.

And now, as reported today in the Journal Nature, the same team has found evidence of sex and internal fertilisation in the most primitive placoderms that lived 385 million years ago.

Getting into the groove

The ancient placoderms that the team studied are known as the Antiarchi. This group of armoured fish had  bony plates covering the front portion of their bodies, and scaly or naked body and tail.

They measured about eight centimetres long and lived in ancient lake environments 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.

'We now know that internal fertilisation is the general condition for placoderms, and is the most primitive type of reproduction for vertebrates,' said Dr Johanson.

Gender differences

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

'This is the first time in vertebrate evolution that males and females developed separate reproductive structures, with males developing claspers and females developing fixed plates to lock the claspers in for mating,' said Prof Long.

A lot more to learn

The Antiarchi group is well known and widespread, and this research highlights how new discoveries can still be made from groups we previously thought were well known.

'The Antiarchi are well represented in museum collections around the world, and it is surprising that these features had not been observed previously,' said Dr Johanson.

'Now, however, we can return to these collections with a new research focus. New and exciting discoveries will not be far behind.'