On The Origin Of Copulation

Insects often lay their eggs on leaves in small clusters

Sexual reproduction is responsible for the diversity of life. Its gene shuffling powers introduce variations into a species, providing durability against the erosive forces of selective pressures.

It most likely evolved just over a billion years ago, almost with the dawn of multicellular organisms. Indeed multicellularity is a requirement for reproduction as only multicellular creatures can produce cells specifically designed to fuse and create a complete set of genetic instructions for building. Yet sexual reproduction, in most cases, is not conducted in the fashion familiar to us.

Penetrative sex is in the minority. Instead the females in most sexually reproducing species will deposit their eggs somewhere safe, such as in the crevices of a reef or the underside of a leaf. The males will then simply eject sperm or the equivalent thereof over the eggs. Monogamous couples will conduct this affair in quite a precise fashion, utilising complex rituals and manoeuvres in order to increase the chance of successful fertilisation. Communally breeding males, in particular fish species, will eject with careless abandon. The primary aim is to reach as many different eggs as possible.

Penetrative sex, whilst harder to coordinate is favoured as it reduces the chance of waste and increases the chance of successful fertilisation. Yet how long ago it evolved, however, until recently remained a mystery. In 2013 a single fossil bone was rediscovered in the collections of the University of Tallinn in Estonia by Professor John Long from Flinders University, Australia. Belonging to a species known as Microbrachius dicki, the bone was part of a now extinct group of armoured fish known as the placoderms, inhabiting the basin lakes of the Caledonides and the Rheic ocean during the Silurian and Devonian periods.

An artist's impression of  mating Microbrachius, showing the use of the 'arms'

Microbrachius itself was a lake- dweller, coming in at just 8 centimetres in length - insignificant compared to the 8 metre bulks of the largest placoderms - but it possessed a rather interesting feature from which it derived its name: two small arm-like structures positioned on the underside of the body.

'Microbrachius means little arms but scientists have been baffled for centuries by what these bony paired arms were actually there for. We've solved this great mystery because they were there for mating, so that the male could position his claspers into the female genital area,' said Professor Long.

The use of arms for positioning during mating proves that Microbranchius engaged in a form of penetrative sex; the male using his claspers to transfer sperm to the female. 'Until this point in evolution, the skeletons of jawed vertebrates couldn't be distinguished because males and females had the same skeletal structures,' added Dr Brian Choo, also from Flinders University. '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.'

Claspers in the placoderm family suggest one of two alternative, but equally intriguing, scenarios: either an unprecedented loss of internal fertilisation in vertebrates or the coherence of the armoured placoderms as a single branch in the tree of life. Either way, the origin of penetrative reproduction is immensely revealing. Discoveries made over the past few years have shown that paired limbs, jaws and teeth in vertebrates originated with the placoderms. The use of penetrative reproduction by placoderms, alongside these previous discoveries, suggests that this group of bizarre armoured fish played a significant role in vertebrate evolutionary history, more than we previously gave them credit for.