For as long as animals have been around on our planet, they’ve been having sex - most often with a partner, but sometimes on their own. In fact, sex is thought to pre-date animals themselves, at least sex in the simplest sense of sharing genetic material.
What was the first species to reproduce?
According to researchers, the first living organisms to participate in some kind of genetic exchange were bacteria, around two billion years ago and a long, long time before the first animals emerged.
The first animals to get involved in the act were probably sea sponges, some 800 million years ago. Just like today’s sea sponges, these ancient sea sponges likely reproduced by shooting eggs and sperm into the water column, which combined with eggs and sperm from other sea sponges to form larvae.
What was the first animal to have sexual intercourse?
This indiscriminate method isn’t quite the intimate method we’re familiar with. To find the first evidence of this kind of sex, termed ‘sexual intercourse’ by biology professors or ‘coitus’ by anyone born before the 1950s, we have to travel back in time 385 million years.
In 2014, a team of researchers announced they had discovered the first clear evidence of unique male and female sex organs in vertebrate animals. These belonged to the aptly named Microbrachius dicki - an ancient, armoured fish that lived during the Devonian Period in lakes in what is now Scotland and Estonia.
The males of this species had L-shaped genital limbs, known as claspers, while the females had reciprocal genital plates. These adaptations point towards a fish that reproduced by having sexual intercourse, rather than by the more popular, tried-and-tested method of the period, spawning. It’s thought Microbrachius dicki had sex by swimming side by side with their bony arms - and other bony bits - locked together.
Microbrachius dicki belongs to a group of fish known as placoderms. These long-extinct fish are not only ancestors of today’s fish, but distant ancestors of all living terrestrial vertebrates too, including us. It’s from placoderms like Microbrachius dicki, and other ancient, armoured fish, that we inherit some of our most defining features, from jaws and paired limbs to complementary, male-female genitals.
Asexual reproduction vs sexual reproduction
There are many benefits to sexual reproduction, which produces genetically unique offspring, over asexual reproduction, which produces genetic clones of the parents. For starters, sexual reproduction produces much greater genetic variation amongst animal populations. It also combines advantageous genetic mutations that may have occurred in different individuals.
This swapping and sharing of different genes increases animals’ chances of survival in unpredictable, ever-changing environments. There’s also less chance of populations being wiped out by diseases that may have otherwise affected a population of genetically identical individuals.
The first animals to have sex certainly weren’t aware of these benefits; simple instincts drove their desires to procreate. Nevertheless, by having sex and sharing genes and advantageous genetic mutations, these animals, and those that followed them, quite literally drove the process of evolution, ultimately giving rise to the incredible diversity of animals we see around us today.