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The Invention of Sex

12 Jan, 2012

In the latest of our shortlisted entries to the 2011 Wellcome Trust Science Writing Prize, Daniel Swindlehurst compares sexual and asexual reproduction.

Every generation thinks they invented sex and it’s well established that it’s impossible to conceive of your parents having sex. Of course deep down we know this isn’t the case, we know sex was happening long before we existed, and that at some point our parents must have indulged, at least once. But back in the depths of geological time one generation did invent sex.

Sex is pretty important. We’ve devoted countless plays, songs, reality television shows and even wars to it. Without it we’d quickly cease to exist. But for all of the attention we lavish upon sex, few of us know when it was invented and why it became so popular?

Before sex, reproduction was a more prosaic affair. Organisms grew until they attained a certain mass, then simply split into two identical clones. Such ‘asexual reproduction’ is still carried out today by many organisms, mostly microscopic life-forms like amoeba and bacteria, but also by a number of animals and plants, ironically including the aphrodisiacal strawberry, which can create offshoots, called stolons that grow into independent copies of the original plant.

At a point in the history of life at least one pioneering species struck on an innovative method of reproducing. Rather than cloning themselves they began to swap genetic material with other members of their species, which could then be gestated and developed into a new individual. This innovation, branded sexual reproduction, became rather fashionable and can now be found throughout the evolutionary ‘tree of life’.

The question as to why sex became so popular has entertained biologists for some time. After all, asexual reproduction has its advantages – all offspring are ‘born’ without the need to undergo further development and population sizes can increase at rates far beyond even the most energetic species of rabbit.

Sex also carries a number of disadvantages. It requires a lot of energy in the form of It’s a great way to spread disease, it usually involves a search for a mate, seduction rituals and lying about your job, and can end badly for those involved – particularly for the males of many species of spider.

However, sexual reproduction offers one clear evolutionary advantage. It allows greater degrees of variation to be achieved between individuals of the same species. Variation is the fuel of evolution – it provides the raw material for natural selection to occur.

One mechanism that causes variation between individuals is mutation. As DNA sequences are copied mistakes can be made, at times leading to the development of new processes or structures within an organism. This occurs in all species, irrespective of how they choose to reproduce, and appears to have driven much of the evolution of life for billions of years.

But sex offered new possibilities for variation, as the genes from two parents are split and recombined in their offspring, leading to the creation of unique genomes and a greater degree of genetic difference between individuals of the same species. This may have endowed such species with a greater stock of evolutionary innovations that allowed them to better conquer environmental adversities, giving rise to the process of ‘true’ speciation witnessed today in plants, animals and fungi.

So when was sex actually invented? What sort of fossil evidence would be required to prove sex was occurring?

The British palaeontologist Nicholas Butterfield believes that the best evidence for sex is to be found in multicellularity. For over 2 billion years all life was single celled, but likely around 1.7 billion years ago it became multicellular. This allowed cells to become specialised, which in turn allowed more structurally complex life to emerge. Butterfield makes the case that sexual reproduction was the vital step required to kick-start cellular differentiation, as the processes required for sexual reproduction are similar to those required for the development of specialised cells. Where you find differentiated cells you’ll find sexual reproduction

In 2000, Butterfield described a 1.2 billion year old algae-like multicellular fossil with clear cellular specialisation. Furthermore, it had what appeared to be spore-like structures that allowed different male and female specimens to be identified. This is arguably the earliest multicellular fossil discovered with specialised cells and is considered to be the earliest evidence of a sexually reproducing species.

In true British style, Butterfield named the fossil specimen Bangiomorpha pubescens, pubescens from puberty – as life on Earth was passing through its evolutionary infancy and emerging into its sexual maturity – and Bangiomorpha, partly because it resembles a modern red algae called Bangia, and partly, well, I’ll leave that to your imagination.

Daniel Swindlehirst

This is an edited version of Daniel’s original essay. Views expressed are the author’s own.

Find out more about the Wellcome Trust Science Writing Prize in association with the Guardian and the Observer and read our ‘How I write about science‘ series of tips for aspiring science writers.

Over the coming months, we’re publishing the shortlisted essays in this year’s inaugural competition.

Image Credit: Wellcome Images
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