The Science Behind Why Sex First Became a Thing

Sex is the primary means of reproduction for the vast majority of life on Earth. But when did it start and why?
Christopher McFadden
origins of sexromana klee/Flickr

Sex. The vast majority of multicellular organisms do it, and most of us think about it all the time, but how did it become a thing? 

Somewhere in the region of 99% of all multicellular organisms reproduce sexually. Each and every species have their own mating rituals and mechanisms for it too.


But how the whole thing started is something of a mystery to science. The great Charles Darwin more than a little confounded by it himself.

"We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements. The whole subject is as yet hidden in darkness", wrote Darwin in 1862.

But clearly, it is important and developed as a reproductive strategy for a reason. But just why it began, and when, is not well understood by science.

origin of sex bed
Source: Depositphotos

How were the first living things created?

First of all some housekeeping.

This article will be dedicated to the scientific examination of the origins of life. We will, therefore, not discuss, or examine, any of the myriad explanations provided by religious doctrines.

The origin of life is one of the oldest and most widely studied questions in science. To date, there are various theories on the subject.

Whilst they all have their own differing mechanisms and explanations, the basics are the same: -

- Organic molecules were somehow created through natural processes.

- These built up over time and at some point were sufficiently complex to become self-replicating. This eventually led to the so-called RNA world.

- RNA provided the means for self-replication, and self-assembly, that would kick-start the slow and unstoppable process of evolution.

The rest, as they say, is history. 

Here are some of the most prevalent theories on the origin of life. 

1. The clay hypothesis

One theory of the origin of life postulates that clay was the main reason life started on Earth. It was first suggested by organic chemist Alexander Graham Cairns-Smith at the University of Glasgow.

As the story goes, clay acted like prototype ribosomes to help concentrate and combine organic molecules together. This would also help explain why the vast majority of organic molecules, like amino acids in organisms, are sinistral or left-handed in their structure. 

Cairns-Smith suggests that mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.

2. The spark of life

One widely held hypothesis is that life was started by lightning. Electrical discharges helped generate amino acids and sugars from the Earth's abundant resources of water, methane, ammonia, and hydrogen.

The theory was first proposed and demonstrated by the famous 1953 Urey-Miller experiment. 

If true, the process would have continued unabated for millions of years building ever more abundance and complexity of organic molecules. This would have provided all the building blocks essential for life to begin. 

Since the 1950s scientist has been able to show that the early Earth's atmosphere was probably hydrogen poor. If true this might be a thorn in the theory's side. 

However, it might well be possible that volcanic clouds in the early atmosphere could well have supplied methane, ammonia, and hydrogen. 

3. Deep sea-vent hypothesis

Yet another hypothesis is that life started in the deep oceans around submarine hydrothermal vents. These vents, just like today, provide a lot of energy and hydrogen essential for the formation of organic molecules. 

These molecules could have accumulated in the nooks of the vents leading, eventually, to the formation of larger and more complex organic structures. 

4. We are all aliens

Another widely discussed theory is the Panspermia hypothesis. This theory suggests that life, or complex organic molecules, were delivered to earth by comets or asteroids that picked up 'hitchhikers' as they traveled through space.

Organic molecules are actually pretty abundant in space but they may have also developed on other worlds, like Mars, were blown into space during asteroid impact, and were eventually transported here. Some go as far as to suggest that complete microbes were also transported in this fashion to Earth (and around the cosmos). 

If true, this would suggest that life should be very common indeed around the cosmos. But it also doesn't really provide a satisfying answer to the question of the origin of life.

It merely moves the birthplace of life somewhere else.

When did the first multicellular organisms appear on Earth?

The short answer is we simply do not know. We may, in fact, never find out.

The problem is one of preservation. The fossilization process is very hit and miss and requires very particular conditions to occur. 

This is such a problem that the fossil record is full of gaping holes that may never be filled.

origin of life fossil
Source: Amanda Kelley/University of Alberta, Canada

That being said, we have found some tantalizing evidence of early life. To date, the earliest known life forms (that have been found fossilized) are micro-organisms preserved in ancient chert deposits in Australia.

These fossils are about 3.5 Billion years old, and if true, suggests that life began and evolved more quickly than previously expected.

How did sexual reproduction start?

Before sex existed, most life appears to have reproduced asexually. That is to say, that is was primarily a process of cell division just like most single-celled organisms still do today.

This is a 'simple' copy-and-divide process that most plants, bacteria, and animals do today. Compared to sexual reproduction, it is a much less 'messy' affair and doesn't require the vast amounts of energy required for sexual reproduction. 

Sexual reproduction, on the other hand, requires an organism to invest far larger amounts of energy, and resources, to achieve. It is more hit-and-miss and can even be dangerous or potentially fatal to the organism. 

A prime example is spiders. In many cases, the actual act of copulation will result in the male being killed, and often eaten, by his mate. But even in higher organisms, mating tends to lead to conflict and can also be potentially fatal.

So why on Earth (pun intended) did organisms develop such an energy intensive and dangerous method of reproduction? When did it start?

Sexual reproduction does, it turn out, have many benefits over asexual reproduction. One of the main ones is that it shuffles genetic materials around and allows for much greater variety in outcomes.

For this reason alone, it is one of the main driving forces behind evolution and biodiversity.

However, it should be born in mind that many single-celled organisms also reproduce sexually if environmental conditions demand it

Another benefit of sexual reproduction is that it is a means of storing and transferring information between generations. Sexually reproducing organisms basically "learn" to cope with the world and pass on what they have learned to their children.

But when did it all begin? This is much harder to tackle and is another question we may never be able to answer with any real satisfaction. 

origin of sex dummies
Source: Daniel Lugo/Flickr

What was the first organism to have sex?

Some of the earliest fossil evidence for sexual apparatus is from a 385 million-year-old fish called Microbrachius dicki ("Little Arms"). Scientists believe that these little arms were used by the male to latch on to their mate to copulate.

But the origin of sex is probably far older than that. Afterall we know that all sexually-reproducing organisms probably had a common ancestor at some point in time.

And we might just have a candidate for that. In 1.2 Billion-year-old rock deposits in Canada scientists may have found early evidence for sexual reproduction.

They contain a fossil called Bangiomorpha pubescens which is a form of red algae or seaweed, that appears to have sexually reproduced. This is, to date, the oldest reported occurrence in the fossil record. 

And it appears to have evolved this strategy in response to the harsh climate of the time.

"With respect to climate, it appears that the Bangiomorpha pubescens fossils appeared about the same time that hundreds of millions of years of relative environmental stasis had come to an end. We see major perturbations in the carbon and oxygen cycles at this time, suggesting major environmental shifts", says Galen Halverson at McGill University in Montreal, Canada.