The first life 3.8 billion years ago was relatively simple. They were creatures known as prokaryotes: single-celled microscopic organisms where DNA strands floated openly around the cell, increasing the risk of it getting damaged. But they survived, and the bottom of Earth’s oceans began to become overcrowded with living little blobs.
To overcome the shortage of ‘real estate’ 3.4 billion years ago, some prokaryotes evolved to live near the surface of the oceans, no longer keeping themselves warm with underwater volcanoes. Instead, these new microscopic blobs evolved to use the Sun’s energy. They used photosynthesis, converting water, sunlight and carbon dioxide (CO2) in the atmosphere to feed themselves. Just like plants today.
And just like plants today, they pumped oxygen (O2) into the atmosphere as a waste product. The problem is that oxygen is highly turbulent, can create violent chemical reactions and in large quantities could kill fragile primitive life like those ancestral microbes that had evolved on an early Earth where there was very little oxygen in the atmosphere.
By 2.5 billion years ago, microscopic photosynthesisers had increased the level of oxygen in the atmosphere from next to nothing to 2.5 per cent. This nasty chemical killed off scores of microbes in an event known as the Oxygen Holocaust, Earth’s first known mass extinction event and one of the only mass extinction events (before humans) kicked off by living organisms rather than an asteroid impact or a super-volcanic eruption.
The surviving microbes evolved an increasing tolerance for oxygen in the atmosphere. Some of them even evolved the ability to consume oxygen instead of carbon dioxide, reversing which chemical was food and which waste. These were the first aerobic species, microscopic creatures similar in that respect to humans and other animals. We inhale oxygen, we exhale carbon dioxide.
But photosynthesisers remained the majority of living things on Earth and continued to create unmitigated disasters by pumping out oxygen. Enough O2 had gathered in the atmosphere 2.2 billion years ago that the oxygen atoms began to group together in threes, creating ozone (O3). The resulting ozone layer that blanketed Earth reflected a lot of the Sun’s rays back into space. While we desperately require the ozone layer to protect us from solar radiation today, in the short term 2.2 billion years ago, this was not a good thing. Photosynthesising life continued to increase the thickness of the ozone layer.
As a result, Earth got colder and colder. The oceans froze at the poles. Then the ice spread down towards the equator, encasing the entire planet in a frozen prison, in the first ‘Snowball Earth’ event, roughly 2 billion years ago. The average global temperature would have been around -50°C.
Snowball Earth imposed a strain on the microbes living in the now ice-covered oceans. As a result of the strain, a new kind of microbe evolved: the eukaryote. These are ‘beefier’ cells, 10 to 1000 times the size of prokaryotes, which also evolved to protect their DNA by keeping it in a central nucleus instead of letting the strands just float around the cell. We humans are descended from these eukaryotes, as are all the ancestors and descendants in the evolutionary tree of the plant and animal kingdoms. And it is these tiny eukaryotic blobs that were the first creatures to have sex.
In the same disastrous Snowball Earth period, these microscopic eukaryotes began to engage in carnal relations with one another – as does 99.9 per cent of all eukaryotic life today. The habit stuck and has grown only more thrilling and perplexing. But the question of how and why our microbial ancestors began to feel compelled to exchange genetic information, in the same way two people might exchange phone numbers at a bar, remains shrouded in mystery.
The above is an excerpt from The Shortest History of Sex, an exploration of sexual evolution, tracing it from approximately two billion years ago until the present time, by David Baker.
David Baker is a history and science writer.