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The first life, 3.8 billion years ago, was relatively simple. They were creatures known as prokaryotes: single-celled microscopic organisms where strands of DNA floated openly around the cell, increasing the risk of damage to the cell. But they survived and the bottom of the Earth’s oceans began to be populated by small living masses.
The Shortest History of Sex, David Baker, Pan Macmillan India, 2023.
To overcome the shortage of “real estate” 3.4 billion years ago, some prokaryotes evolved to live near the surface of the oceans, without keeping warm with underwater volcanoes. Instead, these new microscopic spots evolved to use energy from the Sun. They used photosynthesis, converting water, sunlight, and carbon dioxide (CO2) from the atmosphere to feed themselves. Like today’s plants.
And like today’s plants, they pumped oxygen (O2) into the atmosphere as a waste product. The problem is that oxygen is very turbulent, can create violent chemical reactions, and in large quantities could kill fragile primitive life, such as those ancient microbes that had evolved on a primitive Earth where there was very little oxygen in the atmosphere.
By 2.5 billion years ago, microscopic photosynthesizers had increased the level of oxygen in the atmosphere from almost nothing to 2.5 percent. This nasty chemical killed dozens of microbes in an event known as the Oxygen Holocaust, the first known mass extinction event on Earth and one of the only mass extinction events (before humans) initiated by living organisms instead. from an asteroid impact or a supervolcanic event. rash.
The surviving microbes developed an increasing tolerance to oxygen in the atmosphere. Some of them even developed the ability to consume oxygen instead of carbon dioxide, reversing which chemical was food and which was 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 photosynthesizers remained the majority of living things on Earth and continued to create absolute disasters by pumping out oxygen. By 2.2 billion years ago, enough O2 had accumulated in the atmosphere for oxygen atoms to begin grouping together in threes, creating ozone (O3). The resulting ozone layer covering the Earth reflected many of the Sun’s rays back into space. While today we desperately need the ozone layer to protect us from solar radiation, in the short term 2.2 billion years ago this was not a good thing. Photosynthesizing life continued to increase the thickness of the ozone layer.
As a result, the Earth became increasingly colder. The oceans froze at the poles. The ice then spread towards the equator, locking the entire planet in an icy prison, in the first ‘Snowball Earth’ event, about 2 billion years ago. The global average temperature would have been around -50°C.
Snowball Earth put pressure on microbes living in the now ice-covered oceans. As a result of the strain, a new type of microbe evolved: the eukaryote. These are “robust” cells, 10 to 1,000 times the size of prokaryotes, which also evolved to protect their DNA by keeping it in a central core rather than letting the strands simply float around the cell. Humans are descended from these eukaryotes, as are all ancestors and descendants of the evolutionary tree of the plant and animal kingdoms. And it was these small eukaryotic masses that were the first creatures to have sexual relations.
In the same disastrous period as Snowball Earth, these microscopic eukaryotes began to engage in carnal relations with each other, as 99.9 percent of all eukaryotic life does today. The habit persisted and has become increasingly exciting and disconcerting. But the question of how and why our microbial ancestors began to feel compelled to exchange genetic information, the way two people might exchange phone numbers in a bar, remains a mystery.
The above is an extract from The shortest history of sex. an exploration of sexual evolution, tracing it from approximately two billion years ago to the present day, by David Baker.
David Baker is a history and science writer.