The origin of oxygen: When did we start breathing? 


Let's look at a feature of the Earth that is indispensable for life, and unique to our planet: an oxygen-rich atmosphere. But how did that come about? Because a few million years ago, there was no oxygen at all. Slowly, oxygen began to accumulate in the atmosphere and this had major consequences for life on Earth.

Author: Kathelijne Bonne.

Humanity is not standing above the earth. We are a part of it. That all sounds rather hippie-like but it is pure science. The way our bodies work is programmed for us to live on the Earth only, under the present oxygen-rich conditions. We are the product of 4 billion years of adaptation. Our metabolism depends on other organisms (which we have to eat), and on the gases in the atmosphere, with which we interact by breathing. That is how we generate energy. 

The air we breathe is made up of a mixture of gases. The percentage of nitrogen in the atmosphere is 78% and the rest is oxygen, water vapor, argon gas, carbon dioxide and a few more gases. Oxygen is now indispensable, but there have been times when there was hardly any gaseous oxygen.

For completeness, the geologist in me must say that the oxygen that is now in the atmosphere, was locked away in the minerals of the rocks, and it is also an atom in carbon dioxide.

Long haul

Blue sky with clouds (water vapor). The atmosphere was once completely different.
Blue sky with clouds (water vapor). The atmosphere was once completely different.

The story of oxygen is a long-winded one. We must go back in time 2.4 billion years. Then there were only bacteria and archaea in the world, both single-celled organisms without nucleus. They had different ways to produce energy and used various sources. But when one source was exhausted, they had to look for other sources. therefore, they had to diversify.

Cyanobacteria for example wanted to take advantage of the abundant sunlight and the high concentrations of carbon dioxide present in the young atmosphere. They basically invented the process of photosynthesis, a form of metabolism, or a way of generating energy (humans do that by eating and breathing).

Making sugars

Photosynthesis involves harvesting energy from sunlight and absorbing carbon dioxide. With those ingredients, cyanobacteria made sugars and other organic building blocks. Oxygen was emitted into the sky as a byproduct. Hence, the atmosphere became increasingly enriched in oxygen through the action of photosynthesizing bacteria. Yet, the oxygen content of the atmosphere was still nowhere near as high as it is today.

Oxygen revolution

Scientists refer to this episode the Great Oxygenation Event. But it is also sometimes called the Oxygen Crisis. Why crisis? Because the presence of oxygen caused problems for many creatures, it was actually a poison. Many anaerobic organisms became extinct. Oxygen is highly corrosive and a strong oxidant. It has a strong tendency to react chemically. The "new" oxygen required a great adaptation of life. But it survived and learned new things. Actually, this oxygen revolution proceeded rather slowly, but then, bacteria also have a quite slow life and it took a long time before these tiny creatures had produced sufficient oxygen.

What were the long-term consequences of the increasing oxygen levels?

Ozone layer

Oxygen is normally bound in pairs: each oxygen molecule consists of two oxygen atoms, therefore oxygen in chemical language is O2. But very high up in the atmosphere, oxygen reacts with ultraviolet radiation, and binds in groups of three, O3. That's ozone, another gas with different properties. It shields the earth from dangerous solar radiation, which is deadly to life. The ozone layer would later allowed life to colonize the land (that happened about 430 million years ago).

Breathing oxygen: a good invention

But oxygen had other advantages. While in the beginning oxygen seemed harmful for many organisms, some organisms developed methods to make use of oxygen. They started extracting oxygen from the air, called breathing. That was a good "invention". Dbiochemical reason. For some biochemical reasons, aerobic respiration is much more efficient than anaerobic respiration. That is, for the same amount of breathing, an oxygen-breathing organism gets much more energy out of it.

This gave them the opportunity to evolve rapidly and experiment. One of the results was the emergence of the eukaryotes. These came into existence about a billion years ago. In a eukaryote, each cell has a nucleus and several organelles, such as mitochondria. Bacteria and archaea, no matter how efficient, diversified and long-lived, do not have a cell nucleus and organelles. The DNA is swimming around loose in their cells.

Lots of energy needed

Oxygen respiration eventually allowed eukaryotes to assemble into multicellular creatures. The maintenance of a multicellular being requires an enormous amount of energy, which is provided by oxygen respiration. Animals, and eventually humans, breathe. And consider that we must breathe continuously to keep our bodies alive. And as a by-product, we exhale carbon dioxide.

Circle of life

We extract oxygen from the atmosphere and plants and algae return the oxygen. The opposite happens with carbon dioxide. So there is a balance, a cycle. But the oxygen and carbon dioxide contents fluctuate on large timespans. Because the supply and withdrawal of both gasses fluctuate. Today humans have a high impact on the concentration of each gas in the atmosphere. By burning fossil fuels, carbon dioxide is on the rise. Meanwhile, the biomass is declining and we are shifting the balance between of oxygen and carbon dioxide at a faster than usual rate.


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Some sources: 

Hemp et al, 2010, Astrobiology Science Conference. Evolution of Aerobic Respiration.

Santos-Bezerra et al, 2021, Scientific Reports, Evolution of a key enzyme of aerobic metabolism reveals Proterozoic functional subunit duplication events and an ancient origin of animals. 11, 15744.

some keywords: great oxygenation event, oxygen, origin of oxygen, bacteria and archaea, oxygen revolution, breathing, anaerobic bacteria, early earth, young atmosphere,