Photosynthesis is the process by which plants, algae, and cyanobacteria make food from water and carbon dioxide, using the energy from the Sun. As a waste product, oxygen comes into being. Oxygen is a colourless, odourless, and highly reactive gas. It constitutes 21% of the atmosphere on Earth and is essential for most forms of life.

The atmosphere of early Earth was filled with nitrogen, greenhouse gases, and hazy vapours from erupting volcanoes, and the temperature on the surface was blistering – hostile for any living being. Yet, simple microbial life was evolving in the oceans, and perhaps as early as 3.5 billion years ago, some of them (cyanobacteria) began using the energy of sunlight to make their food from carbon dioxide and water (a process known as photosynthesis). As a waste product, oxygen came into being.

At first, oxygen began filling oceans. For some tiny beings, it was too toxic, killing those who couldn’t adapt or hide in the seabeds or underground. For others who could exploit the odourless and colourless gas, it eventually became a lifeline, helping them to turn food into energy needed to survive and grow¹.

Around 2.5 billion years ago, there was enough oxygen in the water and more and more of it began to spread into the atmosphere (the Great Oxygenation Event), replacing greenhouse gases and clearing the air for a blue sky. Temperatures began to fall, too, and Earth entered an ice age for the first time (2.3 BYA).²

Over time, more and more oxygen came into being, enabling life to grow into animals (600 MYA), plants, fungi, and algae. Animals and fungi began to consume oxygen, while the two others began to produce it, too.

In the atmosphere, oxygen atoms teamed up into ozone molecules, and the ozone layer began to protect organisms from the harmful ultraviolet radiation of our Sun, allowing life to occupy the barren ground (500 MYA). First fires soon ignited when oxygen reacted with heated vegetation, perhaps due to lightning strikes or lava from erupting volcanoes.

Before settling to current levels, there was much more oxygen in the atmosphere, reaching up to 35% roughly 300 million years ago. This enabled invertebrates once ruling the Earth to become giants – insects with a wingspan more than a metre flew in the sky, and millipedes as long as 2.5 metres roamed in the forests. However, it seems oxygen did not explain the size of dinosaurs a bit later (240-65 MYA), the largest land animals ever to walk on our planet, since oxygen in the air already approached the present rate.

Today, the air we breathe holds around 21% oxygen, 78% nitrogen, and small amounts of other gases, such as carbon dioxide.

¹ Oxygen efficiently turns food into energy, enabling bigger sizes for those exploiting the gas. Oxygen eventually became the only way for animals, plants, and most fungi to turn food into energy; therefore, they can't survive without it. Yet, too much oxygen is toxic for us, too, and our bodies contain only a tenth of the oxygen available in the air, using it not only in metabolism but also to kill invading bacteria.

² Oxygen is a highly reactive element that can create bonds with almost all other elements. Therefore, it took so long for oxygen to spread from the oceans into the atmosphere. In the water, oxygen teamed up with others until it was hard to find others to bond with. Free oxygen could spread and eventually rise to the air. In the air, oxygen began to team up with hazy vapours, creating new, heavier elements that fell to the ground, clearing the air for a blue sky.

Bibliography

Lane, N. 2016. Oxygen the Molecule That Made the World. Oxford, United Kingdom: Oxford University Press. 374 p. ISBN 9780198784937. Pages 9, 11, 16, 20, 22-24, 35, 47-48, 52-53, 65-66, 73-75, 83, 101.

Panciroli, E. 2022. The Earth a Biography of Life The Story of Life On Our Planet Through 47 Incredible Organisms. London, United Kingdom: An Hachette UK company. 255 p. ISBN 9781529413984. Page 15-22, 96-99.

Dartnell, L. 2019. Origins How the Earth Shaped Human History. London, United Kingdom: Penguin Random House. 346 p. ISBN 9781784705435. Pages 169-174, 265.

Bryson, B. 2003. A Short History of Nearly Everything. London, United Kingdom: Transworld Publishers. 668 p. ISBN 9781784161859. Pages 362-367.

Frankopan, P. 2023. The Earth Transformed an Untold History. London, United Kingdom: Bloomsbury Publishing. 695 p. ISBN 9781526622563. Pages 27-28.

Brusatte, S. 2019. The Rise and Fall of the Dinosaurs. London, United Kingdom: Picador. 404 p. ISBN 9781509830091. Pages 113-117.

BBC Earth. 2023. Earth Atmosphere. Available at: https://areena.yle.fi/1-66606587 (Accessed: 12 March 2024).

Kilpua, E. & Koskinen, H. 2023. Aurinko Tuttu ja Tuntematon Tähtemme. Helsinki, Finland: Gaudeamus. 318 p. ISBN 9789523452329. Pages 263-264.

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