Asked by: Aaron Cottingham, Chiddingly
Free oxygen is so reactive that it doesn’t last long in the atmosphere unless plants and other photosynthetic organisms continuously replenish it. This means that on any planet, including Earth, life must initially evolve without any oxygen to breathe.
There are lots of modern bacteria that ‘breathe’ many other elements and compounds, including sulphur, carbon dioxide, iron, manganese, cobalt and uranium. All of these are much less common in the Universe than oxygen though, which is the third most common element.
There are bacteria that can metabolise hydrogen gas, which is the most abundant element, but like all of these alternative metabolisms, it is much less efficient than oxygen-based systems. This might mean that simple bacterial life is common across the galaxy using lots of different biochemistries, but complex multicellular life only evolves where it has access to the boost in energy provided by oxygen reactions.
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