I’ve noticed that most of, if not all living things rely on oxygen in one way or another to breath, live. We humans need oxygen so then is carried to our bloodstream to oxygenate our internals, same with a fish for example where the oxygen is obtained from the water. But why oxygen? Why live couldn’t have happen with other elements like hydrogen for example? Why oxygen is any life’s fuel in our universe?
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So, first of all, not all life needs oxygen. In fact, the first life is thought to have not used oxygen, indeed actively avoiding it. This is because oxygen is one of the best oxidizers. Which is why that’s what it’s called when atoms are stripped of electrons in a chemical reaction. This all changed when the cyano bacteria attacked. In fact, earth almost lost all life because of oxygen killing everything.
Now, to fully answer the question, life needs energy. That’s basically the definition of life. The way life gets usable energy is by taking a fuel of some sort and reacting it in some way to turn the fuel into a lower energy state waste plus excess energy. Then it uses that energy to do things. I was intentionally extremely vague with that sentence because there are a lot of ways life does this. Relevant to the current discussion however, life uses a hydrocarbon fuel, reacts it with oxygen in an oxidation reaction, and that produces lower energy wastes like CO2 and energy (We are ignoring ATP for this because it’s irrelevant to the question). So now we have to think of what is the best molecule to use that oxidize the fuel? Well, look at the top right of the periodic table. Ignore the noble gases. Further right you go, the better the element is at oxidizing. Further up you go better it is at oxidizing. Where is hydrogen? It’s on the left. That’s out. Oh look there is an element that is better! Fluorine! Chlorine too. So why aren’t those used? Well they are….sometimes. but, both of them are so good at oxidizing that they are never really found on their own. So oxygen is the best oxidizers that is ever found as a pure element in significant quantities in nature. And there you go, that’s why. It’s the best that exists.
A side note. There are also some other significant advantages that oxygen has. It’s very abundant in water, it’s easily made as a waste product in photosynthesis, it has excellent gas properties making diffusion very easy with it compared to a lot of other molecules, there is a lot there. But the two reasons it is used as the “fuel” (really the oxidizer) for life are that it was a waste product for early microbes and thus was very abundant, and that it is the best oxidizers that can be found naturally in any large quantities because the stronger oxidizers all decay too fast and in an uncontrolled way.
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