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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Oxygen has two properties (well, it has a lot, but we’ll focus on two) that make it incredibly useful for life.
a ) It’s incredibly reactive, making it very useful in things like the controlled burning of sugars that give your cells the energy they need to do stuff, facilitating many of the chemical reactions in your body. What we call “burning” is just code for “stuff rapidly reacting with oxygen”. Most of the things that burn burn because of an oxygen reaction, from a log fire to sugars converting in your cell to rust forming on iron. All of it is cut from the same cloth at different speeds and intensities.
b ) it’s abundant in nature, meaning that the lifeforms that depend on it don’t have to struggle with obtaining oxygen: it’s already a large portion of the atmosphere. You could use other molecules for similar purpose, but then you’d have to find a near-never-ending supply of that molecule.
Now, oxygen in itself isn’t strictly “needed” for life if you limit your life to fairly simple lifeforms : many of the first lifeforms that developed on earth were anarobic, meaning they could survive in environments with very limited or no oxygen. In the dawn of life the earth was much more hostile to life and nearly all oxygen was locked up in other molecules, like water or Co2. The life that lived then didn’t need molecular oxygen, but instead used other molecules that could perform similar jobs (albeit not always as effectively). Things like nitrate or sulfate (Which, technically, do have an oxygen atom in them but we’ll sidestep that technicality). Anarobic cells may even *dislike* free oxygen. The first massive extinction event was caused by single-celled organisms that started producing oxygen, accidentally poisoning the earth for the other cells that couldn’t use it.
Oxygen is not the fuel per se. Oxygen is what is required for the fuel to burn. Oxygen is the “burn” molecule. It’s why liquid oxygen is so very dangerous.
Hydrogen is fine as a fuel. It’s just rare to find it on its own, so not much really wants it. But hydrogen+oxygen = water + energy and that is what life wants: making energy.
It’s not about the fuel being present. It’s about the chemical reaction that the fuel provides, and fuel alone doesn’t just self-destruct into energy for life. Substitute hydrogen for sugars, fats, and you have energy for humans. But we still need oxygen for it to happen.
Oxygen is semi-stable in a diatomic molecule (O2), and so can float around like that all over the place without much fuss. However, oxygen would prefer to be bonded in a different way, like to hydrogen (H2O) or carbon (CO2), which are lower energy states. We use that difference in energy as energy; when O2 converts to H2O or CO2, we steal some of the energy that gets released for our own purposes.
There aren’t many other elements (if any) that are very common, semi-stable, and easily convertible to a more stable, energetically preferable form with other common elements.
To make fire (AKA burn) you need an oxidizer and a combustible. Combustible can be wood, coal, grass, or many other things. Oxidizer is oxygen. Like, that’s the whole list, you need oxygen for an oxydizer, which is necessary to burn things.
To produce energy you need to “burn” things inside your body. Which mean that you’ll always need oxygen. If you don’t have oxygen, you can’t burn things. Can’t burn things can’t move. Can’t move, can’t live.
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.
Oxygen is crazy. It is a very destructive atom that will wreck molecules with abandon.
Why? Because it is electronegative. This means that it wants to hog electrons. When a single oxygen meets a protein or DNA or almost anything, oxygen gives it a wedgie and steals an electron (usually it does this by bonding to the thing it reacts with).
Billions of years ago, there was almost no pure oxygen molecules around. At some point, an enterprising algae figured out how to get carbon off of CO2 and poop out oxygen as waste. This was cool fit that algae, but poisoned the atmosphere with oxygen and killed like 95% of all life on earth. Eventually, new life just had to figure out how to deal with it.
So, oxygen really messes stuff up…but we need it. Why? Precisely because of how powerful it is. If you know how to use oxygen’s reactivity you can power lots of cellular functions with it. That’s what happens in the electron transport chain in the mitochondria.
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