Oxygen produced by the first life this planet had for over 2 billion years was a waste byproduct. And the amount of oxygen it put off actually caused the first of the 5 major extinctions the planet has seen, which then made it possible for life that required oxygen to flourish.

We aren't looking for oxygen to prove life per say, but for certain elements which if in certain amounts with combination of other elements would indicate a strong suggestion of life.

Hope that helps.

Edit: Oh, also, almost forgot: it is believed by some that the overabundance of oxygen in our atmosphere helped accelerate the extinction of the non-avian dinosaurs due to the atmosphere being volatile and susceptible to longer hotter fires. Oxygen is garbage lol

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Oxygen is not strictly necessary for life, as there are many forms of life on Earth that do not use oxygen or use it in very small amounts. For example, some microorganisms can use other gases, such as hydrogen or sulfur, as a source of energy.

However, oxygen is an exceptionally good source of energy for life. When oxygen is present, organisms can use it in a process called aerobic respiration, which is a very efficient way of producing energy from nutrients. This is why many forms of life on Earth, including humans, rely on oxygen for their energy needs.

It is also possible that life on other planets could use other gases or elements as a source of energy. For example, some scientists have speculated that life on other planets might use methane or ammonia as a solvent instead of water, or that it could use other chemical reactions to produce energy. However, we have no concrete evidence of this, as we have not yet discovered any life on other planets.

In summary, while oxygen is a very important element for life on Earth, it is not necessarily the only element that could be used by life on other planets. It is possible that life on other planets could use other gases or elements as a source of energy, or that it could use completely different chemical reactions to produce energy.

Well the "proof it works for us" is part of why our searches for life (and habitable planets) center on life lthat functions like our own. It's easier to detect and confirm carbon based, oxygen breathing, water drinking lifeforms (especially at a distance) than looking for exotic life that doesn't use those things. As noted in the comments by greypowerOz, there is other reasoning besides that, oxygen is great for metabolic processes and complex life, but other respiratory gases like hydrogen has been considered.

If you're looking for other forms of life, you're really looking for chemical disequilibriums, an abundance of certain elements (like oxygen, and methane) that on their own, would dissipate quickly, and so need some process like life to sustain that abundance. However, false positives are possible, and natural processes could create these same disequilibriums under the right conditions, and look like life. So that's a big thing we need to keep in mind when looking for biosigniatures, especially ones different from life like on Earth.

>why are we so obsessed with oxygen when we only have proof that it works for us?

The Copernican Principle: the Earth is not the center the universe, and we humans do not occupy any special place in it. Since there's nothing special about us, then any particular trait of our biology is most likely to be common rather than rare or exotic. Since we breathe oxygen, it isn't because oxygen breathing is rare, but because it's common.

It is an assumption, yes, but your assumptions are more likely to be correct (or closer to correct) the less dependent they are on rare or exotic conditions or properties.

Also, since the only forms of life we have any experience observing is the kind that occurs here on Earth, the easier it will be to recognize life elsewhere the more closely it resembles life here.

That's not to say other chemistries which sustain life are impossible, but it will probably be harder for us to recognize them as life the first time we observe them.

I'm far from an expert in astrobiology and I have forgotten some useful terms,but I haven't seen anyone else talking about the fact that a large part of why oxygen is useful to us is its high electronegativity. Essentially, this means that it attracts electrons.

In aerobic respiration, Oxygen is the final electron acceptor, this means it is the particle that ends up with an extra electron after all the energy producing reactions are done. Without having an element with a high electronegativity, there are, essentially, fewer energy yielding reactions that can be performed on a given substance. Thus, life doesn't need Oxygen, but there aren't all that many options for elements with a higher electronegativity.

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From my quick research it seems that only fluorine has a higher electronegativity than Oxygen, but it is also the 24th most abundant element in the universe, compared to #3 for oxygen. This is a huge difference, oxygen makes up about 1% of the matter in the universe, whereas fluorine makes up around (4 × 10^−5)%. Fluorine is also more reactive than oxygen which makes it more dangerous, thus, any lineage utilizing it as the final electron acceptor would have to evolve a metabolism capable of using it safely, and the energy benefits would have to outweigh the costs of potentially running out of environmental fluorine.

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Of course, some planets are relatively rich in Fluorine, on earth, for example, fluorine is the 13th most abundant element rather than the 24th, so perhaps there are planets where the abundance of fluorine negates the issue of scarcity. So it seems possible, and there is another comment here saying that the conceptual possibility of hydrogen-respiring lifeforms is agreed upon by many scientists( and I remember discussing an extremophile bacteria or archaea that uses sulfur as their final electron acceptor in one of my undergraduate courses). But it seems that complex life probably requires high metabolic efficiency, and for that you need an element with high electronegativity. And it just so happens oxygen is the second most electronegative element and the 3rd most abundant element. So, if the assumption that an element with high electronegativity is needed in relatively large amounts for complex life to form, we would expect the vast majority of complex life in the universe to use oxygen, even if there are other technical possibilities.

There could be life out there so different from us we might not recognize it as life, imagine silicone based life instead of carbon based life like here on Earth. It makes sense for us to look for what we know. This is why we’re so interested in rocky planets the right distance from there star for liquid water to be present. Life exists without oxygen on Earth, but not without water.

Because all life that we know of have evolved using water/oxygen and are carbon-based life forms. So there is no proof of it evolving in other ways, so to assume would just be pure speculation based on zero previous hints or evidence. However, if you check out extremophiles, it will show you the range in which life can evolve through our current knowledge.

The only form of life that we know of is ours. We understand our life chemistry and what it requires. There are other theoretical life chemistries but they’re only theoretical.

I’m not going to pretend I know how those other chemistries work but it makes sense to first check for the conditions that we know actually work before we look for theoretical conditions.

I think that really if we look at the spectrum of any planetary atmosphere and find it to be out of equilibrium with respect to the partial pressure of any gas, that would be evidence worth investigating as being from a life or a life-like source. Oxygen isnt really a requirement for life at all but some people probably assume its a requirement for complex multicellilar life but that is still just an assumption nothing that follows from any fundamental principles

Anyone that thinks they know what can generated on literally trillions on planets is silly. The reality is we’ve found creatures on earth that consume rock, breath poisonous gas, etc. Life can exist in many forms, the question is what is more adaptable? Given the environment and such. Oxygen was a biproduct of our life here but that doesn’t mean it will be this way everywhere.

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Oxygen supports combustion. Most other gases don't. You can't combust hydrogen without oxygen, it's already the simplest element. Helium is an inert gas like nitrogen. For life to take place chemical reactions need to take place.

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Oxygen actually caused the first great extinction. It was basically a poison to the earliest life forms

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I take the principal that Humans, although unique in detail, are more or less typical for what we should expect for the type of life that develops ET. I say this not that we are special or super stars, but that we are probably more normal and and typical rather than some sort of extreme edge case.

The whole us using oxygen thing is probably nothing special and if we ever get our hands on Encyclopedia Galactica and get to browse the countless known sentient species that consuming oxygen will be more or less the norm.

We can have two takes. Oxygen is something that makes us special, it is our unique, or at least very, thing in the galaxy that makes life on Earth very special, and very rare. Something that makes humans special is our ability to breath oxygen.

And then the other take. Oxygen consuming animals are super common, most life that isn't microscopic consumes Oxygen. The Great Oxidation Event is probably a fairly typical thing to happen early in a planet's history. We are not special because we consume oxygen.

My question is that if mars was like earth many many years ago that means life was there on Mars and life ended just like we are gonna end, so could there actually be fossils of life like we had dinosaurs

I think scientists are wrong. Just because we need oxygen and water to survive doesn't mean other lifeforms out there need it too. I'm willing to bet there is life out there that wouldn't survive on earth and vice versa. The universe is huge and old. I wouldn't be surprised if some life forms need to live in 300 degree heat to survive in other parts of the universe.