Early earth history had almost no free oxygen, but plenty of life-forms.
If you really look closely, Oxygen is a toxic waste byproduct of some forms of early life (and plants presently)
Oxygen itself is quite poisonous to many life-forms. However, a certan number of life-forms evolved to use oxygen as part of their energy process, such as ourselves. They evolved after the first life-forms started producing significant quantities of oxygen.
Oxygen is quite incidental to the development of life on a planet, and simply indicates that it is likely to have occurred. However life can exist that does not produce oxygen as a by-product, examples include organisms that existed in early earth history, but also include specialised organisms in some high temperature locations on earth today.
Oxygen is toxic to all life forms, IF the concentration is high enough!
Early photosynthetic organisms (probably related to the cyanobacteria) were responsible for releasing oxygen from CO2 and other chemically-bound sources. However, oxygen did not reach all parts of our world, and as pointed out we do still have organisms that cannot survive in the presence of even tiny amounts of oxygen. However, these can exist not only in geothermal areas, but also at low temperatures typically in sediments at lake bottoms. Most of these do not release oxygen, but move it from one chemical state to another. The best example are some of the bacteria that produce methane; others produce hydrogen sulphide. It is thought that these are direct decendants of early life, sometimes being referred to as archebacteria.
On the point of burning water; H2O can be oxididised and exist as H2O2 (peroxide). This reaction is one of the things that kills oxygen-using organisms. But as they say on TV, don’t try it out at home…
Going from H[sub]2[/sub]O to H[sub]2[/sub]O[sub]2[/sub] absorbs energy, rather than liberating it, so it can’t really be called “burning”.
And I’m not quite certain what you mean about oxygen being “toxic to all life forms, IF the concentration is high enough”. Breathing pure dry oxygen isn’t exactly a good long-term plan for humans, but it can certainly be done.
Toxic doesn’t mean it kills you right away, it means that it is bad for you. Which you agree with when you say that breathing high-concentration O[sub]2[/sub] “isn’t exactly a good long-term plan for humans”.
Yup--simple example. Scuba divers have big problems with nitrogen as they go deep. Various exotic mixtures are used as you go deep, but not pure oxygen because it would kill. The deeper you go, the smaller the percent of oxygen in the mix. One mix used at extreme depth is oxygen and hydrogen. It won't burn, though--not enough oxygen.
Yeah, if there is the wrong sort of contamination, it will spontaneously decompose. Boom!
However, I was reading an interesting thing about it recently. Somebody has designed a helicopter powered by it. It's a *VERY* simple system--no engine, no electricity. It's basically a seat & fuel tanks under a rotor.
The hydrogen peroxide is pumped (initially *BY HAND*) up to the rotors, it flows out to the tips where it hits a catalyst. The result is a rocket engine.