Actually scientists suspect that most (if any) ET would breathe methane, not oxygen.

Oxygen really likes to react with other elements and compounds which makes it well suited for metabolic processes, which means an organism gets more energy out of the matter it consumes. (Its reactivity also made it kind of damaging/toxic to early organisms). Organisms that were lucky and got a rudimentary ability to use oxygen and be resistant to its negative effects gained an evolutionary advantage and could thus become dominant. Over time this turned into a dependance as the use of less efficient energy sources got minimised and eventually “phased out”.

@ragingloli So oxygen is that person in biology’s Facebook universe, the one with the million friends? Lol.

@mazingerz88 I think perhaps a better way of putting it is that oxygen is literally what makes stuff burn. Your body actually does sort of burn the stuff it eats.

As @ragingloli, there were organisms before the atmosphere was rich in oxygen. Plantlike organisms that got energy from sunlight are actually the ones responsible for putting all the oxygen in the atmosphere. Originally, when this started happening, oxygen was highly poisonous to life, and most living things died in what is sometimes called the “oxygen holocaust.” The surviving organisms evolved to adapt to an oxygen-rich atmosphere, and luckily for them, oxygen is super-reactive and so ended up helping them use energy more efficiently.

Well all living things are a majority made up of water. H20. Dihydrogen Oxide. Perhaps it has to correlate with constantly reloading the Oxygen within the body?

Yup, Rags hit the nail on the head exactly correct. Qingu is also correct.