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Most organisms depend on water for metabolisms. Even the higher organisms need to maintain their daily water intake. Why water and not something else (for example like H2S)? I know H2S is poisonous for our body but isn't that just because our body is not compatible with it? Is there a possibility that we could have been compatible with H2S and not H2O? What are the factors that prevent this?

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    $\begingroup$ "I really don't know why it is that all of us are so committed to the sea, except I think it's because in addition to the fact that the sea changes, and the light changes, and ships change, it's because we all came from the sea. And it is an interesting biological fact that all of us have, in our veins the exact same percentage of salt in our blood that exists in the ocean, and, therefore, we have salt in our blood, in our sweat, in our tears. We are tied to the ocean. And when we go back to the sea - whether it is to sail or to watch it- we are going back from whence we came." John F. Kennedy $\endgroup$ – AMR Jan 2 '16 at 6:21
  • $\begingroup$ I am confused as to why water and not something else.... $\endgroup$ – Shirin Riana Jan 2 '16 at 6:29
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    $\begingroup$ Water is an abundant compound that exists as a liquid at a temperature range amenable to complex organic chemistry. It's not too reactive, or too inert. It's polar which allows it to dissolve salts and other ions, while also driving hydrophobic compounds into aggregates like micelles or bilayers. Other simple liquids like NH3 would only exist at very cold temperatures where reaction rates are slow, or dissolve too much stuff, or not enough stuff, or react with too many functional groups, or just aren't abundant enough. Heavier atoms, like Sulfur, aren't as common as lighter ones like Oxygen. $\endgroup$ – user137 Jan 2 '16 at 6:38
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    $\begingroup$ We evolved in a water environment. From the LUCA (the last universal common ancestor) through to when the first fish came onto land we lived in the sea... 3.5 Billion years of evolution. That and all other hydrides except fluorine are gasses. 2012books.lardbucket.org/books/… $\endgroup$ – AMR Jan 2 '16 at 6:38
  • $\begingroup$ Sources say that dna and everything else was formed due to complex reactions which took place in the oceans. Thus, we prefer water or H2S $\endgroup$ – Tusky Jan 2 '16 at 7:41
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One source says,

Reactions cannot happen without water molecules.

It's not the most impressive source, but here's another source that has the same theme, with more details:

Cosmo Quest Forums

The water molecule forms non-covalent bondings. It is bipolar and easily 'gets inbetween' other molecules which want to marry by charing their electrons more permanently. This makes water a great dating site where for example aminoacids quickly can try out their fit to many different parts of an RNA molecule. If they don't make good enough covalent bondings, water will immediately be there to separate them until the next one comes along for a try.

Methane (on Titan) is as liquid as water and does transport stuff around. But it lacks the ability to interfer with how other molecules bond. Biochemical processes would be much slower in methane liquid than in liquid water.

Water is also a fabulous solvent; so many things can be dissolved in it. As others have mentioned, the simple abundance of water in the universe may be a factor.

However, I don't know anything about H2S' bonding ability or its power as a solvent.

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  • $\begingroup$ H2S is a gas until -50°C, or there abouts... not a great solvent... Also how the macromolecules came about is less relevant to the fact that Life evolved in an aqueous environment. Cells are mostly water, they evolved in water, that is why it is water. $\endgroup$ – AMR Jan 2 '16 at 9:51
  • $\begingroup$ In fairness, Shirin Riana more or less asked if life could have evolved on Earth IF it had been covered with H2S (or something else) other than water. In fairness, I didn't really answer that. I basically said "Water does some cool things!" but could there be living things on other worlds that substitute something else for water? $\endgroup$ – David Blomstrom Jan 3 '16 at 1:04
  • $\begingroup$ @DavidBlomstrom there could be, but such speculation is off-topic here. Try Worldbuilding. $\endgroup$ – MattDMo Jan 3 '16 at 3:06

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