Non-Water Biological Solvents
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Basic Information

Life as we know it requires water. Water acts as a biological solvent, dissolving various chemicals into solution, so that they may be transported more readily via the body's cells. It's an important component in photosynthesis. It's polar, forming hydrogen bonds that allow it gather other materials from solution and clump them together - which may have been a factor in kick-starting life on earth. Water is in every cell of life here on earth (see also Cytosol). Water also helps regulate temperature within the body.

However, it's possible that alien biology might subvert that assumption. There are places in the universe without water, or where the water exists only in the frozen form of ice. In such places, life may exist that doesn't use water. Just as alien life might be based on silicon instead of carbon, it's possible that alien biochemistry might find another solvent. Such life-forms would be very alien - the need and use of water is common to pretty much all life on earth.

Most Likely Alternatives to Water

Much has been written about those three candidates, and their suitability for life.

Less Likely

These alternatives are briefly mentioned by Wikipedia, but given no real details and each is quickly refuted. So, a GM might be able to make use of this, but you're gonna have some hand-waving to do.

Non-Polar Biological Solvents

Water is polar, which is very helpful in allowing materials dissolved in solution to accumulate in a particular region within the water. This is a likely step in how life got started on earth - with organic material accumulating and thus having an increased chance of reacting and replicating. However, it's not the only possibility - Clay Theory suggests that life on earth started because of silicon clay, which has similar properties to water in that it can form a substrate upon which organic materials can accumulate. If this is true, then it seems likely life could form on worlds where water is rare and the only available solvents are non-polar.

It has been suggested (by Isaac Asimov, who was a professor of biochemistry, in addition to being an author) that poly-lipids could form a substitute for proteins in a non-polar solvent.

This opens up several additional options for life that would otherwise not be available:

Sources

Game and Story Use

  • Silicon-based Life and Starfish Aliens are likely to use something other than water as a biological solvent.
  • The planets where such life evolves are likely to be very hazardous to humanity. Most of the potential solvents listed above are toxic to us, especially in the high quantities required to support an ecosystem based on them. In several cases, for this state of affairs to evolve and be stable, the planets would need to have very little liquid water (or oxygen in general), and be either very cold, very hot, or have immense pressure.
    • Which means the Uninhabitable Planets might not actually be uninhabitable. Provided that life is weird enough, it could be everywhere.
  • Diplomacy is strained when the atmospheres and conditions native to life prevents two species from ever being in the same room. However, at least they won't be competing for the same resources, so it shouldn't be too hard to come up with a non-aggression treaty.
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