Question

Why can't life have started in an ammonia solution?

Indeed, the chemistry of an ammonia solution can account for many things. Initial planetary gases are right for Haber chemistry, ammonia condenses, aqueous solutions are fluid to minus 80 C. This would explain early Martian rivers. Further, essential catalysts involving Cu, Ni, Zn, etc. are soluble in ammonia solutions. One could reply by listing all of the advantages of water, but perhaps one of the strongest and clearest reasons why water is superior over ammonia deals with its high polarity and therefore its ability to maintain a very strong hydrophilic/hydrophobic dichotomy with non-polar, fat-soluble, hydrocarbon-like molecules. Or, to use more familiar terms, oil forms a slick on water because the two are energetically incompatible liquids. This immiscibility is very important because it allows cellular membranes to remain stable in aqueous solution, and it drives proteins and nucleic acids to fold upon themselves and create biologically-active structures. Ammonia forms a much weaker dichotomy with non-polar substances than does water, therefore the large biomolecular structures so necessary for life would probably not be possible with ammonia. If life in ammonia is not impossible, it would be a LOT more difficult!
May 1, 2002