Ahhh, but in order to make such a claim, you *must* assume that there is
more than one possible biochemical pathway for carbon-based life to form.
Why shouldn't I? From a chemical point of view it all looks like a small
random selection from a tremendous wealth of possibilities. For example, who
needs both leucine and isoleucine, or why do some 800 amino acids occur in
some organisms, but at most 22 are encoded in any genome? Why aren't there
any pyridine bases which could form metalloribozymes or
even -deoxyribozymes? Why this rather unstable sugar-phosphate backbone
instead of, say, one formed of peptide bonds? (Artificial "PNA" has been
produced and is used for certain biochemical experiments.) Why are cell
membranes of non-archaeans made of both phosphate-glycerol-fatty acid
molecules and sphingosines (which mimic the shape but not the chemical
properties)? Last but not least, why is there so much homology across all
life? Why is the tubulin in our cytoskeletons such a plausible homologue of
the FtsZ that bacteria and archaea use for cell division, to use an example
that is not immediately obvious?