Why does all life use the same 20 amino acids?
An even bigger question: why is DNA/RNA always expressed in the same way? I.e. the codon->aminoacid table is constant for all living things. I always found this very puzzling. Of course, the easiest explanation is that life really did start from a single cell. But it sounds unlikely that a) there really was that single origin cell, and not a few that popped up around the same time (on a geological scale), and b) no organism has drifted in the millions of years of evolution.