concept

Frozen accident

A feature of life that chemistry did not force to be that way - one of several workable options - but which was picked early by chance and then "froze" in place, because everything built afterward came to depend on it and it could no longer be changed. The set of twenty amino acids, how the genetic code maps each triplet to an amino acid, and the use of ATP as the energy currency all look like frozen accidents. Its opposite is a universal optimum - a feature chemistry forces, so any life would converge on it.