 Evolutionary biology also followed this reductionistic trend down to molecules. Molecular evolution emerged as a field of study where evolutionary changes were tracked at the level of DNA sequences, and DNA became thought of as a ledger of evolutionary history. Most genetic mutations, it became clear, had little to no impact on an organism’s fitness—either because the mutation occurred in such a way that the same amino acid was produced anyway (a product of redundancy in the genetic code) or because the mutation occurred in a region of the genome where no proteins were coded; this, in turn, forced biologists to rethink the extent to which DNA sequences could be understood as the product of natural selection alone (Kimura 1968; Dietrich 1994). Mutation rates in homologous DNA sequences shared by different species (say, humans and chimpanzees) also allowed for inferring how far back those two species diverged from a common ancestor (entries:  ecological genetics;  genetic drift). 