The molecular clock hypothesis, first advanced in 1965 by Linus Pauling (1901-1994) and Emile Zuckerkandl, suggests that the mutation driven changes (substitutions) in the amino acid sequences of proteins has a characteristic substitution rate that is related to time. The identification of such clocks would allow biologists to assign accurate dates to evolutionary history.

Upon comparing the amino acid sequences for hemoglobin molecules taken from different species, Pauling and Zuckerkandl noticed that the number of amino acid substitutions increased with the presumed evolutionary distance between species (i.e., how high up on the "evolutionary tree" a species appeared). One proposed mechanism that might account for such changes was an evolutionary clock in which the ticks of the clock are the number of amino acids substitutions that occur over time. Such molecular clocks are a predicted consequence of the neutral theory of molecular evolution (i.e., the theory...