In 1964, Emile
Zuckerkandl and Linus Pauling christened the approximate
rate constancy in the rate of amino acid substitutions in
proteins the molecular clock. Since then immunological comparisons
and DNA sequence data have also been used to infer the existence
of molecular clocks. The molecular clock has been no stranger
to controversy, however. Issues of variability in rates, the
universality of the clock, its reliability in phylogenetics,
and the correct units for its timescale have marked its history.
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