Molecular clock hypothesis.

In 1962 Émile Zuckerkandl and Linus Pauling published evidence that the rate of amino acid substitution in proteins is constant over time. In 1965, after several protein sequences (cytochrome c, hemoglobin, and fibrinopeptides) seemed to show this pattern, they proposed the molecular clock hypothesis (MCH). According to their hypothesis, mutations leading to changes in the amino acid sequence of a protein should occur at a constant rate over time, rather than per generation, as previously assumed. In other words, if the sequence of cytochrome c were determined 1,000,000 years ago, 500,000 years ago, and in the present, the rate of amino acid substitution would be the same between the first two samples as it would be between the second and third. To state this more accurately, they considered the rate approximately constant, which means that one protein may display some variation, but if the average rates of change for several were considered as a group, they would be constant.