Molecular Clocks

    [Left] Inferred pairwise nucleotide substitutions among 17 pairs of mammal species from seven gene products, as estimated from protein studies, plotted against date of divergence, as estimated from the fossil record. The line is drawn from the origin through the oldest comparison at the upper right (the well-dated marsupial / placental divergence at 125 MYA). The strong linear relationship suggests that molecular differences between pairs of species are proportional to the time of their separation, not the degree of organismal difference.  Therefore, measures of genetic divergence can be used to date the time of divergence for species pairs for which no fossil data are available: genes function as "Molecular Clocks" (from A. C. Wilson 1976).

    [Right] Measured pairwise nucleotide divergences in the mitochondrial DNA (mtDNA) genomes among modern and extinct hominids, calibrated as a log-linear plot. The San2 L1 sequence is the most divergent from all other living humans; the M vs N divergence is the basal separation for all non-African humans. Both separations are well dated. Radiometric dating of the Neanderthal and Denisovan fossils places their separation from modern humans at 500 and 1,000 KYA, respectively. The log-linear curve fitting provides a "multiple hit" correction, where DNA sites in the older comparison have undergone more than one change.