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 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).
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 Neandertal 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.