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.
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.