Molecular Clocks

    Prior to the 1960s, estimates of the time of divergence of various organismal groups were based entirely on fossil evidence. Such inferences required several assumptions, notably that a particular fossil could be placed reliably as the ancestor of an extinct or living group. For example, there was for many decades argument as to where the 10 MYA fossil Oreopithecus was in the lineage of Anthropoid apes or Hominoid humans. If the latter, this would push back the origin of human bipedalism and cranial characteristics, and our divergence from other Anthropoids. The earliest molecular studies of Chimp / Gorilla / Human differences suggested a much more recent ancestor < 5 MYA. These data and date were rejected by most paleontologists, until the discovery in 1974 of the intact 4.2 MYA Australopithecus skeleton of "Lucy", with many features of modern hominids. Oreopithecus is now recognized as a large anthropoid ape.

      [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 (AC Wilson, 1976). The line is drawn from the origin through the oldest comparison at the upper right (the well-dated marsupial / placental divergence 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. Accumulated genetics differences function as "Molecular Clocks".

    [Right] Measured pairwise nucleotide divergences in the mitochondrial DNA (mtDNA) genomes among modern and extinct hominids, calibrated as a log-linear plot. The M vs N divergence is the basal separation of all non-African humans. The San2 sequence in the African L1 lineage is the most divergent from all other living humans. Both separations are well dated. Complete mtDNA genomes have been recovered from fossil Neandertals and Denisovans from Central Asia. Radiometric dating places their separation from Anatomically Modern Humans (AMHs) 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.