A New Molecular Clock of Mitochondrial DNA and the Evolution of Hominoids

The approximate constancy of the rate of change is one feature of molecular evolution. It is consistent with the neutral theory1 and serves as a clock to date a divergence time of organisms.2>,3) Since Sarich and Wilson,4~ many researchers have estimated the divergence time between man and the African apes using the molecular clock (for review, see ref. 5). In spite of diverse materials and methods used, their results are similar and show a recent divergence of 4-& million years (Myr) ago. This dating does not contradict with any fossil evidence, if Ramapithecus is placed on orangutan line.5~ '0> The molecular clock, however, is not yet free of any contradiction with hominoid fossil records. The clock estimate of the divergence time between orangutan and man is between 8 Myr4~ and 10±3 Myr ago,5~ though recent finding indicates that orangutan and man diverged at a minimum of 13 Myr, probably more than 14.5 Myr. 7 ) The clockk dated the split of gibbons from man between 10 Myr4~ to 12±3 Myr ago.5~ But the fossil evidence indicates that Epipliopithecius and Micropithecus are 16 and 20 Myr old, respectively, and are ancestors of gibbons.8~ Since the previous estimates of genetic distances are indirect, we performed a direct comparison among mitochondrial. DNA (mtDNA) sequences. Our data are the L-strand mtDNA sequences of 896 nucleotides from five species of hominoids,9~ from bovine,10~ and from mouse.'1 This segment contains genes for three tRNAs and parts of two proteins. In animal mtDNA, it is known that transition predominates over transversion,9~ and that the substitution at the third colon position is much more frequent than other nucleotide positions. The numbers of transition and transversion type differences between species are listed in Table I separately for the third colon positions (class 1 sites) and for the rest (class 2 sites). Table I indicates that mouse (splitting date : t1 Myr ago), bovine