Man, the conservative and revolutionary mammal. Molecular findings on this paradox

Abstract

Man's antecedents in the stem line of the Primates advanced by an anagenetic process which increased the informational content of genomic DNA and decreased the rate of divergent molecular evolution among genomes. A glimpse of this process was obtained from phylogenetic trees of homologous sets of sequenced mammalian polypeptide chains. The trees were constructed by the maximum parsimony principle, i.e. for each set, the branching arrangement and codon sequence ancestors were sought which yielded the fewest nucleotide replacements over the link paths describing the phylogeny of that set. Contrasting primates with ungulates, the trees revealed little genetic divergence in the branching of catarrhine primates and an especially close cladistic and genetic relationship of chimpanzee, Man, and gorilla. Moreover, the mutation lengths of the link paths demonstrated that in the descent of the stem line from the primitive placental ancestor to Man the rate of nucleotide replacements was first rapid and then by the later Tertiary markedly decelerated. This pattern was attributed to selection for longer gestations and generation times and for larger ontogenetic arrays of intricately coadapted molecular structures, the former decreasing the sidereal-time based mutation rate and the latter reducing the proportion of new mutations neutral to natural selection. While the molecular adaptations directed inwards to the internal environments of the organisms in the advancing lineage were best specified by homozygous gene combinations, the molecular adaptations directed outwards to the challenges of an expanding external environment were best specified by heterozygous combinations. These two conflicting selective tendencies have produced further maturational delays to post-gestational ages of the polymorphic macromolecular systems effecting the outward-directed adaptations, and have further braked the rate of divergent molecular evolution in human populations.