Genetic similarities and dissimilarities in primates: Implications for the understanding of human evolution [Somiglianze e differenze genetiche nei primati e loro implicazioni]

This review is mainly addressed at the genetic similarities and dissimilarities between humans and their closest living relatives: the orangutans, gorillas and chimpanzees. Two main concepts are discussed,i.e. the hierarchical arrangement of diversity and coalescence. Special attention is given to the importance of considering the evolution of coding regions when interpreting experimental data accumulated in the latest years also as a result of genome projects. The selection of comparative experimental data that are discussed cover three main areas: population genetics, comparative genomics and gene expression profiles. Population genetics studies converge in showing that each of the orangutan, gorilla and chimpanzee subspecies harbour an amount of genetic diversity far larger than any human population. This indicates a long-lasting genetic isolation as well as population effective sizes larger than that of human populations ancestral to the extant continental gene pools. The complete sequencing of the chimpanzee genome revealed a 1.23% divergence as compared to human for orthologous regions that can be directly matched. An additional 3% is accounted for by genome regions which are either present or absent in one species only. Some examples of genes for which a prominent role in the emergence of human-specific traits has been advocated, are also discussed. Altogether, in both the human and chimpanzee lineages, genetic novelties mainly consisted in quantitative changes for already existing structures and developmental pathways. Finally, gene expression profiles have been explored to test the hypothesis of a mainly regulatory evolution. In this case, too, the data show a major role for purifying rather than directional selection. Taken together, all the above data display rates and modes of the evolutionary change that are also observed in other mammalian lineages. The final section deals with the taxonomic implications of the recent genetics findings, and the questioned attribution of the species or even higher ranks to extinct forms which mark the lineage leading to modern humans. © 2007 Accademia nazionale dei Lincei