Human Genetics for the Social Sciences

Evolutionary History of the Great Apes

The great apes consist of the gibbons, siamangs, orangutans, gorillas, chimps. bonobos, and us humans. Past classification, based largely on anatomical and behavioral features, placed gibbons (and their close cousins, the siamangs), orangs, gorillas, and chimps into a single family called Pongidae and humans into a separate family termed Hominidae. More modern classification, relying on cytogenetic and DNA technologies not available in the past, paints quite a different picture. The figure given below relies on various sources to depict the evolutionary history of the great apes.

The first branch in evolution occurred when gibbons and siamangs separated from the other apes. That places gibbons and siamangs into one category and we humans, chimps, gorillas and orangs into another. If these results hold up, they imply that we humans have more in common with an orangutan than orangutans have with gibbons! The next split occurred when the orangs branched off. Today's orangutan inhabits the two southeast Asian islands of Sumatra and Borneo. Genetically, these two populations are quite different and there is some debate as to whether they are different subspecies or different species.

The next split is controversial. In the figure, the gorilla line splits off from the human/chimp branch, but the actual genetic similarity among us humans, chimps, and gorillas is a matter of debate. It could be that the human line or perhaps even the chimp line was the first to break off. In any event, all the evidence points to the fact that we humans are genetically more similar to gorillas and chimps than gorillas and chimps are to orangutans. Gorillas are subdivided into two subspecies, the mountain gorilla (Gorilla gorilla berinnei) and the lowland gorilla which is further divided into the Western (Gorilla gorilla gorilla) and Eastern (Gorilla gorilla graueri) populations. Recent attempts at classification have considered the mountain and lowland gorillas as separate species.

Available data suggests that the split between chimps and bonobos occurred after we humans broke off from the gorilla and chimp/bonobo line. Bonobos inhabit the dense rainforests south of the Zaire (or Congo) river while the populations of chimps live north of the river, suggesting that reproductive isolation promoted by that geographical boundary may have been a major factor in speciation. Three subspecies of common chimps are recognized based on their genetic similarity and their location in Africa. The are the Western African chimp (Pan troglodytes verus), the Central African chimp (Pan troglodytes troglodytes), and the Eastern African chimp (Pan troglodytes schweinfurthii).

Most genetic research has compared the genomes of the great apes on a few loci here and a few loci there, so a general perspective on genetic similarities and differences must await a comprehensive review of the literature. The recent sequencing of the human genome has prompted calls for sequencing of the genomes of the other great apes. Comparison of the sequence data across a large number of loci will permit much greater insight into the classification and evolutionary history of the great apes than is possible with today's data. Undoubtedly, the figure presented above will require modification after these data are at hand.

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