Chimpanzees, and Gorillas," A. L. Zihlman points to biochemical as well as anatomical similarities between humans and their primate relatives when he notes, "Phylogenetic relationships have previously been determined mainly by anatomical similarities, but now, biochemical similarities provide independent criteria for evolutionary relationships" (1). Figure 1 gives a molecular cladogram showing genetic relationships of the ape and human lineages. If this cladogram is correct, then an ancestor for the earliest hominids must also be a suitable ancestor for chimpanzees and gorillas. Zihlman's evidence derives from the fact that the scant hominid fossil record and the absence of any fossil chimpanzees or gorillas forces us to work backwards from the living hominoids to reconstruct a prototype of the common ancestor. Pigmy chimpanzees are more "generalized" in body size, body build and sexual dimorphism than the common species, and may provide a good prototype for studying the derivation of the earliest hominids and African apes.
The hypothesis that the pigmy chimpanzee is the common ancestor (prototype) of humans, chimpanzees, and gorillas is bold in light of the fact that the records available for reconstructing primate phylogeny are meager, consisting of individuals of about two hundred living species and occasional bony remains of several hundred extinct species drawn from various parts of the world at various times during the past 65 million years.
Despite their differences in size, diet, and social organization, the three living African apes "are quite similar in many aspects of cranial and skeletal anatomy and virtually identical in many biochemical assays" (2). Are their differences merely a corollary of their differences in body size, then? As Fleagle points out, a simplistic summary of this view is that gorillas are overgrown chimpanzees and that pigmy chimpanzees (also known as bonobos) are morphologically similar ...