Anthropoid Evolution
One of the ultimate goals of evolutionary anthropology is to identify the genetic changes responsible for primate evolution, especially evolution of the craniofacial complex. Knowledge of the genotype-phenotype relationship is critical to understanding human origins beyond the descriptive level. Evidence points to large numbers of genes involved in craniofacial development, any of which may be involved in a given pattern of variation within or between species.
Remarkably, years of research in experimental biology and genetics have demonstrated the consistently deep phylogenetic conservation of developmental networks. New shapes do not need new genes, and shape can evolve quickly by tinkering with an existing system. The generation and formation of such traits almost certainly involves many genes organized into networks of interaction. Craniofacial variation results from variation in the parameters of these generative processes (e.g., signals, receptors, activators, inhibitors, second messengers, transcription factors, structural genes). Fortunately, like the genes themselves, these networks are highly conserved among mammals.
Baboons are an excellent model species in which to investigate the role of potential candidate genes in producing craniofacial form, because they are plentiful and adaptable to captivity. As anthropoid primates (the broad group that includes monkeys and apes), baboons also share with humans many aspects of their development and genetic makeup. Comparative genomic studies of humans, baboons, and other mammals will eventually allow us to design tests for specific hypotheses regarding the role of a particular gene in the evolution of a given phenotype within a defined clade. Baboons belong to a group of Old World Monkeys called the papionins which includes macaques, mangabeys and their close relatives.
Figure 1. Molecular phylogeny of the extant papionins.
Papionin phylogeny: using baboons to model Hominid craniofacial evolution
The fossil record of Plio-Pleistocene papionins in Africa is excellent and crania are relatively common. Mitochondrial DNA studies suggest that the two African papionin clades diverged about 6-8 Ma (million years ago), similar to times suggested by molecular clocks for the split between chimpanzees and humans. The Mandrillus and Cercocebusclade split from the Theropithecus, Lophocebus and Papio clade, giving a “baboon-like” and a “mangabey-like” lineage in each (Figure 1). Thus, molecular and morphological evidence points to independent evolution of “baboon-like” genera. Species of Parapapio are morphologically close to the expected ancestral morphotype of both lineages and the earliest specimens are from about 6.5 Ma. The dominant east African genus at 6.5 Ma was Theropithecus (Figure 2), now restricted to T. gelada, but with a modest radiation during the Plio-Pleistocene. The genus Papio replaced Parapapio and Theropithecus ecologically during the Pleistocene. Modern Papio are adaptable omnivores. The mandrill/Cercocebus clade can be contrasted with the Papio/Lophocebus clade in both of which “mangabeys” and “baboons” have evolved independently. There are considerable cranial differences between Theropithecus and Papio that have been recognized as being similar to those seen between Homo and Australopithecus and these are of considerable interest. The papionin primate radiation is thus an appropriate choice for modeling hominid craniofacial evolution.
Figure 2. Lateral view of a skull of the extinct baboon Theropithecus oswaldi.
Salient craniofacial features
It is now generally recognized that the large faced “baboons” and the mangabeys are polyphyletic. Morphologists have found that they can support both a diphyletic origin of baboons (Papio) and mandrills (Mandrillus) and a concordant diphyletic origin of mangabeys (Cercocebus and Lophocebus). Large faced “baboons” and hard object eating mangabeys have evolved twice from a primitive condition. Similarly, extreme sexual dimorphism has arisen convergently in the two main large faced African groups and similarities in cranial size and shape resulted independently. Dimorphism has seemingly been reduced and increased in different hominid lineages, and several differences between Papio and Gelada mirror some of those seen between chimpanzees and Australopithecus and Homo (e.g. facial shortness, petrous orientation, height and angulation of mandibular rami, cranial base flexion, relative incisor and cheek tooth size) (Figure 3).
Figure 3. Comparison of skull form in the Pan-Homo clade (above) and in the papionins (below).
The papionin radiation, then, shows a suite of evolutionary changes that have also occurred or been reversed in hominid evolution. If we can find candidate genes for certain traits that have occurred independently in the living papionins, we will understand similar traits in hominid evolution. Identifying the developmental gene networks responsible for these traits will allow us to establish the likely first appearance of such networks in fossil lineages. This will give time depth information to the molecular results, and strengthen research in papionin paleontology.
