Cranial anatomy and baboon diversity.

Abstract

The following article by Frost et al. (2003) exemplifies an exciting application of new ways of using anatomical evidence to reconstruct evolutionary history and embodying significant innovations on both the practical and the theoretical fronts. Their chosen subject is cranial variation in the large, extant African monkeys known as baboons, which, together with the mangabeys (their close and probably primitive relatives), comprise a zoological subtribe, the Papionina. The baboons are a particularly interesting example of mammalian evolution in tropical Africa. They have been extensively watched in the wild, so their natural history is well known. They include one genus that is very widespread and diverse, and two others that are much more restricted in diversity and distribution, thus representing different patterns of evolutionary change. Finally, for those concerned with human evolution, the study of baboons has the added interest that the place, time-frame, and ecological setting of their evolutionary emergence and diversification coincide quite closely with those of the human species and its close, extinct relatives (the Tribe Hominini). For this reason, it has been suggested that the baboons provide a uniquely apt set of analogies to help us understand human evolutionary history (Jolly, 2001). Another great advantage of the baboons as a subject of study is that they have been quite extensively “collected”; that is, they have been the object—some might say the victims—of the curious custom of taking heads as trophies. This practice, a favorite pastime of the Celts of Caesar’s day, among other human groups, appears to be underlain by a more or less implicit belief that a head (or a skull) embodies an individual’s essence. And if the skin also is collected, this completes the trophy by capturing its outward appearance too. Whatever the murky cultural roots of the practice of decapitation and flaying, it is fortunate for science that early zoological collectors followed it so enthusiastically. The scientific headhunters of a bygone era achieved a comprehensive geographical coverage that is now documented by the skin and skull collections of traditional museums of natural history. It could never be duplicated in the pitifully impoverished world ecosystem of today. All systematists agree that these collections are a treasure chest of evidence about biological diversity, but how are they best exploited? Some researchers delight in shocking their colleagues by suggesting that the best use for such museum materials would be to grind them up as a source of DNA, an organism’s quintessence in today’s popular mythology. Those of a more traditional bent see scientific justifications, not unlike the headhunter’s mystical ones, for valuing skulls as structures. For an evolutionary biologist, the “essence” that a cranium embodies is information about phylogenetic relationships, encoded in an anatomical complexity that results from evolutionary adaptation to the skull’s unique multifunctionality. A mammalian cranium, of course, houses the front end of the digestive and respiratory systems, the major component of the central nervous system, and most of the receptor organs for the special senses. Its form is also influenced by locomotion, social communication, and even reproduction. Each species’—indeed each individual’s—cranial form is a compromise between the demands of multiple functional roles, mediated by developmental patterns from its evolutionary heritage. Because the exact configuration of the compromise is unlikely to be duplicated in another evolutionary lineage, detailed resemblance in craniodental anatomy is a likely indicator of a phylogenetic relationship. Some remarkable similarities in cranial form can certainly be explained by functional parallels, similarity in absolute size, or chance, rather than by shared evolutionary ancestry, but it is most unlikely that cranial anatomy is so dominated by such homoplasies that no evidence of evolutionary history can be found. Extracting this evidence in useable form presents three challenges: how to select measures that capture the essential features of complex anatomical shapes, how to make these measurements accurately and reproducibly, and how to process the data to yield evolutionary information. For the task of identifying critical morphological variables, the human eye is a wonderfully sensitive and subtle instrument, but it is also idiosyncratic, and notoriously easy to mislead when judging similarities and differences among the shapes of objects of different sizes and (as occurs especially in paleontology) varied superficial textures and states of preservation. One practical solution to the problem of achieving some quantification and reproducibility, while not losing the value of the informed eye, is to combine observation and measurement. Characters