Human Ancestors Hall: The Robust Debate

The "Robust" Forms

When the fossil record was still poorly understood, it was thought that there existed two distinct branches of early human. These were the "gracile", meaning small, and "robust", meaning large, forms of the genus Australopithecus. The gracile forms were represented by the species A. afarensis and A. africanus; the robust forms by the species A. robustus and A. boisei.

You may be saying "Hey, those last two species were called "Paranthropus robustus" and "Paranthropus boisei" in the phylogenetic tree!" And you're right. That's the debate.

The word "robust" originally referred to the larger body that the members of these two species were supposed, by paleoanthropologists, to have had. Few postcranial bones existed at the time for either, and inferences were based on the size of their teeth. Now that there are more fossils from which to draw our conclusions, it is apparent that the average body size was not significantly different between any of the species. But the term does accurately reflect the difference in the size of the cheek teeth (molars and premolars) of these species. Compare the size of the cheek teeth of the "robust" P. boisei on the left and the "gracile" A. africanus on the right. These photos are to scale.

These robust forms were "megadont" -- they had huge, broad cheek teeth with thick enamel. The incisor teeth were by contrast small. The emphasis was on the rear teeth, and they were designed to support the stresses of heavy chewing. Combined with the morphology of the other parts of the skull -- large zygomatic arches to allow the passage of large chewing muscles and a large sagittal crest to provide a large area to anchor these muscles to the skull -- these early humans showed adaptations to chewing tough, fibrous foods, giving us clues as to their diet. Microscopic studies of the teeth show wear indicative of a vegetarian diet for both P. robustus and P. boisei. Similar expansion of chewing structure can be seen in the evolution of many groups of animals, for example wild pigs.

Now the debate . . .

All of these species had been assigned to the genus Australopithecus. Yet, because of the extreme heavy chewing adaptations of the "robust" forms, it is thought that they represent a group of species (or clade) separate from that which led to modern humans, and eventually this clade became extinct. As such, many researchers believe that they should be placed in their own genus Paranthropus, separate from the australopiths that gave rise to later humans. This is not universally accepted by researchers, although we use that designation for our presentation of the human lineage. This division would place all of the robust forms together, with the early P. aethiopicus species as the probable ancestor to both later robust species, P. robustus in southern Africa and P. boisei in eastern Africa.

However, some researchers suggest, based on subtle facial features, that the eastern African forms P. aethiopicus and P. boisei form a lineage separate from the South African species P. robustus. In this case, P. robustus would descend from the southern gracile A. africanus, and the genus name Paranthropus would be invalid. This implies that the evolution of similar heavy chewing complexes occurred twice independently. (See a graphic representation of the two implied phylogenies.)

There is still much debate as to the exact ancestor/descendent relationship among the robust species. As such, some species are referred to in the literature by more than one name. As more fossils are found, our understanding of this branch of the human family tree will hopefully become clearer.

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