In 1999, I had the opportunity to travel to South Africa to spend some time in the field with my friend and colleague, Chuck Bellamy, who at the time was serving as Senior Curator in the Coleoptera Department at the Transvaal Museum (now the Ditsong National Museum of Natural History) in Pretoria. During my visit, and learning of my interest in paleoanthropology, Chuck arranged for me a private tour of the 'Broom Room' with its curator, Dr. Heidi Fourie. The Broom Room houses some of the most important fossils of early hominids in the world, including the famous Sterkfontein (STS) 5 'Mrs Ples' and Swartkrans (SK) 48 crania. The bulk of the fossils were discovered by Robert Broom, Raymond Dart, and their successors at the famous Sterkfontein and Swartkrans hominid sites in the northeastern part of the country. Australopithecus africanus, Paranthropus robustus, and some of the earliest known members of the genus Homo (recently described as H. gautangensis) are represented among the fossils, which range from 1.5 to 2.8 million years in age.
When Robert Broom first arrived in South Africa in 1936 and saw Raymond Dart's recently discovered 'Taung child" - the first known Australopithecus fossil, he is said to have knelt at the edge of the table containing the fossil and exclaimed, "I behold my ancestor!" Such was the feeling I had when I first entered the Broom Room and saw the rich wooden cabinets and rows of fossils neatly arrayed on its red felt-lined shelves. I knew which fossil I wanted to see first - Mrs. Ples, the most complete Australopithecus africanus cranium known, discovered at Sterkfontein in 1947 by Robert Broom and John Robinson. Originally named Plesianthropus transvaalensis, it was subsequently regarded to be conspecific with the Taung Child and thought to represent an adult female (most researchers now regard it to represent a sub-adult male). As Dr. Fourie held the cranium for me to look at, I noticed the fossil was about 3.5 feet off the floor - about the presumed height for the species. I suddenly saw Mrs. Ples standing before me in life - a living, breathing being, not an animal, yet not quite human either. I may not have used Broom's precise words, but I whispered something along those lines to myself as the slender, hairy virtual creature stood before me. The Museum Gift Shop was selling plaster replicas of Mrs. Ples, one of which now sits on the desk in my office. I think about that experience at the Transvaal Museum almost everytime I look at it.
Among the other A. africanus fossils I noticed was a partial cranium, also discovered by Broom at Sterkfontein in 1947. While not as complete as Mrs. Ples (missing portions of its left side), it was subsequently associated with a mandible found in the same layer (STS 36) by matching wear patterns on the teeth - making it one of the most complete A. africanus skulls to have been found. Originally classified as a female, this 2.5 million year old cranium is smaller and less prognathous (forward projecting mouth) than other known A. africanus crania. However, its large post-canine teeth and indications of massive chewing muscles suggest it is a male.
Skulls are not the only cranial fossils represented in the collection. Endocranial casts as well have been found in the same deposits from which the crania were taken, and I actually got to hold STS 60 in my own hands! With a chimp-like volume of 428 cubic centimeters, it's a bit on the small side of normal for A. africanus (nearly 60 cc less than the brain capacity of Mrs. Ples, though equaling that of STS 71). Holding it in my hands, I mentally compared its size with that of my own brain and tried to imagine the cognitive differences implied by such difference (okay, no jokes here!).
Broom recognized that the australopithecine fossils he was finding in South Africa represented two distinct morphs - a "gracile" form now encompassed by A. africanus, and a more "robust" form that he described in 1938 as Paranthropus robustus. The fossils from Swartkrans conform to this latter type, with the most complete cranium being SK 48, discovered by Broom and Robinson in 1952 and dated to between 1.5 and 2.0 million years in age. It should be noted that the term "robust" refers not to the size of the body, but rather the characters of the skull that include a more prominent sagittal crest in males, greater sexual dimorphism in body size, and robust zygomatics and mandible with large, thickly enameled post-canine dentition. The Museum Shop had a plaster replica of SK48 as well, which also now sits on the desk in my office.
It should be noted that not everyone in the field accepts Paranthropus as a valid genus distinct from Australopithecus. There is little doubt that the included species represent a derived and specialized form, but whether P. robustus from South Africa and the two east African species P. aethiopicus and P. boisei form a monophyletic clade is still an open question. There is some evidence to suggest that P. robustus is descended from A. africanus because of similarity of some derived features with the latter, which if correct would render Paranthropus paraphyletic and not a valid taxon (Conry 1997). This view, necessarily, suggests also that the hyper-masticatory adaptations of robust australopithecines evolved independently in eastern and southern Africa. While this certainly could have happened, (Strait et al., 1997) argue that a more parsimonious interpretation of multiple morphological traits suggests Paranthropus is indeed monophyletic and that it should be retained as a valid genus. Either way, Paranthropus certainly represents a distinct “grade” of australopithecines, and until more convincing data to the contrary are produced I prefer to recognize it as a valid genus.
The massive mandibles that distinguish P. robustus from the gracile australopithecines are richly represented among the Swartkrans material (one can imagine that these robust bony structures are prone to preservation), including SK23 (discovered at the same time and dated to the same age as the SK 48 cranium) and SK 12 and the SK 52 partial skull (see photos). Presumably these morphological adaptations of the mandible/maxilla and associated musculature are related to a more specialized diet of tough plant material that required considerable chewing to process, compared to the more omnivorous A. africanus that pre-date them.
Also amongst the P. robustus fossils is SK 1585, the only endocranial cast known for the species with a volume of 530 cc. Combined with SK 48 and other partial and cranial remains that have been recovered for the species, it appears that the brain of P. robustus averaged slightly larger than that of the earlier A. africanus. Whether this translates to increased cognitive function is open for debate, although there are some structural differences in SK 1585 compared to A. africanus endocranial casts that suggest this could be the case.
Perhaps the most contentious fossil in the Broom Room is SK 847, a highly fragmentary partial cranium discovered at Swartkrans in 1969 by Ronald Clarke and dated to between 1.5 and 1.8 million years. Originally thought to represent P. robustus due to its association with other fossils of that species, it was eventually associated with a maxilla (SK 80) originally described as Telanthropus capensis and later included in Homo erectus. The affinities of the composite specimen were contentious, and at the time of my visit its classification remained unresolved (Johanson and Edgar 1996). More recent studies have suggested that this and other South African specimens represent a species not previously sampled in east Africa, and the specimen was eventually included as a paratype in the description of a new species, Homo gautangensis, the newest member and earliest representative of its - our - genus (Curnoe 2010).
Conroy, G. C. 1997. Reconstructing human origins: a modern synthesis. New York, Norton.
Curnoe, D. 2010. A review of early Homo in southern Africa focusing on cranial, mandibular and dental remains, with the description of a new species (Homo gautengensis sp. nov.). Journal of Comparative Human Biology 61(3):151–77.
Johanson, D. and B. Edgar. 1996. From Lucy to Language. New York: Simon and Schuster Editions.
Strait, D. S., F. E. Grine and M. A. Moniz. 1997. A reappraisal of early hominid phylogeny. Journal of Human Evolution 32:17-82.Copyright © Ted C. MacRae 2011