The following is taken from Rick Potts' Humanity's Descent
.... let us consider how scientists actually interpret the array of fossil evidence that led to our dream. The skull we held in our hands belonged to the human known as Australopithecus boisei, one of the longest-lived of all known species of hominids. First discovered by Mary Leakey in the Olduvai Gorge, A. boisei belonged to a group known as the robust australopiths.
The term "robust" refers to the great size of their cheek teeth and the inflated appearance of their faces, buttressed against powerful chewing forces. The little certain knowledge we have of their bodies suggests that like the earlier australopiths, they were fairly small, perhaps from three feet seven inches to four feet six, and sixty-five to ninety pounds. Their brains were slightly larger than those of a like-sized chimpanzee, which means only about one-third the size of our own.
The oldest known robust skull, about 2.5 million years, is an extraordinary piece of anatomy. Discovered by Alan Walker on the west side of the Turkana basin, it is known as the "Black Skull" because of its dark fossilized hue. It has an enormous crest for chewing muscles near the back of the braincase, where later robusts have it farther forward. The Black Skull has the pillared, buttressed face typical of the robust clan, but it protrudes around the nose and mouth, unlike the flat, tucked-in physiognomies of the later robusts. For these reasons, many anatomists have favored distinguishing it, and several related finds from Omo and Turkana, with its own species name, Australopitcus aethiopicus.
It seems likely that the long, successful boisei lineage was a transformed descendant of aethiopicus. The strong, flattened face, the ponderous brow, the crested peak, the thick muscular ropes that moved the jaw, the massive molars, the molarized bicuspids, the thick bed of mandibular bone containing the teeth, all comprised the human parallel to megadonty, the sets of large, flat-topped teeth many other Pliocene animals evolved. The chewing factory the robust australopiths evolved suggests that they ate coarse, fibrous plants, and studies of the microscopic scratches on their teeth confirm the importance of these items in their diet.
As boisei thrived in the rift valley, a similar species evolved in southern Africa, Australopithecus robustus. It varies from place to place, and the species name Australopithecus (or Paranthropus) crassidens may apply to one of these variants. These southern australopiths represent a third and possibly a fourth species exemplifying the extreme chewing adaptations of megadonty. The southern and eastern varieties are easily distinguished, and evidence is mounting that the robust species from these two regions attained their powerful dental apparatus somewhat independently. If so, this reflects parallelism within even this small radiation of hominid species. Boisei and robustus represented the eastern and southern terminations of their proud lineages. The last known fossils of the robusts are a little more than I million years old.