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Paranthropus robustus was discovered by Robert Broom in 1938, in the South African caves. It existed from 2 to 1.6 million years ago. Again, in Paranthropus robustus, we see sexual dimorphism, with the males weighing, on average, 10 kg more than the females. Its brain size was larger than that of P. aethiopicus: from 550 to 600 cc. The skull had a low, sloping forehead, meeting with prominent brow ridges. It had flaring zygomatics. As in P. aethopicus, we see a sagittal crest running down the length of the forehead. There is the usual V-shaped dental arcade, with extremely large molars and relatively small incisors and canines. The molars and premolars have thick enamel, and there is no diastema, which probably means that males did not fight anymore for access to females.
Its teeth were evidently accustomed to grinding hard and coarse foods, such as tubers, but not chewing meat. This adaptation to a vegetarian lifestyle corresponds to the global cooling around 2 million years ago. At that time, there occurred a thinning of the rain forests and an ice age, with food becoming less and less abundant. The environment was generally semi-arid savanna with local woodlands. One branch of Australopithecus africanus or afarensis (it's under debate) diverged to produce Homo habilis, who was omnivorous and thus adapted to the climate change by including meat in its diet and, generally, a larger variety of foods. There was another branch which formed the robust australopithecines, who specialized in chewing tough tubers as a way of procuring food. However, such tough chewing cannot be operated on any normal mandible; a large, heavy one with considerable chewing power would be needed. The sagittal crest is needed to support such a heavy mandible. The molars developed cusps grooves to aid in chewing the tough vegetable matter. The thick molar enamel and enlarged molars are also the results of this climate change.
There is also the question of whether Paranthropus robustus was a possible manufacturer of stone tools. The evidence for this proposal lies in Oldowan tools and bones-- found in association with P. robustus skeletons. Scientists believe the bones may have been used as digging tools. Although the stone tools still remain controversial, but an examination of P. robustus hand bones indicates that they were capable of the fine manipulation necessary for the manufacture of stone tools.
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