The original specimen of this species, the "type specimen," is the OH 5 skull found in 1959 by Mary Leakey at Olduvai Gorge in Tanzania. The photograph on the top-right is a reconstruction of OH 5 with the mandible of another individual scaled to fit (the original find did not include the lower jaw).

Below OH 5 are two very important P. boisei fossils; KNM ER 406 and KNM ER 732, thought to be a male and female respectively.

There was some earlier debate in the paleoanthropological community as to the number of early human species in southern Africa between 3 and 1 million years ago. Conventional wisdom had it that two species existed, Australopithecus africanus and Paranthropus robustus. In contradiction to this view, Milford Wolpoff, of the University of Michigan, advocated the"single species hypothesis". It claimed that the differences between the southern forms were caused by age differences and sexual dimorphism of the specimens. Many researchers had problems with this hypothesis. For example, why in southern Africa were the supposed males dying at a different place than the supposed females? And why were they dated to almost a half a million years later? It was clear that a larger fossil record would be needed to prove or disprove this hypothesis.

Interestingly, the answer to the question of the southern African early humans would come from hundreds of kilometers away in East Africa. The discovery of two fossils, KNM ER 406 and KNM ER 732, at Koobi Fora in eastern Africa would provide the necessary expansion of the record needed to disprove the "single species hypothesis". Upon discovery its in 1969, ER 406 showed enough similar morphology to be assigned to the same species as OH 5; with the addition of ER 732, comparisons could be drawn between the two that could shed light on the nature of dimorphism in early humans. As these two specimens were examined, researchers found that the early humans of this period followed what is called the great ape model of sexual dimorphism. Male crania were larger than females, and more heavily constructed. While differences existed between the two skulls, these differences were exactly what would be expected between the sexes in other great apes.

The two southern African forms, however, did not fit this model of the distinction between the sexes. The differences were too great to be the result of sexual dimorphism. This observation favored the idea of two distinct species in southern Africa.

The final blow to the "single species hypothesis" was the 1975 discovery of the cranium KNM ER 3733, assigned now to Homo ergaster, in the same layer as ER 406, the "robust" form Paranthropus boisei. Scientists finally knew for sure that more than one species of early human coexisted in the same geographical area. The old single line of progressive evolution was, once and for all, split into branches. And the human family tree has never looked the same since.