Paranthropus boisei, arguably the best known of the “robust australopithecines,” (the species included in the genus Paranthropus—Paranthropus aethiopicus, Paranthropus robustus, and Paranthropus boisei) is known from eastern African sites dating between 2.3 and 1.4 million years ago. Specifically, P. boisei fossils have been found at sites in Tanzania (Olduvai Gorge and Peninj), Ethiopia (Konso and Omo River basin), and Kenya (Koobi Fora, Chesowanja, and West Turkana). Paranthropus boisei is important because it exemplifies the genus Paranthropus, a group of species with extreme features of the skull and teeth not found in other species. Current evidence suggests that the species in this genus eventually went extinct without leaving any descendants; the remains of P. boisei help to explain why this phenomenon occurred. In addition, P. boisei provides important insights into the evolutionary relationships among early hominins.
Many of the features found in P. boisei are shared with other species in the genus Paranthropus (but see the essay on P. aethiopicus) and are considered adaptations for producing large forces during chewing, particularly on food chewed by the cheek teeth—the premolars and molars. The premolars and molars of this species are very large (especially in width) with extremely thick enamel, and the front teeth are very small. The bones comprising the face are massive and positioned far forward (particularly on the sides of the face) to provide a mechanical advantage to the masseter muscle, a chewing muscle that closes the jaw. This forward position of the sides of these bones gives the face its characteristic “dished” appearance, in which the sides of the face protrude further forward than the center of the face, comprising the opening for the nose. The mandible (lower jaw) of P. boisei is very large; in particular the mandibular corpora (the parts of the mandible below the cheek teeth) are massive and the mandibular rami (the vertical plates of bone at the rear of the mandible, behind the teeth) are very tall. Finally, the places on the skull where the chewing muscles attached are enlarged. The sagittal crest (the ridge of bone on the top of the skull extending from front to back), in particular, is large and positioned more toward the front of the skull.
Other features characteristic of P. boisei are unique to this species. Many of these features are simply extreme versions of the features found in other species in the genus Paranthropus. For instance, the mandible and cheek teeth are extremely large, the premolars (particularly the fourth premolar) are shaped more like molars than in other Paranthropus species, and the front teeth are extremely small relative to the cheek teeth. The face of P. boisei is exceptionally massive and lacks the anterior pillars and surface topography found in P. robustus. The zygomatic arch (the bony arch formed by zygomatic [cheek] and temporal bones) in P. boisei forms a smooth, circular arc and creates a very large opening (temporal fossa) through which the, presumably very large, temporalis muscle (a chewing muscle that closes the mouth) passes. This feature and the massiveness of the bones of the face creates the characteristically “visor” shape of the facial bones in P. boisei, in which the bones below the eye orbits resemble the brim of a visor.
Other unique features found in P. boisei are not necessarily related to the production of chewing forces. The foramen magnum (the hole in the base of the skull through which the brain stem passes) is short and is often characterized as being heart-shaped, as opposed to the longer and more oval- or circular-shaped foramen magnum found in P. robustus. The shape of the mandibular fossa, where the lower jaw connects to the base of the skull forming the jaw joint, in P. boisei is different than in P. robustus—e.g., it is deeper from top to bottom. Lastly, the braincase (the portion of the skull that surrounds the brain) is larger than in P. robustus and the size of the brain in P. boisei is therefore slightly larger than in P. robustus.
As discussed above, the diagnostic features of P. boisei come chiefly from the skull and, while postcranial remains (remains of parts of the skeleton other than the skull) have been found at sites bearing P. boisei skull fossils, these postcranial fossils are not directly associated with the skull fossils. Therefore, the postcranial features of P. boisei are largely unknown. Some postcranial fossils that are more confidently associated with diagnostic P. boisei skull fossils suggest that this species had limb proportions (the relative sizes of the upper and lower limb) similar to those of Australopithecus afarensis and the scientific consensus is that P. boisei was bipedal. Other postcranial fossils that may be those of P. boisei suggest that the anatomy of the hand bones would have permitted this species to manufacture stone tools, a capability usually considered unique to species in the genus Homo; however, this suggestion is tenuous because the hand fossils are not directly associated with diagnostic skull fossils.
The evolutionary relationships among early hominins, including P. boisei, are hotly debated. Likewise, no consensus has been reached on the taxonomy (the scientific names given to species) of the early hominin species. Most scholars recognize a number of unique features that are shared by all of the robust australopiths—P. aethiopicus, P. boisei, and P. robustus—and use the genus name “Paranthropus” to denote the fact that these species are more closely related to each other than any are to other hominin species. Some scientists, however, maintain that the features characteristic of the robust australopiths evolved independently in eastern and South Africa. Furthermore, scholars disagree about the precise phylogeny (evolutionary relationships) among the robust australopiths and other species in the genus Australopithecus (see essays on Australopithecus africanus and P. aethiopicus). Some scholars, in order to specify that the robust australopiths may not, in fact, be more closely related to each other than they are to other species, prefer to use the genus name, “Australopithecus,” to refer to the robust species.
The extreme size and shape of the skull bones of P. boisei have been traditionally considered to be an adaptation for consuming very hard, brittle foods, such as nuts and tubers. Consumption of these foods would have required P. boisei to produce large chewing forces on the molars and premolars to break down these otherwise inedible foods. Therefore, the tacit consensus among scientists has been that the diets consumed by P. boisei and the other robust australopiths consisted largely of hard plant foods. This specialization on hard foods is thought to explain how the robust australopiths were able to coexist with other hominin species (e.g., Homo habilis and Homo erectus) without going extinct. The disappearance of these foods (due to changes in climate or other factors) on which the robust australopiths specialized has also been used to explain their eventual extinction; when these foods disappeared or became less abundant, the robust australopiths went extinct because they had evolved a specialization for a food that was no longer available. Recent evidence from dental microwear (the microscopic markings left on teeth by the food animals consume), however, suggests that, instead of being the primary diet of P. boisei, hard foods were “fall back foods”—that is, foods that were eaten when preferred foods were scarce or unavailable. In fact, some scholars have suggested that P. boisei consumed more meat than early species in the genus Homo, but the evidence on which this suggestion rests is relatively scant.
The habitats in which P. boisei lived have been reconstructed as being close to water sources, such as streams or lakes. Studies of the specific environmental preferences of P. boisei, however, have come to different conclusions; some suggest that P. boisei preferred more closed habitats (habitats that are largely covered by tree canopy), while others suggest a preference for open habitats (habitats not covered by tree canopy).