Fossils attributed to Paranthropus aethiopicus have been found at East African sites that have been dated to between 2.7 and 2.5 million years ago. Specifically, this species has been found in Ethiopia (in the Omo River basin), Kenya (in West Turkana), and Tanzania (at Laetoli). Fossils of P. aethiopicus exhibit a mixture of features in the braincase (the parts of the skull that enclose the brain) that resemble Au. afarensis with facial and dental features that are very similar to those in Paranthropus boisei and Paranthropus robustus. Together with its age (which succeeds Au. afarensis and precedes P. boisei and P. robustus), these features of P. aethiopicus have helped scientists understand the origins of the robust australopiths (P. aethiopicus, P. boisei, and P. robustus). Consensus on the precise evolutionary relationships among these species and between these species and earlier hominins, however, has not been reached.
Aethiopicusis known only from remains of the skull; no postcranial (parts of the skeleton not including the skull) remains have been attributed to this species. Although a maxilla and several teeth (some of which were found with associated bone of the lower jaw) representing P. aethiopicushave been found, the most informative fossil is a nearly complete cranium (skull minus lower jaw) from Kenya. This largely edentulous (toothless) cranium (catalog number KNM-WT 17000) is nicknamed the “Black Skull” because the sediments in which it was buried stained it black. The cranium of P. aethiopicus bears an interesting mix of features that are more similar to Au. afarensis and features that more closely resemble P. robustus and P. boisei. The features shared with Au. afarensis include a prognathic (forwardly jutting) face and a relatively small cranial capacity (an estimate of brain size based on volume of the brain case; the estimated cranial capacity of P. aethiopicus is in the lower end of the range of Au. afarensis). The morphology (size and shape) of the temporomandibular joints (the joint between the lower jaw and the cranium) in P. aethiopicus and Au. afarensis is also very similar. While the dentition as a whole differs greatly from that found in Au. afarensis, the anterior (front, flat) teeth in P. aethiopicus, like those of Au. afarensis, are relatively large, compared to those found in the robust australopiths. The sagittal crest (a bony ridge on the top of the skull extending from front to back in the middle of the skull to which the temporalis muscle—a large chewing muscle that closes the mouth—attaches) in P. aethiopicus is also similar to that found in Au. afarensis. Specifically, the P. aethiopicus sagittal crest is more pronounced in the back of the skull, as it is in Au. afarensis, suggesting that, like Au. afarensis, P. aethiopicus emphasized the back part of the temporalis muscle; this fact is corroborated by other similarities between Au. afarensis and P. aethiopicus in the markings made on the back of the skull by the temporalis muscle. It is important to note that the features that P. aethiopicus shares with Au. afarensis are not otherwise seen in species in the genus Paranthropus and these features, by and large, are not seen in Australopithecus africanus.
Many features in the P. aethiopicus cranium more closely resemble those exhibited by P. boisei and P. robustus. For instance, the zygomatic (cheek) bones are positioned very far forward and the outer margins of the face project far forward of the middle of the face, creating the appearance of a “dished” face (in which the outer parts of the face project so far forward that they obscure the nose hole when viewed from the side), characteristic of the other robust australopiths. As in the other robust australopiths, the bones of the palate in P. aethiopicus are thick. The premolar and molar teeth in P. aethiopicus are very large—similar in size to the other robust australopiths and much larger than in Au. afarensis—and the premolars are shaped much more like molars than in Au. afarensis.
The morphology of P. aethiopicus has a direct bearing on the evolutionary relationships among early hominin species. In particular, P. aethiopicus is important to understanding the origin of the robust australopiths as well as the relationship of these species to Au. afarensis and Au. africanus. Prior to the discovery of the “Black Skull,” researchers contended that Au. africanus was the probable ancestor of both P. robustus and P. boisei. This discovery, however, casts doubt on this scenario because it represented a hominin species that was both contemporary to Au. africanus and that exhibited much more primitive (shared features found in its ancestor, in this case Au. afarensis) cranial morphology. Some researchers now suggest that two distinct lineages of robust australopiths existed—one in South Africa, represented by P. robustus, whose ancestor, according to this scenario, is Au. africanus, and another in East Africa, represented by P. boisei, whose ancestor is P. aethiopicus (whose ancestor, in this view, is Au. afarensis). The similarities between P. robustus and P. boisei, from this point of view, are evidence that both species independently acquired features related to chewing hard foods. However, most phylogenetic (relating to the evolutionary relationships among species) analyses group P. boisei and P. robustus together and suggest that the common ancestor of these two species was likely more derived (possessing features not shared with its ancestor, in this case, features shared with the P. robustus and P. boisei) than P. aethiopicus.
Regardless of the perspective that scientists adopt, a consensus has emerged that the hominin fossil record between 3.0 and 2.0 million years ago witnessed a relatively high degree of homoplasy (the development of organs or other bodily structures within different species, which resemble each other and have the same functions, but did not have a common ancestral origin) and reversal. In other words, whichever phylogenetic scenario is correct, the current evidence suggests that some of the features shared by different species do not reflect close evolutionary relationships (for example, features related to the production of large chewing forces, found in the robust australopiths and, to some extent, in Au. africanus; see essay on Au. africanus for more detail) and some features evolved from one state to another and then back to the original state (reversal). A more complete fossil record of the time span between 3.0 and 2.0 million years ago (in particular, a better fossil record of P. aethiopicus), as well as a better understanding of the morphological diversity subsumed within Au. africanus—(see essay on Au. africanus) will help clarify these unresolved phylogenetic issues.