The discovery of A. aethiopicus and the implications surrounding it remain one of the most central questions concerning the evolution of the hominid line in paleoanthropology today.
The first specimen of this species was an edentulous mandible found by Camille Arambourg and Yves Coppens discovered in southern Ethiopia in 1967. It was first named Paraustralopithecus aethiopicus. Because of this new distinction, the find was largely ignored or overlooked in the field. It was the discovery of "The Black Skull" (KNM-WT 17000) in 1986 that brought interest to the species. It's commonly called "Black Skull" because during fossilization, manganese absorbtion gave the skull a blackish color.
Classification of this species is still highly contested by most researchers. The specimen found is very similar to A. afarensis, yet with a cranial capacity of only approximately 410 cm3. Generally, the face resembles the species A. boisei, in that it is extremely indicitive of the robust Australopithecines. However, the top and back of the cranium itself is similar to A. afarensis.
It has very large molars, flared cheek bones, a dished face, and a prominent sagittal crest running along the midline at the top of the skull.1 This set of anatomical features is most unusual in terms of categorization. Research in the field has come to a wide range of conclusions:
"Australopithecus aethiopicus became important in phylogenetic considerations soon after the discovery of the Black Skull. The species is generally accepted to have shown that the genus designation Paranthropus is polyphyletic and invalid, though some still vocally argue against that fact. One of the earliest important cladistic analysis was by Walker and Leakey (1988), which they claim shows aethiopicus is at the base of the boisei lineage, is more primitive than robustus, and that aethiopicus is not ancestral to robustus. However, Strait et al. says that this phylogeny requires 39 extra steps above the most parsimonious tree, and most cladists do not favor this phylogeny. Skelton and McHenry (1992) and Lieberman et al. (1996) both came to the same conclusions regarding aethiopicus and Paranthropus using different character traits. Both see aethiopicus as a dead-end side branch and Paranthropus as polyphyletic and invalid. On the other side of the coin, Strait et al. (1997) see all the robusts sharing a recent common ancestor (aethiopicus), with Paranthropus monophyletic." 2
Attempting to reach a conclusion about the specific classification of this species with respect to other early human ancestors is difficult. Certainly, the discovery of this species and dating it to coexist with other known early Australopithecus shows that species diversity during this time was much greater than originally thought.
1 See http://www.archaeologyinfo.com/australopithecusaethiopicus.htm for a photograph of "The Black Skull" and an interesting visual reproduction of A. aethiopicus.