Paranthropus aethiopicus

Locations of P. aethiopicus discoveries

In 1968, French palaeontologist Camille Arambourg and Breton anthropologist Yves Coppens described "Paraustralopithecus aethiopicus" based on a toothless mandible (Omo 18) from the Shungura Formation, Ethiopia. The name aethiopicus refers to Ethiopia.[2] In 1976, American anthropologist Francis Clark Howell and Coppens reclassified it as A. africanus.[3]

In 1985, the skull KNM WT 17000 dating to 2.5 million years ago was reported from Koobi Fora, Lake Turkana, Kenya, by anthropologists Alan Walker and Richard Leakey. A partial jawbone from a different individual, KNM-WT 16005, was also discovered. They clearly belonged to a robust australopithecine.[4] By this point in time, much younger robust australopithecines had been reported from South Africa (robustus) and East Africa (boisei), and been variously assigned to either Australopithecus or a unique genus Paranthropus.[5] Walker and Leakey assigned KNM WT 17000 to the boisei clade. They noted several anatomical differences, but were unsure if this stemmed from the specimens' archaicness or represented the normal range of variation for the species. If the former, they recommended classifying them and similar specimens into a different species, aethiopicus (and recommended that Paraustralopithecus be invalid). The discovery of these archaic specimens overturned previous postulations that P. robustus was the ancestor of the much more robust P. boisei (a hypothesis notably argued by palaeoanthropologist Yoel Rak in 1985) by establishing the boisei lineage as beginning long before robustus had existed.[4]

In 1989, palaeoartist Walter Ferguson recommended KNM WT 17000 be classified into a different species, walkeri, because the holotype of aethiopicus comprised only the jawbone and KNM WT 17000 preserves no jaw elements.[3] Ferguson's classification is almost universally ignored,[6] and is considered to be synonymous with P. aethiopicus.[7]

Several more lower and upper jaw specimens have been unearthed in the Shungura Formation,[5]^{: 112–113} including a juvenile specimen, L338y-6.[8] In 2002, a 2.7–2.5 Ma maxilla, EP 1500, from Laetoli, Tanzania, was assigned to P. aethiopicus. Also found was the upper portion of a tibia, but it cannot definitively be associated with EP 1500 and thus with P. aethiopicus.[9]

The genus Paranthropus (from Ancient Greek παρα para beside or alongside, and άνθρωπος ánthropos man,[10] otherwise known as "robust australopithecines") typically includes P. aethiopicus, P. boisei, and P. robustus. P. aethiopicus is the earliest member of the genus, with the oldest remains, from the Ethiopian Omo Kibish Formation, dated to 2.6 million years ago (mya) at the end of the Pliocene.[11] It is possible that P. aethiopicus evolved even earlier, up to 3.3 mya, on the expansive Kenyan floodplains of the time.[12] P. aethiopicus is only confidently identified from the skull KNM WT 17000 and a few jaws and isolated teeth, and is generally considered to have been ancestral to P. boisei which also inhabited East Africa, making it a chronospecies. Because of this relationship, it is debatable if P. aethiopicus should be subsumed under P. boisei or if the differences stemming from archaicness should justify species distinction. The terms P. boisei sensu lato ("in the broad sense") and P. boisei sensu stricto ("in the strict sense") can be used to respectively include and exclude P. aethiopicus from P. boisei when discussing the lineage as a whole.[5]^{: 106–107}

It is also debated if Paranthropus is a valid natural grouping (monophyletic) or an invalid grouping of similar-looking hominins (paraphyletic). Because skeletal elements are so limited in these species, their affinities with each other and to other australopithecines is difficult to gauge with accuracy. The jaws are the main argument for monophyly, but such anatomy is strongly influenced by diet and environment, and could in all likelihood have evolved independently in P. boisei and P. robustus. Proponents of monophyly consider P. aethiopicus to be ancestral to the other two species, or closely related to the ancestor. Proponents of paraphyly allocate these three species to the genus Australopithecus as A. boisei, A. aethiopicus, and A. robustus. British geologist Bernard Wood and American palaeoanthropologist William Kimbel are major proponents of monophyly, and against include Walker.[5]^{: 117–121}

This species, originally named Paraustralopithecus aethiopicus, cannot retain the species epithet aethiopicus if moved to genus Australopithecus because Australopithecus aethiopicus is already a junior synonym of Australopithecus afarensis. Such a classification would have to use the name Australopithecus walkeri for this species. The change of species epithet would also happen in a taxonomy that classifies all hominins as Homo.[14]