Australopithecus/Paranthropus aethiopicus (2.7–2.3 mya)

(“southern ape” / “beside human” / Ethiopia)

INTRODUCTION

Australopithecus aethiopicus is the most primitive of the robust species. I use genus Australopithecus because it is thought to be descended from Au. afarensis. In addition, Paranthropus was the genus name assigned to the South African robust form, P. robustus, and questions remain as to whether the two species are related.

PHYLOGENY

There are multiple lines of evidence to support Au. aethiopicus as a descendent species of Au. afarensis. While some believe that Au. aethiopicus gave rise to P. boisei, others link P. boisei with P. robustus in a different clade, with Au. africanus as their common ancestor. More recently discovered material within the geographic range of Au. aethiopicus supports the Au. aethiopicus → P. boisei evolutionary scenario. The dates of the new fossils fall between the two species, and they possess intermediate or transitional characteristics. Figure 17.2 shows one cladistic schema that illustrates how some researchers suggest these species were related. This particular scenario shows the authors’ belief that Au. africanus is a robust form.

DISCOVERY AND GEOGRAPHIC RANGE

In 1967, the earliest Au. aethiopicus fossils were discovered by Yves Coppens and Camille Arambourg in the Shungura deposits at the site of Omo in southern Ethiopia. They assigned them to a new genus and species, Paraustralopithecus aethiopicus. While it was debatable as to whether they actually had a new species, the discovery of the “Black Skull” (see Figure 17.1) in the West Lake Turkana region of Kenya by Alan Walker in 1985 put any doubts to rest. At that time, the species was added to genus Australopithecus because it was thought to be descended from Au. afarensis. There were then three recognized species of robust australopiths in Africa, and efforts to determine their phylogenetic relationships began.

The Black Skull or KNM-WT (Kenya National Museum – West Turkana) 15000 was a magnificent find. The almost complete skull was stained from manganese, but it is always fun to sing scary movie music to my students when introducing … THE BLACK SKULL! (Figure 17.3 … for fun!)

PHYSICAL CHARACTERISTICS

A unique characteristic that ties Au. aethiopicus to P. boisei is a heart-shaped foramen magnum, as opposed to the more ovoid form seen in Au. africanus and P. robustus. Primitive characteristics shared with Au. afarensis are the flat cranial base, small brain (~410 cc), long molars (mesiodistally, i.e. front to back versus side to side), and the degree of prognathism in the lower face. Because their faces were so broad and their brains so small, they exhibit a high degree of postorbital constriction (also known as waisting), i.e. the area between the face and braincase is narrow. Derived robust characteristics are buttressing of the skull, face, and mandible. Their muscles of mastication were incredibly strong, as evidenced by the sagittal crest running down the midline of their skull where the temporalis muscle originated. The sagittal crest was higher and more posteriorly placed than in the two more derived robust species. The zygomatics were large and flared to allow for passage of the temporalis muscle from the sagittal crest to insert on the mandible and to expand the attachment site for the masseter muscle, the other large muscle of mastication (see Figure 17.4). The zygomatics were more forwardly flared than in the other two robust species. They also had heavy nuchal (posterior neck) muscles to support the weight of their heavy face and skull, and the attachment sites of those muscles on the posterior skull was also an expanded crest that converged with the sagittal crest, i.e. a compound sagittal-nuchal crest. Large brow ridges in the robust species offset the stress generated by biting down on hard foods. However, Au. aethiopicus’s brows were smaller than the other two species. Their third maxillary molars were convergent, meaning they were positioned more medially than the first and second. While some researchers disagree, others find evidence for a more derived venous sinus system in the crania of the robust forms. The system consists of large collecting veins that ultimately empty into the jugular veins, allowing for rapid, gravity-fed blood drain from the brain, in order to keep fresh blood pumping in via several arterial systems. The largest and most superficial of those sinuses (see Figure 17.5) leave impressions on the inside of the skull vault. Dean Falk’s “Radiator Theory” argues that our ancestors needed to keep their brains cool as they increased in size in a hot, open environment. The system became more complex after the time of the australopiths.

ENVIRONMENT AND WAY OF LIFE

As mentioned, there is evidence that Au. afarensis was more terrestrial than the southern australopith clade, suggesting that the classic ape environment had diminished. We know that grasslands were expanding and woodlands were shrinking. Since it is assumed that Au. aethiopicus is descended from Au. afarensis, the environment was favoring species with dietary adaptations that allowed them to survive.