Two very different specimens were found in sediments of the same age (1.7 million years) in Koobi Fora, Turkana Basin in nothern Kenya:

• A complete but toothless cranium of Paranthropus boisei, KNM ER 406, discovered by Richard Leakey in 1968.
• A complete cranium of Homo ergaster, KNM ER 3733, discovered by Bernard Ngeneo – in Richard Leakey’s team – in 1975.

ER 406 P. boisei and ER 3733 H. ergaster. Photo credit: Roberto Sáez

These finds were important because they were breaking the ‘simple species hypothesis’ in human evolution. According to this principle, only one species can inhabit a specific ecological niche. Those two specimens virtually coexisted, but they were really different in terms of morphology, cranial capacity and type of resources consumed:

• The massive masticatory features and teeth associated to Paranthropus boisei are designed for a repeated activity chewing and crushing hard vegetables.
• The Acheulean tools associated to Homo ergaster suggest a hunting or scavenving activity to get richer meat resources.

On other hand, the coexistence of P. boisei and H. ergaster has traditionally made very difficult the assignment of the postcranial fossils to either one or the other species.

Meet the Paranthropus boisei ER 406

• Posterior sagittal chest and nuchal crest
• Deep palate and massive tooth roots
• Very large and very projecting zygomatic process, with anterior position and the root far forward over the second upper premolar, maximising the masticatory area
• Dish-shaped face, very wide face but short in height
• Remarkable postorbital constriction
• Projecting glabella
• 510 cc cranial capacity

KNM ER 406 Paranthropus boisei. Photo credit: Roberto Sáez

All the features above are shared with the other complete P. boisei skull, the iconic OH 5 (Nutcracker Man). And like OH 5, this specimen ER 406 is suggested to be a male.

The bone shows pathological evidence of a small, round hole on the frontal bone, suggesting a metastatic abscess.

Meet the Homo ergaster ER 3733

• Double-arched supraorbital torus, which is some gracile but projects more than those in earlier Homo habilis / rudolfensis 
• The neurocranium has considerable height and length
• Flattened sides of the braincase instead of arching like in modern humans
• Very broad face, little prognatic lower face, overall flatness showing a modern architecture
• Broad anterior dentition, but overall reduced teeth
• Lack of postorbital constriction
• Slight sagittal keeling
• Development in the posterior skull region
• 850 cc capacity

KNM ER 3733 Homo ergaster. Photo credit: Roberto Sáez

All the previous features are typically found in the Homo ergaster specimens as well as in the Asian Homo erectus. However, the Asian Homo erectus has other features that differs from those in ER 3733 such as having thicker skull bones and the presence of an angular torus.

ER 3733 is considered an adult because of the closed cranial sutures, the degree of the dental wear and the eruption of the third molars. It is suggested to be a female because the face bones are less robust than those of WT 15000.