The outer and inner morphology of the one-million-year-old UA 31 cranium from Buia, Danakil Eritrea: an updated comparative record

Abstract

In the northernmost sector of the African Rift Valley, late Calabrian human remains have been discovered in the Dandiero Rift Basin of the Danakil (Afar) depression of Eritrea, in two 4.7 km apart but stratigraphically correlated sites of the Aalat succession nearby the village of Buia: Uadi Aalad and Mulhuli-Amo. The assemblage from the c. 1 Ma old Homo site at Uadi Aalad consists of an adult cranium including the face (UA 31), two permanent teeth, and three pelvic portions. Compared with other Eastern African fossil specimens, including the chronogeographically close calvaria from Daka, as well as with the Asian H. erectus s.s. hypodigm, UA 31 displays a blend of H. erectus-like and derived morphoarchitectural features more commonly found in Middle Pleistocene specimens. Together with an anteroposteriorly projecting supraorbital torus, marked postorbital constriction, an increasingly posterolateral enlargement of the parietal contour, a smooth profile of the parietoccipital region with no occipital torus, its absolutely long, narrow and proportionally high braincase is characterized by modest lateral protrusion of the temporo-mastoid region, high positioning of the maximum parietal breadth and from sub-vertical to slightly downwards converging parietal walls. The endocast (995 cc) shows a general plesiomorphic phenotype, with some individual features (dolichocephaly, rounded lower parietal areas) confirming a remarkable degree of variability within the H. erectus/ergaster hypodigm. The cranial base in UA 31 is more flexed than measured in Sambungmacan 4, but less than in Daka. Recent suggestions that, more than with wider parietal expansion, the peculiar non "tent-like" outline characterizing this specimen may be associated with narrow endocranial base and temporal lobes. Together with CT-based evidence about its vault thickness topographic variation and relative bony structural arrangement, here we provide comparative information about basicranial structural morphology and proportions in UA 31 and assess their relationships with global neurocranial architecture.