Neurocranial evolution in Middle Pleistocene: a paleoneurological study of Jebel Irhoud 1

Abstract

The late Middle Pleistocene human fossil record is rather scanty and geographically fragmented. In Europe, after archaic morphotypes like Ceprano and Sima de los Huesos, the endocranial morphology of specimens like Saccopastore 1 display clear Neandertal traits. In Africa the situation is more heterogeneous, and evolutionary hypotheses must be more tentative. Despite its good preservation and possible relationship with the origin of the modern humans, the phylogenetic position of Jebel Irhoud 1 remains debated. Here we compare the neurocranial and endocranial shape and features of this specimen with other Middle and Upper Pleistocene specimens to provide further paleoneurological information on the early stages of the evolution of modern human brain. Although facial traits suggest a phyletic relationship with early modern human dispersals, the neurocranial and endocranial morphology of this specimen is definitely nonmodern, resembling instead the contemporary European morphotypes. The fossil’s slight bulging of the frontal squama is probably associated with bone’s orientation and facial reduction rather than actual changes of the frontal profile. The midsagittal neurocranial geometry fits comfortably within African and European Middle Pleistocene human variation; lateral proportions are somewhat similar to those of Neandertals. If Jebel Irhoud represents western relic populations of an early, proto-modern dispersal, we must assume that the origin of this proto-modern lineage emerged before the evolution of a fully modern brain.