Neurocranial evolution in modern humans: the case of Jebel Irhoud 1

概要

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Jebel Irhoud 1 represents an almost complete cranium from the North African late Middle Pleistocene. Despite the good preservation of most of its anatomical regions, its phylogenetic position is still uncertain, particularly its relationship to the emergence of the modern human lineage. The present paper supplies a basic morphometric description and comparison of the endocast of Jebel Irhoud 1. The endocast's maximum width is large when compared with the hemispheric length, with values similar to those of Neandertals. Conversely, the frontal width is less pronounced, showing proportions compatible with modern and non-modern human taxa. The vertical proportions are similar to those displayed by Homo erectus, while the lateral proportions are comparable to Neandertals. Furthermore, the upper parietal areas show a certain parasagittal lateral bulging, as in European Neandertals. It remains to be established if this trait evolved independently in both the Neandertal and modern human lineages, or if it was already present in a common ancestor of these two groups. Given that Jebel Irhoud 1 in North Africa and Herto in East Africa have similar geological age, similar facial morphology, but different vault proportions, it seems likely that the origin of the modern human lineage may have predated the origin of many aspects of the modern human brain.