Brain size and organization in the Middle Pleistocene hominins from Sima de los Huesos. Inferences from endocranial variation.

The Sima de los Huesos (SH) endocranial sample includes 16 complete or partial endocasts corresponding to European Middle Pleistocene hominins. Different anatomical and molecular studies have demonstrated that these hominins are phylogenetically related to Neanderthals, thus making them the earliest unquestionable representatives of the Neanderthal lineage. The description of endocranial variation in this population is fundamental to shedding light on the evolution of the Neanderthal brain. In this contribution, we analyze and describe endocranial variation in this sample, including aspects related to brain size (endocranial volume and encephalization) and brain organization (through qualitative descriptions and quantitative analyses). Our results indicate that the SH hominins show a transitional state between a primitive hominin endocranial configuration (which is found in Homo erectus and non-SH Middle Pleistocene Homo) and the derived configurations found in Neanderthals and modern humans, without a clear anticipation of classic Neanderthal endocranial traits. In comparison with other cranial and postcranial traits that show a fully Neanderthal or clear pre-Neanderthal condition in the SH collection, endocranial variation in these hominins is surprisingly primitive and shows no Neanderthal affinity. These results and the comparison with other cranial traits confirm that Neanderthals evolved in a mosaic fashion. Traits related to mastication (dental, facial and mandibular anatomy) led the Neanderthalization process, whereas neurocranial anatomy must have acquired a fully Neanderthal condition considerably later.

Postcranial morphology of the middle Pleistocene humans from Sima de los Huesos, Spain.

Current knowledge of the evolution of the postcranial skeleton in the genus Homo is hampered by a geographically and chronologically scattered fossil record. Here we present a complete characterization of the postcranium of the middle Pleistocene paleodeme from the Sima de los Huesos (SH) and its paleobiological implications. The SH hominins show the following: (i) wide bodies, a plesiomorphic character in the genus Homo inherited from their early hominin ancestors; (ii) statures that can be found in modern human middle-latitude populations that first appeared 1.6-1.5 Mya; and (iii) large femoral heads in some individuals, a trait that first appeared during the middle Pleistocene in Africa and Europe. The intrapopulational size variation in SH shows that the level of dimorphism was similar to modern humans (MH), but the SH hominins were less encephalized than Neandertals. SH shares many postcranial anatomical features with Neandertals. Although most of these features appear to be either plesiomorphic retentions or are of uncertain phylogenetic polarity, a few represent Neandertal apomorphies. Nevertheless, the full suite of Neandertal-derived features is not yet present in the SH population. The postcranial evidence is consistent with the hypothesis based on the cranial morphology that the SH hominins are a sister group to the later Neandertals. Comparison of the SH postcranial skeleton to other hominins suggests that the evolution of the postcranium occurred in a mosaic mode, both at a general and at a detailed level.