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- Paleobiology and comparative morphology of a late Neandertal sample from El Sidrón, Asturias, Spain

Presently, most of the scholars agree that Neandertals constitute a distinct human evolutionary lineage not involved in the initial ancestry of modern humans. The causes promoting the Neandertal anatomical pattern, nevertheless, are still under debate (3739). Climatic adaptation is a major factor invoked to accounting for most of the Neandertal anatomy (40, 41), although biomechanical adaptations (42, 43) and stochastic genetic processes (44) are raised for the explanation of specific traits. Clarification of these aspects has focused the research interest on the evolution of this archaic human group, and the amount of interbreeding between Neandertals and early modern humans after the arrival of the latter to Europe is a matter of contention (4547).

Given the establishment of a Neandertal evolutionary lineage in Europe, we are beginning to address issues regarding the population history of Neandertals sensu stricto. Genealogical analyses of the ancient mtDNA sequences are showing well defined genetic groups, suggesting the existence of different lineages within the Neandertal gene pool (8, 9). In the context of Quaternary ecoclimatic instability, the evolution of Neandertals has been long enough to produce regional diversity of populations. If Middle Paleolithic human populations behaved as part of the Paleartic bioma, it would not be surprising if they exhibited the marked north–south faunal provinciality detected in Europe at least during the OIS 3 (48).

Morphometric Implications. The analysis of mandibular variability conforms to a pattern of geographic distribution in a north–south polarity within Neandertal samples, in which southern populations may have developed a slightly distinctive craniofacial pattern. This conclusion may find support in recent research (49), showing that mandibular morphology records geographic patterning in modern humans.

Two sets of traits seem to be involved in the north–south differences. On the one hand, there is the breadth of the mandible, indicated by classic distances (e.g., Bi–M3 breadth), 3D procrustes distances, and the variation in discrete features, especially in the eversion of gonion. On the other hand, there is the relative position of the ramus with respect to the corpus (e.g., M3–lingula length). The latter is associated with the retromolar space size, irrespective of the ramus breadth. Retromolar spaces are moderately developed in the El Sidrón, despite the large size of the mandibles, even though this feature is partially determined by allometry in Neandertals and other species (49, 50). Metrics and discrete features point to a reduced midfacial prognathism in the El Sidrón individuals, and the same have been noted in other Iberian Neandertals (e.g., Valdegoba) (51). Therefore, southern Neandertals seem to present broader and shorter faces, with the architecture of the middle face (broader zygomatic arches with gonion eversion) directly involved in this pattern.

Neandertal populations were largely isolated by geographic barriers (5255), and at the peak of glacial events the European population was mainly concentrated in the south of the continent (54). In this framework, two properties should be expected for the southern populations. First, they should have had a larger temporal continuity, and in consequence more time for developing morphological variants. Second, a larger amount of variability should be found in these populations. The first expectation may be substantiated by the fact that northern samples are morphologically more similar to those from the Middle Pleistocene; that is, Neandertal populations from the north would maintain a more primitive condition within the European lineage. If so, the slightly distinctive morphology of southern samples may be interpreted as derived. Yet, southern samples (e.g., Krapina, El Sidrón) illustrate local variation that might be explored from this perspective.

Summary. The Iberian Neandertal sample from El Sidrón substantially improves the fossil record of these humans, allowing new explorations into the geographic context of Neandertal evolution. The analysis of mandibular variability supports the hypothesis for the identification of northern and southern varieties of Neandertals, in which southern populations may have developed a slightly reduced midfacial prognathism. At the same time, the paleobiology and human-induced modifications of the eight individuals identified in the El Sidrón sample fit well with the established Neandertal patterns. The El Sidrón sample is therefore both expanding our knowledge of Neandertals in Iberia and modifying our perceptions of Neandertal population biology and evolution.

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