Tempo and mode in the Neandertal evolutionary lineage: A structuralist approach to mandible variation

The long-standing debate around the origin and evolutionary mode of the Neandertal lineage is connected to the understanding of the large morphological diversity found in the European Middle Pleistocene Hominin (EMPH) samples. In practical terms, this problem can be approached through two questions: How many morphs can be distinguished in EMPH and how do they relate to the origin of Neandertals? These questions are addressed in this paper by means of geometric morphometric comparative analyses in combination with principles of craniofacial biology. We use the mandible as a model system because of its relative abundance in the fossil record, the presence of diagnostic features relevant to the process under analysis, and the key role played by some specimens. Among them, three samples are central in the debate: the Mauer mandible (as the holotype of Homo heidelbergensis), and the Arago and Atapuerca-SH samples. Our comparative analyses conclude that the Mauer and Arago samples derived from a single deme, and that Neandertal derived features that represent an incipient phase of the evolutionary process. The specimen AT-950 from the Atapuerca-SH sample has a number of structural similarities with Mauer, Arago and Montmaurin, which allows linking of this latter group with the complete Atapuerca-SH hypodigm. Consequently, all of these specimens are considered members of the Neandertal lineage. The fossil record illustrates a long evolutionary Neandertal lineage going back at least 0.6 million years. Against populational-based interpretations, we favor a structuralist approach and follow Enlow's counterpart growth analysis and compensatory growth mechanisms for interpretation of the EMPH diversity. In this framework, morphometric cranio-mandibular variation in hominin samples from the European Middle Pleistonece must be contemplated within the head form vertical variation pattern (e.g. vertical dolicho- and brachyfacial form spectrum) widespread in Hominoid species.
This work was supported by the Ministerio de Ciencia, Innovación y Universidades, Spain projects CGL2012-36682, CGL2016-75109-P, and CGL2015-63648-P.