Phylogenetic position of the Gran Dolina-TD6 hominins in the context of the human evolution in Europe

Abstract

The so-called “Aurora Archaeostratigraphic Set” of the TD6 level of the Gran Dolina cave site in Sierra de Atapuerca, northern Spain has yielded about 150 human fossil remains, attributed to Homo antecessor (Bermúdez de Castro et al., 1997, Science 276), and dated to the MIS 21 or MIS 25. Our aim is to present a brief analysis of the main features of this sample and to discuss the phylogenetic position of this hominin assemblage in the framework of the evolutionary scenario of the settlement of Europe. The TD6 human fossils exhibit a particular mosaic of primitive and derived features for the Homo clade that reinforces the taxonomic identity of H. antecessor. Some dental features are primitive regarding the Homo clade, whereas other are derived and shared with Pleistocene Eurasian hominis. The mandibles show a primitive structural pattern shared with other African and Asian specimens. However, they have lost the massive aspect characteristic of most Homo African mandibles, and present some progressive features also shared with Middle Pleistocene African and, particularly, Asian specimens. None of the mandibular features considered apomorphic in the European Middle and Late Pleistocene hominins are present in the TD6 mandibles (perhaps except the great development of the medial pterygoid tubercle). In spite that the TD6 hominins are nearly one million years old, it can be stated that they show a clear tendency towards what might be called “modernity”. In fact, their brain size was higher than 1000 cubic centimeters and the postcranial skeleton shares most of the features with European Middle and Late Pleistocene hominins, including modern humans. The stature estimations reveal that TD6 hominins were tall, about 175 centimetres. The pattern of dental development is definitively modern, whereas the very complete face ATD6-69 (adolescent) represents the earliest occurrence of a modern face in the fossil record. The remodeling pattern of this specimen is also similar to that of modern humans. The TD6 hominins also exhibit some postcranial and dental features that are shared with Neanderthals and the European Middle Pleistocene hominins. Obviously, these features cannot be considered as Neandertal apomorphies, but traits that appeared in an old hominin population and were inherited by both H. antecesor and H. neanderthalensis. Since we no longer support our previous hypothesis about the phylogenetic position of H. antecessor as the last common ancestor of the Neandertals and modern humans, it is necessary to reconcile the present evidence with an alternative hypothesis. We propose a cladogenetic event of the genus Homo, previous to the chronology of H. antecessor, from which gradual branching of hominin lineages (species) would have occurred throughout time. This cladogenesis would have been characterized, among other features, by a cranial size increase and the appearance of a derived modern like face morphology. This means that we need to revise the Eurasian fossil record with a different perspective. H. antecessor would represent a side branch confined to Western Europe. Interbreeding between individuals of Homo antecessor and those of other branches of the same cladogenesis, which colonized Europe in later times, cannot be ruled out.