Evolutionary interpretation of the modern human-like facial morphology of the Atapuerca Gran Dolina-TD6 hominins

Abstract

The Early Pleistocene human fossil remains recovered from the TD6 lithostratigraphic unit of the Gran Dolina cave site in the Sierra de Atapuerca, northern Spain, show a mosaic of primitive and derived features. Among the latter, the modern human-like midfacial topography, as well as several synapomorphies shared with some European Middle Pleistocene hominin and Neanderthals, represents a challenge for the phylogenetic interpretation of Homo antecessor. Using an ontogenetic approach of the maxilla ATD6-69, Freidline et al. (Journal of Human Evolution, 65: 404-423 (2013)) have confirmed previous observations that H. antecessor adults had a set of facial features characterizing H. sapiens. However, Freidline and collaborators proposed that the evolution of modern-looking facial morphology occurred independently in Africa, Asia, and Europe and at several times during the Early and the Middle Pleistocene. Following their line of reasoning, the presence in H. antecessor of some features shared with the European Middle Pleistocene hominins and the Neanderthal lineage could also be interpreted as convergences. However, instead of supposing multiple, parallel evolution, we suggest that a more parsimonious interpretation envisages the hypothetical existence of an Early Pleistocene hominin population, from which several hominin lineages originate and inherit particular combinations of derived features. The TD6 hominins probably represent a side branch of this cladogenetic event, which evolved in Western Europe.

The origin and evolution ofHomo sapiens

“…Bermú dez de Castro & Martinó n-Torres [106] concluded that antecessor was a side-branch of a lower Pleistocene radiation of lineages in Eurasia that eventually gave rise to Neanderthals in Europe and to H. sapiens in Africa. If heidelbergensis (sensu Stringer [25]) is not the LCA for sapiens and neanderthalensis because of a derived facial morphology and because the known fossils post-date the actual divergence date, then what did the LCA look like (Ancestor X in figure 2c).…”

“…For some, this morphology in a fossil dated at more than 800 ka may reflect the ancestral condition for H. sapiens [29]. Previously [2], I questioned whether this morphology would have been maintained in the adult, and whether there would have been population variation in its expression, but it does seem to be expressed in three more fragmentary subadult and adult specimens from Gran Dolina [106]. Issues have also been raised about allometric effects on the zygomaxillary area such that larger archaic faces would show an inflated maxillary morphology more like that of the massive Bodo and Petralona crania [107].…”

“…It has been suggested that the antecessor material also displays a derived sapiens-like pattern of dental development [111], as well as some similarities in postcranial morphology [106], but in other respects there are Neanderthal-like features such as in the mastoid region, hypertrophied medial pterygoid tubercle, M 1 shape, clavicle and humerus. Bermú dez de Castro & Martinó n-Torres [106] concluded that antecessor was a side-branch of a lower Pleistocene radiation of lineages in Eurasia that eventually gave rise to Neanderthals in Europe and to H. sapiens in Africa. If heidelbergensis (sensu Stringer [25]) is not the LCA for sapiens and neanderthalensis because of a derived facial morphology and because the known fossils post-date the actual divergence date, then what did the LCA look like (Ancestor X in figure 2c).…”