No known hominin species matches the expected dental morphology of the last common ancestor of Neanderthals and modern humans

Abstract

A central problem in paleoanthropology is the identity of the last common ancestor of Neanderthals and modern humans ([N-MH]LCA). Recently developed analytical techniques now allow this problem to be addressed using a probabilistic morphological framework. This study provides a quantitative reconstruction of the expected dental morphology of the [N-MH]LCA and an assessment of whether known fossil species are compatible with this ancestral position. We show that no known fossil species is a suitable candidate for being the [N-MH]LCA and that all late Early and Middle Pleistocene taxa from Europe have Neanderthal dental affinities, pointing to the existence of a European clade originated around 1 Ma. These results are incongruent with younger molecular divergence estimates and suggest at least one of the following must be true: (i) European fossils and the [N-MH]LCA selectively retained primitive dental traits; (ii) molecular estimates of the divergence between Neanderthals and modern humans are underestimated; or (iii) phenotypic divergence and speciation between both species were decoupled such that phenotypic differentiation, at least in dental morphology, predated speciation.

Keywords: European Pleistocene; geometric morphometrics; morphospace; node reconstruction; phylogeny.

Fig. 1.

Phylogeny used for ancestral state reconstruction and candidate species. Numbers in parentheses represent the averaged chronology of the corresponding species (although see ref. 56); italic numbers represent branch lengths in millions of years; bold gray numbers represent ages of nodes. Branch lengths corresponding to a 1-Ma divergence between Neanderthals and modern humans are represented to the left of the slashes and values corresponding to a 0.45-Ma divergence to the right. H. erectus has been dated to 1 Ma (values before the slash in the H. erectus branch) or to 0.5 Ma (values after the slash). Candidate species are represented below the phylogenetic tree with bars showing their approximate temporal spans (black, candidates for the [P-H]LCA; dark gray, candidates for the [H]LCA; light gray, candidates for the [N-MH]LCA). H. erectus and the SH group have been included in the framework phylogeny in four (H. erectus) and six (SH) phylogenetic scenarios (Fig. S1) and evaluated as candidate species in the remaining scenarios. H. ergaster has been evaluated as a candidate species for the [H]LCA and the [N-MH]LCA. H. habilis has been evaluated as a candidate for the [P-H]LCA and the [H]LCA. Vertical guides indicate the age of the evaluated nodes. Fig. S1 shows individualized representations of the 12 phylogenetic topologies.

Fig. 2.

Dental morphology of the [N-MH]LCA. Estimated ancestral morphologies are represented as TPS-grids showing the deformation from the base of the phylogeny into the estimated ancestral shape. Photographs (with landmarks represented as blue points) show examples of typical morphologies found in the Neanderthal lineage (NEA) and in the modern human (H. sapiens) lineage (SAP). Mesial margins are represented to the left, distal margins to the right, buccal margins to the top, and lingual margins to the bottom of the figure. Not to scale.

Fig. 3.

Projection of the simplified hominin phylogenetic tree (represented by lines connecting terminal species and nodes) with terminal species (large dark-gray filled circles), ancestor shapes (small light-gray filled circles) and candidate species (open circles) into the upper and lower first molar morphospaces. Confidence ellipses (95%) around the three nodes discussed in the text are represented with the three ancestors located in their centers (solid line, confidence ellipse of the [N-MH]LCA; dashed line, confidence ellipse of the [H]LCA; dotted line, confidence ellipse of the [P-H]LCA). (A) Upper first molar morphospace. (B) Lower first molar morphospace. Plots corresponding to premolars and second and third molars are provided in Fig. S4. Note that confidence ellipses in these graphs correspond only to the first and second principal components (PCs), whereas P values in Table 1 are based on all of the PCs. Phylogeny: BOI, P. boisei; ERE, H. erectus; NEA, H. neanderthalensis; ROB, Paranthropus robustus; SAP, H. sapiens; SH, SH group. Candidates: AFA, A. afarensis; AFR, A. africanus; ANT, H. antecessor; ERG, H. ergaster; GEO, Homo georgicus; HAB, H. habilis; HEI, H. heidelbergensis.