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Home , Homo antecessor, Homo ergaster, Homo habilis, Homo rhodesiensis, Homo rudolfensis, Paranthropus

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

Aida Gómez-Roblesa,b,1, José María Bermúdez de Castroc, Juan-Luis Arsuagad, Eudald Carbonelle, and P. David Pollyf

aDepartment of Anthropology, The George Washington University, Washington, DC 20052; bKonrad Lorenz Institute for Evolution and Cognition Research, A-3422 Altenberg, Austria; cCentro Nacional de Investigación sobre la Evolución Humana, 09002 Burgos, ; dCentro de Investigación Sobre la Evolución y Comportamiento Humanos, Universidad Complutense de Madrid–Instituto de Salud Carlos III, 28029 Madrid, Spain; eInstitut Català de Paleoecologia Humana i Evolució Social, Universitat Rovira i Virgili, 43007 Tarragona, Spain; and fDepartments of Geological Sciences, Biology, and Anthropology, Indiana University, Bloomington,IN 47405

Edited by Jukka Jernvall, University of Helsinki, Helsinki, Finland, and accepted by the Editorial Board September 19, 2013 (received for review February 9, 2013)

A central problem in is the identity of the last ancestral reconstruction that arises from lability of the evolutionary common ancestor of Neanderthals and modern humans ([N-MH] process and alternative hypotheses of the phylogeny on which it is LCA). Recently developed analytical techniques now allow this based. To implement this, we used geometric morphometric data to problem to be addressed using a probabilistic morphological frame- reconstruct ancestral dental shapes and their confidence ellipsoids work. This study provides a quantitative reconstruction of the using the phylogenetic generalized linear model (GLM) approach expected dental morphology of the [N-MH]LCA and an assessment (ref. 6; see Materials and Methods and SI Text for detailed explan- of whether known species are compatible with this ancestral ations). Ancestral reconstructions are based on an independent position. We show that no known fossil species is a suitable candi- phylogeny, ideally one built from nondental characters to avoid date for being the [N-MH]LCA and that all late Early and Middle circularity (Fig. 1 and Fig. S1). Note, however, that the phylogenetic taxa from have dental affinities, relationships of hominin species are vigorously debated and that pointing to the existence of a European clade originated around they are based mainly on craniodental traits, although not on the EVOLUTION 1 Ma. These results are incongruent with younger molecular diver- quantitative dental shape traits we use (Figs. S2 and S3). The an- gence estimates and suggest at least one of the following must be cestral dental reconstructions were based only on species whose true: (i) European and the [N-MH]LCA selectively retained phylogenetic position is relatively uncontroversial; species whose primitive dental traits; (ii) molecular estimates of the divergence phylogenetic position is contested were evaluated as potential candidate ancestors for nodes of relevant age. We considered 12 between Neanderthals and modern humans are underestimated; phylogenetic topologies that are collectively representative of the or (iii) phenotypic divergence and between both species range of credible alternative hypotheses about hominin rela- were decoupled such that phenotypic differentiation, at least in tionships (7). The ages of fossils and nodes were calibrated using dental morphology, predated speciation. comprehensive reviews of the chronology, geographical location, morphology and ecology of hominin species (8). In addition to phylogeny | node reconstruction | geometric morphometrics | the [N-MH]LCA, we evaluated ancestor candidates at two other morphospace | European Pleistocene important nodes of hominin phylogeny, the last common ances- tor (LCA) of the (excluding transitional hominins ncertainty about ancestry is typical for many nodes of the such as and ) and the LCA of Uhominin phylogeny. Species are usually suggested as com- and Homo, as controls for whether our statistical mon ancestors based on their geography, chronology, and mor- framework is too strict. Our method is not intended to be phological affinities (1–3), but these criteria can be unsatisfactory for two fundamental reasons: first, the inherent incompleteness Significance of the hominin fossil record precludes a clear understanding of the spatial and temporal range of variation of hominin species; The identity of the last common ancestor of Neanderthals and and second, comparative morphological analyses are frequently fi modern humans is a controversial issue. This debate has been descriptive in , making them dif cult to evaluate objectively. often addressed by means of descriptive analyses that are Fossil dental remains are central to our understanding of difficult to test. Our primary aim is to put questions about hominin evolution due to their commonness in archaeological evolution into a testable quantitative framework and and paleontological sites, their normally good state of preserva- to offer an objective means to sort out apparently unsolvable tion, and the minor influence that environmental factors—apart from wear—have on their morphology during an individual’s life. debates about hominin phylogeny. Our paper shows that no Teeth provide the most consistently complete representation of known hominin species matches the expected morphology of the hominin fossil record and are thus fundamentally important this common ancestor. Furthermore, we found that European for assessing the plausibility of competing evolutionary scenarios representatives of potential ancestral species have had af- and avoiding inadvertently biased interpretations (Tables S1 and finities with Neanderthals for almost 1 My, thus supporting S2). The combination of quantitative evolutionary theory, geo- a model of early divergence between Neanderthals and metric morphometrics, and dental anthropology provides a pow- modern humans. erful framework for reconstructing statistical expectations of the morphology of unknown ancestors (ref. 4 but see also ref. 5) and Author contributions: A.G.-R. and P.D.P. designed research; A.G.-R., J.M.B.d.C., J.-L.A., for making posterior comparisons with actual fossil remains. E.C., and P.D.P. performed research; P.D.P. contributed new reagents/analytic tools; The main aim of our study is to evaluate whether several known A.G.-R. analyzed data; and A.G.-R. and P.D.P. wrote the paper. candidate species have morphologies that are statistically consistent The authors declare no conflict of interest. with them being the last common ancestor of Neanderthals and This article is a PNAS Direct Submission. J.J. is a guest editor invited by the Editorial Board. modern humans ([N-MH]LCA). To be a credible ancestor, a fossil 1To whom correspondence should be addressed. E-mail: [email protected]. must have lived at the appropriate time and be statistically close to This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the ancestral morphology, taking into account the uncertainty of the 1073/pnas.1302653110/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1302653110 PNAS Early Edition | 1of6 Downloaded by guest on September 30, 2021 and Discussion). We also considered two dates for , 1 Ma or 500 ka, to account for the different age estimates of key Javanese (16) and Chinese fossils (17). We included H. erectus as a potential candidate species for the [N-MH]LCA in some phylogenetic scenarios. For each candidate, we evaluated its compatibility with the statistical reconstruction of the morphol- ogy of the LCA under each of the 12 phylogenetic scenarios. Species were considered implausible as ancestors if one or more of their teeth was incompatible with the expected ancestral morphology under all 12 of the phylogenetic scenarios. Results The statistical reconstruction of the dental morphology of the [N-MH]LCA is characterized by symmetric and lingually re- duced upper (P3 and P4) and by slightly asymmetric 1 lower premolars (P3 and P4). The upper first (M ) has a slightly skewed morphology and the lower first molar (M1), a Dryopithecus pattern (also called “Y5”)withfive - developed main cusps. The Dryopithecus pattern is the shared ancestral condition for all hominin lower molars, and consists of a five-cusped molar in which the mesiolingual (metaconid) and the distobuccal cusp (hypoconid) are in contact and create a Y-shape fissure pattern. The upper second molar (M2) of the reconstructed common ancestor has a mildly reduced hypocone, Fig. 1. Phylogeny used for ancestral state reconstruction and candidate whereas the upper third molar (M3) has strong reduction such species. Numbers in parentheses represent the averaged chronology of the that a high proportion of individuals are expected to lack the corresponding species (although see ref. 56); italic numbers represent branch distolingual cusp. Lower second molars (M2) of the recon- lengths in millions of ; bold gray numbers represent ages of nodes. structed [N-MH]LCA are expected to have marked reduction of Branch lengths corresponding to a 1-Ma divergence between Neanderthals the fifth cusp and frequent occurrence of non-Y5 morphologies and modern humans are represented to the left of the slashes and values (mainly cruciform patterns). The lower third molar (M3)of corresponding to a 0.45-Ma divergence to the right. H. erectus has been the expected ancestral phenotype has five cusps that are not dated to 1 Ma (values before the slash in the H. erectus branch) or to 0.5 Ma arranged in a Y pattern, and a reduction of the lingual half of the (values after the slash). Candidate species are represented below the phy- logenetic tree with bars showing their approximate temporal spans (black, molar relative to its buccal half (Fig. 2). This reconstruction of candidates for the [P-H]LCA; dark gray, candidates for the [H]LCA; light gray, the dental morphology of the [N-MH]LCA is generally consis- candidates for the [N-MH]LCA). H. erectus and the SH group have been in- tent with all 12 phylogenies, one of which is shown in Fig. 2 (the cluded in the framework phylogeny in four (H. erectus) and six (SH) phylo- one that includes the SH group as a pre-Neanderthal population, genetic scenarios (Fig. S1) and evaluated as candidate species in the that uses the earlier date of 1 Ma for the [N-MH]LCA, and remaining scenarios. H. ergaster has been evaluated as a candidate species which dates H. erectus to 1 Ma; Fig. 3 and Figs. S4 and S5 are for the [H]LCA and the [N-MH]LCA. H. habilis has been evaluated as a can- based on the same phylogeny). didate for the [P-H]LCA and the [H]LCA. Vertical guides indicate the age of Our results reveal that no known species of the relevant age the evaluated nodes. Fig. S1 shows individualized representations of the 12 is statistically consistent with the expected morphology of the phylogenetic topologies. [N-MH]LCA, even given the considerable uncertainty of the reconstructed ancestral morphologies. Graphical reconstructions of the nodes in dental morphospace are shown in Fig. 3 and Fig. a phylogenetic reconstruction technique, but a means to evaluate S4, along with confidence ellipsoids that show the associated compatibility of potential ancestral species within the framework uncertainties that arise from the stochastic nature of the evolu- of established phylogenies. tionary process. The size of each confidence ellipsoid is directly Phylogenetic scenarios differed from one another in the related to the lengths of the branches connected to the node. At number and age of tip species and in the age of the nodes least two postcanine teeth fall outside the 95% confidence (branch lengths; Fig. 1 and Fig. S1). European Middle Pleisto- intervals of the [N-MH]LCA reconstruction for all candidate cene fossils were grouped into one of two operational taxonomic species under all 12 phylogenetic scenarios (Table 1). Further- units based on previously recognized morphological differences more, taxa such as H. heidelbergensis have several postcanine (9): the group (represented in our study teeth that are morphologically consistent with the [N-MH]LCA by fossils from Arago ) and the Sima de los Huesos (SH) only in one or two phylogenetic scenarios (Table S3). The ad- group. SH hominins, unlike other European Middle Pleistocene dition of other European fossils to the H. heidelbergensis sample, fossils (such as those from Arago, Mauer, and Petralona), are such as those from Mauer and Petralona, could alter this result, widely considered to be an early chronospecies of classic Nean- but this is unlikely because they are similar (not only in dental, derthals (3, 9–11). However, the markedly derived dentitions of but also in cranial and mandibular morphology) to the Arago SH hominins (12) appear to contradict their placement as an specimens relative to other taxa in our analysis (2, 3, 9, 18). Our early member of the Neanderthal lineage and their morphology results demonstrate that morphological evidence for any of these substantially influences the reconstruction of the [N-MH]LCA; taxa being the [N-MH]LCA is, at best, weak. For these species to therefore, we evaluated SH both as a taxon on the phylogenetic be credible ancestors, the age of the Neanderthal–modern hu- tree and as a potential candidate for the [N-MH]LCA. For the man split would have to be radically different from the range we former, SH was considered to be a node in the Neanderthal considered or selection on the dentition would have to have lineage at 350 ka following ref. 13, an age that is less contro- caused the ancestor to radically deviate from the expectations of versial than a subsequent, older assessment of circa 600 ka (14). Brownian motion evolution. In addition to ruling out these We considered two alternative estimates for the divergence time candidate species, our results indicate that all three European between Neanderthals and modern humans (1, 11). One scenario candidate groups are statistically closer to the Neanderthal lin- dates this divergence to 450 ka based on molecular clock esti- eage than to the modern human lineage (Table 2 and Table S4), mates (11, 15); the other dates this divergence to 1 Ma based on which suggests that European hominins have had significant morphological affinities observed in the fossil record (see Results dental affinities with Neanderthals for almost 1 My (the oldest

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1302653110 Gómez-Robles et al. Downloaded by guest on September 30, 2021 show that the fit between candidates and their corresponding nodes is substantially better at the older nodes than at the [N- MH]LCA (Table 1). The main exception is the SH group, which has some teeth that are comparatively close to the [N-MH]LCA in terms of Procrustes distances, but which is incompatible as an ancestor because some of its teeth are strongly derived and have a very low probability of conforming to the expected ancestral morphology. Discussion Our results demonstrate that, even though there are plausible candidate species for deep nodes of the hominin phylogeny (A. afarensis, A. africanus, and H. habilis for the [P-H]LCA and H. habilis and H. ergaster for the [H]LCA), we still do not have a good fossil candidate for the phenotypic LCA of Neanderthals and modern humans. This result is striking because confidence intervals of ancestral reconstructions are very broad, which means that existing candidates are very incompatible. Species

A EVOLUTION

Fig. 2. Dental morphology of the [N-MH]LCA. Estimated ancestral mor- phologies 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.

B species, , has been dated to ca. 950 ka; ref. 19). African and Asian H. erectus do not share this European affinity, but they have nearly equal dental affinities with Neanderthals and with modern humans (Table 2 and Table S4). H. ergaster was evaluated as a [N-MH]LCA candidate be- cause it is the youngest African species in our sample, despite it being considerably older than this ancestor is expected to be. Interestingly, H. ergaster fits the expected ancestral phenotype better than some of the younger candidate species (Table 1). Asian H. erectus was also evaluated as an ancestral candidate because the fossil specimens included in our study lived during the right time interval (0.5–1 Ma), but it has generally a worse fit with the [N-MH]LCA than H. ergaster. Our method identified compatible candidates for other nodes in the phylogeny (Fig. S5). Our data show that H. ergaster and, especially, H. habilis fit the expected morphology of the LCA of the genus Homo ([H]LCA) at all tooth positions in one or more Fig. 3. Projection of the simplified hominin (represented of the 12 phylogenies (Table 1 and Table S3). Similarly, both by lines connecting terminal species and nodes) with terminal species (large afarensis and Australopithecus africanus fit the dark-gray filled circles), ancestor shapes (small light-gray filled circles) and expected morphology of the LCA of Paranthropus and Homo ([P- candidate species (open circles) into the upper and lower first molar mor- H]LCA) at all tooth positions in most or all of the phylogenetic phospaces. Confidence ellipses (95%) around the three nodes discussed in scenarios (Table 1 and Table S3). Despite being age-in- the text are represented with the three ancestors located in their centers appropriate, we also evaluated H. habilis as a candidate for the (solid line, confidence ellipse of the [N-MH]LCA; dashed line, confidence el- fi lipse of the [H]LCA; dotted line, confidence ellipse of the [P-H]LCA). (A) [P-H]LCA because of its suggested af nities with Australopithecus fi fi species (20). Interestingly, it has even stronger compatibility with Upper rst molar morphospace. (B) Lower rst molar morphospace. Plots corresponding to premolars and second and third molars are provided in Fig. the expected morphology of this ancestor than do A. afarensis and S4. Note that confidence ellipses in these graphs correspond only to the first A. africanus (Table 1 and Table S3). The fact that several can- and second principal components (PCs), whereas P values in Table 1 are based didates are compatible with deeper nodes is not surprising be- on all of the PCs. Phylogeny: BOI, P. boisei; ERE, H. erectus; NEA, H. nean- cause the long branch lengths leading to older nodes result in derthalensis; ROB, ; SAP, H. sapiens; SH, SH group. broader confidence ellipses than at shallower nodes with shorter Candidates: AFA, A. afarensis; AFR, A. africanus; ANT, H. antecessor; ERG, H. branches (21). Regardless of uncertainty, Procrustes distances ergaster; GEO, Homo georgicus; HAB, H. habilis; HEI, H. heidelbergensis.

Gómez-Robles et al. PNAS Early Edition | 3of6 Downloaded by guest on September 30, 2021 Table 1. Procrustes distances between expected ancestral morphologies and candidate species † Procrustes distance* (P value)

3 4 1 2 3 P P M M M P3 P4 M1 M2 M3

[N-MH]LCA SH group 0.045 0.027 0.024 0.034 0.049 0.051 0.042 0.023 0.031 0.044 (<0.001) (0.072) (0.373) (0.710) (0.392) (0.042) (0.103) (0.432) (0.169) (0.074) H. heidelbergensis 0.046 0.061 0.052 0.072 0.059 0.057 0.036 0.025 0.034 0.042 (0.785) (0.147) (0.006) (0.001) (0.255) (0.310) (0.487) (0.991) (0.194) (0.980) H. antecessor 0.038 0.037 0.051 0.058 N/A 0.081 0.046 0.030 0.048 0.041 (0.234) (0.280) (0.001) (0.001) (0.036) (0.259) (0.934) (0.016) (0.360) H. erectus 0.031 0.023 0.028 0.053 0.067 0.084 0.043 0.029 0.038 0.038 (0.999) (0.805) (0.867) (0.015) (0.030) (0.039) (0.622) (0.733) (0.628) (0.678) H. ergaster 0.051 0.044 0.053 0.077 N/A 0.061 0.032 0.038 0.034 0.042 (0.206) (0.920) (0.013) (0.001) (0.250) (0.608) (0.913) (0.670) (0.811) [H]LCA H. georgicus 0.061 0.028 0.050 0.049 0.091 0.059 0.030 0.033 0.036 0.050 (0.456) (0.580) (0.918) (0.307) (<0.001) (0.009) (0.988) (0.983) (0.955) (0.583) H. ergaster 0.044 0.040 0.050 0.060 N/A 0.057 0.035 0.035 0.034 0.037 (0.394) (0.995) (0.060) (0.170) (0.256) (0.694) (0.894) (0.765) (0.805) H. habilis 0.041 0.037 0.026 0.032 0.039 0.036 0.046 0.021 0.030 0.034 (0.627) (0.596) (0.868) (0.300) (0.602) (0.813) (0.497) (0.965) (0.919) (0.971) [P-H]LCA H. habilis 0.028 0.027 0.024 0.029 0.023 0.046 0.040 0.022 0.031 0.037 (0.847) (0.944) (0.948) (0.751) (0.953) (0.569) (0.724) (0.990) (0.965) (0.947) A. africanus 0.031 0.029 0.029 0.031 0.056 0.043 0.026 0.025 0.035 0.043 (0.970) (0.647) (0.732) (0.934) (0.410) (0.932) (0.992) (0.925) (0.745) (0.573) A. afarensis 0.041 0.033 0.030 0.055 0.049 0.055 0.031 0.037 0.035 0.037 (0.962) (0.868) (0.688) (0.282) (0.509) (0.951) (0.960) (0.664) (0.729) (0.642)

Bold represents P < 0.05 in all phylogenetic scenarios (candidate species is incompatible with ancestry in the specified tooth). N/A, not applicable (no M3 of H. antecessor and H. ergaster are included in the sample). *Mean Procrustes distance obtained in the 12 evaluated phylogenetic scenarios. † Highest P value observed in the 12 different phylogenetic scenarios. P values represent the probability that a fossil species falls within the expected distribution of an ancestral node under the assumption of a Brownian motion model of evolution.

falling outside the confidence intervals for a given node can be 1 Ma suggests three possible scenarios (covered in the next three excluded from ancestry with high confidence presuming the sections), which are not necessarily mutually exclusive. model of Brownian motion evolution is applicable. The use of several different phylogenetic topologies in our study makes the Neanderthals and the [N-MH]LCA Could Be Primitive in Their Dental rejection of these candidates even stronger. Morphology. One possible explanation of our results is that the Even though none of the candidate species we evaluated were constellation of characters shared by European Pleistocene fos- compatible with the [N-MH]LCA, note that our study did not sils represents the primitive condition for the clade Homo include African populations of H. heidelbergensis or African neanderthalensis–H. sapiens and that the true ancestral pheno- specimens dated to ca. 1 Ma. These groups deserve scrutiny as was closer to the Neanderthal condition than expected potential ancestors, especially because they lived at the right under a Brownian motion model. This scenario would require time and their cranial morphological affinities are ambiguous (2, relative stasis in the evolution of dental morphology in the Ne- 18, 22–24). Middle Pleistocene fossils from (sometimes anderthal lineage and either neutral or directional evolution in referred to as ) have been considered by some the H. sapiens lineage. If this were the case, our ancestral re- as part of the lineage leading to H. sapiens (22) and by others construction method would fail to recover the correct mor- as belonging to the ancestral species of Neanderthals and phology because it assumes similar, nondirectional evolutionary modern humans (3, 18, 23, 24). African fossils dated to ca.0.5–1 changes in the lineages stemming from a node and subtending it. Ma thus merit continued study and are currently the most prom- We have tested and rejected this possibility elsewhere (25), and ising source of candidates for the [N-MH]LCA. so consider this scenario unlikely. Whereas some authors have The lack of a plausible [N-MH]LCA candidate among Euro- suggested that Neanderthals are morphologically more primitive pean groups and their dental affinity with Neanderthals back to than modern humans (26), quantitative analysis of evolutionary

Table 2. Morphological affinities of candidate species with Neanderthals and with modern humans 3 4 1 2 3 Candidate P P M M M P3 P4 M1 M2 M3

SH group NEA NEA NEA — SAP NEA SAP NEA NEA NEA H. heidelbergensis SAP NEA — NEA NEA SAP NEA NEA NEA NEA H. antecessor NEA NEA NEA NEA N/A SAP NEA SAP NEA NEA H. erectus SAP NEA SAP NEA NEA SAP NEA SAP SAP SAP H. ergaster SAP NEA SAP NEA N/A SAP SAP SAP NEA NEA

Affinities based on Procrustes distances (Table S4). N/A, not applicable; NEA, higher affinity with Neanderthals; SAP, higher affinity with modern humans; —, higher but not significant affinity with Neanderthals.

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1302653110 Gómez-Robles et al. Downloaded by guest on September 30, 2021 patterns in these lineages are consistent with a Brownian process New paleontological, molecular, and quantitative phenotypic stud- of nonadaptive, neutral evolution in both dental (25) and cranial ies are needed to elucidate the exact nature of these factors. features (27). Moreover, detailed, comprehensive evaluations of postcanine dental morphology have revealed that several dental Conclusions traits shared by European late Early and Middle Pleistocene Our study presents a statistical framework for addressing ques- populations and classic Neanderthals are, in fact, derived (28–31). tions about ancestry and the relationships of particular fossils The suite of dental traits shared by European hominins sug- that can serve as a model for making paleoanthropological hy- gests that a separate European clade with Neanderthal affinities potheses more testable. The quantitative approach we propose existed since at least 1 Ma. This does not necessarily imply offers an objective means to test hypotheses that capitalize on a continuity between all European Pleistocene populations the comparatively rich dental fossil record and address appar- during the last 1 My (32), but it points to all these taxa being part ently unresolvable debates about hominin phylogeny. The same of a Neanderthal phenotypic clade separate from the lineage approachcanbeappliedtootherskeletal parts via 2D and 3D leading to modern humans. It has been suggested that this Eu- quantifications of morphology, and it can be extended to in- ropean group could have been composed of successive migrant clude models of evolution other than Brownian motion pro- populations who shared their common phenotypes via gene flow cesses should there be evidence that they are appropriate. Our (ref. 33, see also refs. 15 and 34), resulting in a lack of linearity in approach thus provides a promising and extensible statistical the pattern of acquisition of Neanderthal traits (33). It is worth context for evaluating fossil remains of uncertain phylogenetic noting that extensive hybridization would hamper ancestral re- position. construction because the expected morphological differences would be attenuated by gene flow between diverging populations. Materials and Methods Materials. Our study analyzed the morphology of 1,200 hominin teeth rep- Molecular Divergence Times May Be Underestimated. The paleon- resenting their 10 postcanine tooth positions (upper and lower premolars and tological estimate of the divergence time between Neanderthals molars) from 13 different species/morphotypes (Tables S1 and S2). We chose and modern humans at ca. 1 Ma is substantially older than most to split controversial named taxa for which all teeth are represented in our molecular estimates, which set the divergence at less than 500 ka samples to evaluate their ancestral status independently on their own (11, 15). The divergence between Neanderthals and modern merits, but we lumped taxa when their dentitions were incompletely rep- humans has also been dated to 300–400 ka using neutrally resented so as to provide more statistical power (e.g., Paranthropus aethio- evolving cranial measurements (35), an estimate that is highly picus was included in Paranthropus boisei and H. rudolfensis in H. habilis). The dependent on assumptions and parameters that are not known only exceptions were H. ergaster and H. antecessor, which were retained as EVOLUTION 3 without error, but which is broadly consistent with the youngest separate candidate species despite their lack of M due to their strong can- molecular clock estimates. In light of the young molecular and didacy as [N-MH]LCA. Although other taxonomic arrangements are plausible cranial dates, several analyses have dismissed the early di- (e.g., merging H. erectus, H. ergaster, and Dmanisi fossils under H. erectus vergence model of Neanderthal origins (1, 3, 11). However, lato), the purpose of our study is not to address taxonomic questions, a late divergence model has been recently challenged on the but to evaluate the plausibility that previously recognized taxa occupy basis of slower mutation rates that result in earlier split times in ancestral positions. In this regard, the most informative approach is a de- – tailed evaluation of morphologically, geographically, and chronologically ape and (refs. 36 38 but see also refs. 39 and cohesive groups. 40). Attempts to reconcile paleontological and molecular dates have to face the fact that alternative molecular estimates for the fi fi Geometric Morphometrics. Dental morphology was quanti ed using 2D timing of the [N-MH]LCA vary considerably, with con dence landmarks and sliding semilandmarks (45) digitized from photographs of the intervals ranging from 300 to 850 ka (11, 37). Our results are occlusal surface of premolars and molars. Depending on the tooth, 4–8 however incongruous with even the lowest boundary of molecular landmarks and 30–40 semilandmarks were used (Fig. S2 and S3). Coordinates confidence intervals, and thus lend support to the early divergence were digitized using tpsDig2 software (46). Orientation of dental specimens model and suggest that molecular estimates of divergence time was standardized such that the occlusal surface was parallel to the lens of can be underestimated. the camera. Unworn and moderately worn teeth were included in our evaluation, whereas severely worn teeth were excluded. A generalized Phenotypic Divergence and Speciation Could Be Decoupled. A third Procrustes analysis (47) and a principal components analysis (48) were carried possible alternative that can conciliate molecular and paleonto- out using the mean configurations of the species (Dataset S1) to provide logical observations is that phenotypic divergence in dental traits shape variables. Ancestral state calculations were carried out on these var- between Neanderthals and modern humans significantly pre- iables in the principal components space (Fig. 3 and Fig. S4) using all prin- dated the completion of speciation. This scenario could explain cipal components (5). why some European Pleistocene populations appear to be derived in dental traits but still show primitive cranial and postcranial Ancestral State Reconstruction. The GLM method for estimating ancestral features. Such mosaicism is particularly evident in H. antecessor’s traits (6, 49, 50) was used, implemented in Mathematica by one of us (51). combination of some derived Neanderthal dental features (29– As is standard practice (52), our implementation of ancestral state recon- 31) and generally plesiomorphic nondental traits (at least for the struction assumes that the traits are evolving under a Brownian motion clade H. neanderthalensis-H. sapiens; refs. 41–43), and in SH’s model of the sort that results either from genetic drift or from selection cranial morphology, which is transitional to the Neanderthal form whose direction and magnitude varies randomly through time and across (10), combined with its strongly derived dental phenotype (12). space. Our previous study indicated that hominin dental evolution is con- sistent with a Brownian motion model, although considerable stabilizing It is most likely that the large discrepancy between molecular selection has been identified in first molars and slight directional trends in dates and phenotypic differentiation results from the wide con- fi lower third premolars and upper third molars (25). Estimates of ancestral dence intervals of the molecular estimates and from the un- states are not badly affected by stabilizing processes, only by long-term certainty associated with dating fossil materials. Regardless, our unidirectional selection that occurs in parallel in different lineages (21, 53). results indicate that dental morphology developed derived fea- Ancestral node reconstruction identifies the single most likely ancestral tures as early as almost 1 Ma in the Neanderthalization process condition and a multivariate confidence ellipsoid (95%) containing other (12), raising interesting questions about which evolutionary plausible but less probable ancestral phenotypes (6, 50, 54). The likelihood forces may have caused this early dental differentiation with distribution of the ellipsoids was used to calculate the probability that fossils of respect to other traits and over what time interval occurred. The appropriate age could feasibly occupy those nodes (SI Text). evolutionary mechanisms behind this process are likely to consist The reconstructed ancestral shape variables were converted back into of a mosaic of random factors (such as genetic drift) and non- landmark configurations (55), after which the Procrustes distances of can- random factors (such as lineage-specific patterns of environ- didate species to the expected ancestor were calculated. Procrustes distance mental selection and rates of evolution of different traits; ref. 44). is a measure of shape difference between superimposed conformations of

Gómez-Robles et al. PNAS Early Edition | 5of6 Downloaded by guest on September 30, 2021 landmarks calculated as the square root of the sum of the squared differences three anonymous reviewers for their help in improving our manuscript. Ac- between the positions of homologous landmarks. The species that falls cess to fossil specimens was made possible by O. Kullmer, B. Denkel, and within the confidence ellipsoid and has the smallest Procrustes distance to F. Schrenk (Senckenberg Institute); D. Lordkipanidze, A. Vekua, and G. Kiladze the hypothetical ancestor can be considered the best candidate to occupy (Georgian National Museum); H. de Lumley, M. A. de Lumley, and A. Vialet (Institut de Paléontologie Humaine); P. Mennecier (Musée de l’Homme); the ancestral position. Procrustes distance was also used to calculate the fi I. Tattersall, G. Sawyer, and G. García (American Museum of Natural History); af nity of candidate species with Neanderthals and modern humans taking L. Jellema and J. Haile-Selassie (Cleveland Museum of Natural History); and into account the phylogenetic topology (SI Text). M. Botella (Laboratory of Physical Anthropology, University of Granada). This work was partially supported by funding by the Spanish Ministry of Science and ACKNOWLEDGMENTS. We thank Emília Martins, James Rohlf, Michelle Lawing, Innovation (Project CGL2009-12703-C03-01-02-03). Fieldwork at Atapuerca is Bernard Wood, Asier Gómez-Olivencia, and David Sánchez-Martín for dis- supported by the Consejería de Cultura y Turismo (Junta de Castilla y León) cussion on several aspects of the manuscript. We also thank the editor and and the Fundación Atapuerca.

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