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Fifty years after the recognition of the species Homo habilis as the earliest known representative of our genus (1), the origin of Homo remains clouded. This uncertainty stems in large part from a limited fossil record between 2.0 and 3.0 Ma, especially in eastern Africa. Some taxa from this time period, such as Australopithecus africanus (~2.8-2.3 Ma) and the less well known A. garhi (~2.5 Ma) and A. aethiopi-cus (~2.7-2.3 Ma), appear too specialized cranially and/or dentally to represent the probable proximate ancestral con-ditions for Homo species known in Africa by ~2.0 Ma (H. habilis and H. rudolfensis). This leaves a thin scatter of iso-lated, variably informative specimens dated to 2.4-2.3 Ma as the only credible fossil evidence bearing on the earliest known populations of the genus Homo (2, 3).

Here we describe a recently recovered partial hominin mandible, LD 350-1, from the Ledi-Geraru research area, Afar Regional State, Ethiopia, that extends the fossil record of Homo back in time a further 0.4 myr. The specimen, se-curely dated to 2.80-2.75 Ma, combines derived morphology observed in later Homo with primitive traits seen in early Australopithecus. The discovery has implications for hy-potheses concerning the timing and place of Homo origins.

The LD 350 locality resides in the Lee Adoyta region of the Ledi-Geraru research area (Fig. 1). Geologic research at Lee Adoyta (4) identified fault-bounded sedimentary pack-ages dated 2.89 Ma to 2.58 Ma. The LD 350-1 mandible was recovered on the surface of finely bedded fossiliferous silts

10 m conformably above the Gurumaha Tuff (Fig. 1). The ma-trix adherent to the specimen is consistent with it having eroded from these silts [for details on stratigraphy and depositional environment, see (4)]. The Gurumaha Tuff is radiometrical-ly dated to 2.842 Ma ± 0.007 Ma (4), a date that is consistent with the normal magnetic polarity of the Gurumaha section, presum-ably the Gauss Chron. An upper bounding age for LD 350-1 is provided by an adjacent, downfaulted younger block that contains the 2.665 ± 0.016 Ma Lee Adoyta Tuff. A magnetostrat-igraphic reversal 12 m conform-ably above the Lee Adoyta Tuff is inferred to be the Gauss/Matuyama boundary at 2.58 Ma (4). As no significant erosional events intervene be-tween the Gurumaha Tuff and the fossiliferous horizon, the age of LD 350-1 can be further con-strained by stratigraphic scaling.

Applying a sedimentation rate of either 14 cm/kyr from the Lee Adoyta fault block or 30 cm/kyr from the Hadar For-mation (5) provides age estimates of 2.77 Ma and 2.80 Ma, respectively, for LD 350-1. Based on the current chronostrat-igraphic framework for Ledi-Geraru, we consider the age of LD 350-1 to be 2.80-2.75 Ma.

The hominin specimen, found by Chalachew Seyoum on January 29, 2013, comprises the left side of an adult man-dibular corpus that preserves the partial or complete crowns and roots of the canine, both premolars, and all three mo-lars. The corpus is well preserved from the symphysis to the root of the ascending ramus and retromolar platform. Sur-face detail is very good to excellent and there is no evidence of significant transport. The inferior margin of the corpus and the lingual alveolar margin are intact, but the buccal alveolar margin is chipped between P3 and M1. The P4, M2, and M3 crowns are complete and well preserved, but the C, P3, and M1 crowns are incomplete (Fig. 2 and text S2). The anterior dentition is represented by the broken root of the lateral incisor and the alveolus of the central incisor.

Given its location and age, it is natural to ask if the LD 350-1 mandible represents a late-surviving population of A. afarensis, whose youngest known samples date to ~3.0 Ma at the neighboring Hadar site (6). Indeed, in overall dental and mandibular size LD 350-1 matches smaller specimens of A. afarensis (figs. S1 to S4 and tables S1 to S3). In addition, LD 350-1 shares with A. afarensis a primitive anterior cor-

Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia Brian Villmoare,1,4,6* William H. Kimbel,2* Chalachew Seyoum,2,7 Christopher J. Campisano,2 Erin DiMaggio,3 John Rowan,2 David R. Braun,4 J. Ramon Arrowsmith,5 Kaye E. Reed2 1Department of Anthropology, University of Nevada Las Vegas, Las Vegas, NV 89154, USA. 2Institute of Human Origins and School of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287, USA. 3Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA. 4Center for the Advanced Study of Hominin Paleobiology, George Washington University, Washington, DC 20052, USA. 5School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA. 6Department of Anthropology, University College London, London WC1H 0BW, UK. 7Authority for Research and Conservation of Cultural Heritage, Addis Ababa, Ethiopia.

*Corresponding author. E-mail: brian.villmoare@unlv.edu (B.V.); wkimbel.iho@asu.edu (W.H.K.)

Our understanding of the origin of the genus Homo has been hampered by a limited fossil record in eastern Africa between 2.0 and 3.0 million years ago (Ma). Here we report the discovery of a partial hominin mandible with teeth from the Ledi-Geraru research area, Afar Regional State, Ethiopia, that establishes the presence of Homo at 2.80-2.75 Ma. This specimen combines primitive traits seen in early Australopithecus with derived morphology observed in later Homo, confirming that dentognathic departures from the australopith pattern occurred early in the Homo lineage. The Ledi-Geraru discovery has implications for hypotheses about the timing and place of the origin of the genus Homo.

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pus, including an inclined symphyseal cross section, a bulb-ous anterior symphyseal face, and a projecting inferior transverse torus that is only slightly elevated above the cor-pus base (Fig. 2A). There are, however, substantial differ-ences between LD 350-1 and the mandibles and teeth of A. afarensis. LD 350-1 lacks the lateral corpus hollow in which the mental foramen is located, which distinguishes A. afarensis mandibles across their size range. Instead, the LD 350-1 corpus is slightly convex inferosuperiorly, with the mental foramen located lateralmost on this arc (fig. S5). Un-like in A. afarensis, the anterior corpus of LD 350-1 bears a distinct symphyseal keel, accentuated by a flattened area between it and a pronounced canine jugum (Fig. 2). It lacks both an incisura mandibulae anterior and a mentum osse-um, and thus does not have a chin. The P3 buccal crown pro-file is symmetrical in occlusal view, lacking the prominent mesiobuccal extension that skews the P3 crown outline in A. afarensis. The P3 occlusal enamel is worn through to den-tine buccally, yet the moderately worn molars retain consid-erable occlusal relief. In A. afarensis a reversed wear pattern is typical: the canine and mesial moiety of P3 remain rela-tively unscathed by wear even when molar occlusal enamel is exhausted (6). The wear disparity in LD 350-1 may signal modification of the relatively primitive C/P3 complex of A. afarensis, which, while non-honing, retained apelike aspects of crown shape and occlusal wear (6, 7). The basally expand-ed hypoconid that creates the characteristic “bilobate” buc-cal crown contour in A. afarensis M1s and M2s is not evident in LD 350-1 (7). Finally, there is a distinct C7 cusp on the LD 350-1 M1 (Fig. 2), yet no such a cusp has been recorded for a M1 of A. afarensis (n = 18) (8, 9).

Despite a well developed lateral prominence that swells the corpus anterior to the ascending ramus, LD 350-1 lacks the great corpus thickening, inflated corpus contours be-neath P3-M1, and anterior dental arch constriction seen in robust australopith mandibles, including Omo 18-1967-18 (holotype of A. aethiopicus, ~2.6 Ma). Postcanine dental ex-pansion, including differential enlargement of lower molar entoconids typical of robust australopiths, is also absent (8) (figs. S1-S3; table S4).

LD 350-1 also differs from Australopithecus in features that align it with early Homo. The inferior and alveolar margins of the corpus are sub-parallel, resulting in similar corpus depths at P3 and M2 (Fig. 2), whereas in A. afaren-sis, A. africanus and A. sediba the corpus is typically deepest under the premolars (10, 11). Some specimens of early Ho-mo, such as SK 45 and KNM-ER 60000 (10, 12), have a deep-er corpus anteriorly, but LD 350-1 resembles many others that are relatively uniform in depth along the corpus (10, 13–16) (figs. S5 and S6 and table S5). In LD 350-1 the mental foramen opens directly posteriorly into a short groove on the corpus. A posteriorly opening foramen is the most common condition in early and modern Homo (13, 17). The foramen always opens anteriorly or anterosuperiorly in A. afarensis mandibles, whereas in A. africanus its orientation

ranges from anteroinferior to directly lateral. The anterior margin of the ascending ramus in LD 350-1 becomes inde-pendent from the corpus opposite the middle of the M3 crown, and in lateral view only the distalmost part of this crown is hidden (Fig. 2 and fig. S5). In the great majority of Australopithecus mandibles the anterior margin of the ra-mus is positioned further anteriorly, between mid- and dis-tal M2. A posterior origin of the ramus margin is most common in specimens of early Homo (fig. S7 and table S6).

The LD 350-1 molars have simple occlusal surfaces, with salient, marginally positioned cusp apices, and broad occlu-sal basins. With the exception of a tiny protostylid on M3, the crowns are devoid of cingular remnants. Estimated M1 length is small (fig. S4). The breadth across the mesial as-pect of M2 and M3 measures less than the distal breadth, giving these crowns a mesially tapered occlusal outline. This distinctive outline is due in part to a relatively confined me-siolingual cusp (metaconid) on these teeth (Fig. 2 and table S4). In A. afarensis and A. africanus, and also in A. sediba specimen MH1, M2 and M3 crown outlines are square or dis-tally tapered (especially M3) (fig. S8). Although some early Homo distal molars show reduced mesial breadths, there is significant overlap with Australopithecus in mesial:distal breadth ratio, and none approaches the uniquely tapered outline of LD 350-1 (table S7). Whereas Australopithecus M2 and M3 crowns usually have a sloping buccal face up to the point of advanced occlusal wear, the buccal walls of these teeth in LD 350-1 descend almost vertically from the occlu-sal rim, as frequently observed in early and later Homo (fig. S8). LD 350-1 departs from the A. africanus dental pattern, which includes buccolingually expanded postcanine teeth (especially M2-3) and flattened molar occlusal surfaces in specimens with similar degrees of premolar wear (6, 7). LD 350-1 possesses a mesiodistally shorter M3 than M2, which is common in Homo but occurs only in 1/16 A. afarensis and 3/14 A. africanus specimens in our sample. In crown length, the LD 350-1 M3 falls outside the distribution of 20 A. afri-canus and 19 A. afarensis M3s (fig. S3).

With an age of 2.80-2.75 Ma, the Ledi-Geraru mandible is younger than the youngest known A. afarensis fossils from Hadar, ca. 3.0 Ma. Although it shares with A. afarensis primitive anterior corpus morphology, the specimen falls outside the range of variation for most other diagnostic mandibular and dental traits of this species. In the majority of traits that distinguish it from this species, LD 350-1 pre-sents morphology that we interpret as transitional between Australopithecus and Homo.

The Homo postcanine dental pattern (18) is present in Members E through G of the Shungura Formation of Ethio-pia (~2.4-2.0 Ma) and is best represented by mandible Omo 75-14, which has a reduced M3 crown and a C7 cusp on M1 (18), features shared with LD 350-1. The A.L. 666-1 maxilla from the Busidima Formation at Hadar also establishes the Homo dentognathic pattern by ~2.3 Ma (19). A small collec-tion of postcanine teeth from Members B and C (~2.9-2.6

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Ma) of the Shungura Formation evinces more derived mor-phology than A. afarensis and has been affiliated with A. africanus/Homo (18, 20). Although A. africanus is usually thought to have been endemic to southern Africa, the corre-sponding time period in the east African record is notori-ously poor in hominin fossils. At ca. 2.80-2.75 Ma, LD 350-1 falls within the Member B-C temporal interval and is broad-ly contemporaneous with A. africanus samples from Maka-pansgat and Sterkfontein (21). Morphologically, however, it is distinguished from A. africanus by its subequal anterior and posterior corpus heights, posterior position of the ante-rior ramus margin, posteriorly oriented mental foramen, steeper premolar-molar wear gradient, vertical buccal walls of M2-3, and reduced M3. In the combination of these fea-tures, LD 350-1 resembles younger east African Homo spec-imens more than it does geologically contemporaneous or older Australopithecus. Summarizing the Shungura For-mation dental evidence, Suwa et al. [(18) p. 270-271] identi-fied the 2.9-2.7 Ma interval as “the transitional period when evolution occurred from an A. afarensis-like to a more ad-vanced species…close in overall dental morphology to the A. africanus condition but also mostly within the A. afarensis and/or early Homo ranges of variation.” We consider the Ledi-Geraru mandible to sample a population from this transition and to point to a close phyletic relationship with Homo at 2.4-2.3 Ma.

A set of teeth comprising the isolated left and right P3-M2 crowns of a single individual (KNM-ER 5431) from the up-per Tulu Bor Member at Koobi Fora, Kenya (22), is germane to the status of LD 350-1. Constrained in age between ~2.7 Ma and ~3.0 Ma (23), these teeth show a mix of australopith (premolar cusp morphology) and early Homo-like (C7 cusp on M1 and M2) characters (15), while its P3 form has been characterized as “intermediate between A. afarensis…and A. africanus or early Homo” [(24), p. 199]. The KNM-ER 5431 teeth could represent the same east African transitional form as the LD 350-1 mandible.

Arguments for the role of 2.5 Ma A. garhi as the direct ancestor of Homo are relevant to interpretations of LD 350-1 (25). The tremendous postcanine dental size and differen-tially enlarged P3 of A. garhi holotype cranium BOU-VP 12/130 are a poor match for the modest-size teeth of the Ledi jaw (in resampling analyses, the LD 350-1 lower second molar and the BOU-VP 12/130 upper second molar were found to be diminishingly unlikely to sample the same spe-cies; see text S3). To include LD 350-1 and BOU-VP 12/130 in the same species-lineage and simultaneously postulate an ancestor-descendant relationship between A. garhi and Homo would require a dramatic increase in tooth size be-tween ~2.8 and ~2.5 Ma, and then an equally dramatic de-crease in tooth size in a ~2.5-2.3 Ma transition to Homo. Although non-robust mandibles with derived corpus mor-phology and smaller teeth are approximately contempora-neous with A. garhi in the Middle Awash of Ethiopia (25, 26), it is unclear whether these represent A. garhi or a se-

cond lineage, potentially of Homo. For all of these reasons, we consider the hypothesis that posits LD 350-1 as repre-senting a population ancestral to ~2.4-2.3 Ma Homo, to the exclusion of A. garhi, to be more parsimonious than ones that include this taxon in the Homo lineage [consistent with the phylogenetic analysis in (27); see text S3].

By ~2.0 Ma at least two species of the genus Homo were present in Africa, H. habilis and H. rudolfensis (3, 28), but primitive anterior corpus morphology distinguishes LD 350-1 from both of them. These species are distinct from one another in dental arcade shape (12, 29), and LD 350-1 sug-gests the primitively arched canine/incisor row seen in H. habilis sensu stricto (fig. S9). For the present, pending fur-ther discoveries, we assign LD 350-1 to Homo species inde-terminate. The identification of the 2.80-2.75 Ma Ledi-Geraru mandible as representing a likely phyletic predeces-sor to early Pleistocene Homo implies that phylogenetic schemes positing the origin of the Homo lineage from Aus-tralopithecus sediba as late as 1.98 Ma are likely to be incor-rect [contra (30); see text S3].

The 2.8-2.5 Ma time period witnessed climatic shifts that are frequently hypothesized to have led to the origin of the Homo lineage (3, 31–33). While the open habitats recon-structed for the Lee Adoyta faunal assemblages provide a new window on these changes (4), too little is known of the pattern of hominin evolution during this period to forge causal links to specific evolutionary events. The Ledi-Geraru specimen confirms that divergence from australopith dental and mandibular anatomy was an early hallmark of the Ho-mo lineage. Additional discoveries are needed to determine whether or not these dentognathic changes were accompa-nied by neurocranial expansion, technological innovation, or shifts in other anatomical/behavioral systems that are familiar components of the Homo adaptive pattern.

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ACKNOWLEDGMENTS

We thank the Authority for Research and Conservation of Cultural Heritage, Ethiopian Ministry of Culture and Tourism, for permission to conduct field work at Ledi-Geraru and laboratory research in the National Museum of Ethiopia, Addis Ababa, in which the LD 350-1 mandible is housed. Research funding provided by the National Science Foundation INSF) (BCS-1157351) and the Institute of Human Origins at Arizona State University is gratefully acknowledged. B.A.V. also thanks NSF (BCS-0725122 HOMINID grant) and the George Washington University Selective Excellence Program for support during this research project. We thank Z. Alemseged, B. Asfaw, L. Berger, F. Brown, C. Feibel, D. Johanson, F. Spoor, G. Suwa, C. Ward, T. White, and B. Wood for discussion, assistance, and/or permission to examine fossils. Constructive comments by F. Spoor, B. Wood, and three anonymous referees substantially improved the manuscript. Supporting data for this paper are presented in the supplementary materials.

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SUPPLEMENTARY MATERIALS www.sciencemag.org/cgi/content/full/science.aaa1343/DC1 Text S1 to S3 Figs. S1 to S10 Tables S1 to S7 References (34–47) 23 October 2014; accepted 13 February 2015 Published online 5 March 2015; 10.1126/science.aaa1343

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Fig. 1. (A) Map of the Lee Adoyta region in the Ledi-Geraru research area. (B) Stratigraphic section of the Lee Adoyta sequence indicating the provenience of the hominin mandible LD 350-1. (C) The LD 350 locality with the position on the surface of the LD 350-1 mandible in relation to the Gurumaha Tuff. Section in (B) is indicated by black line at left in (C). Details in (6).

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Fig. 2. The LD 350-1 mandible. (A) Medial view. (B) Lateral view. (C) Occlusal view. (D) Basal view. (E) Enlarged occlusal view of dentition. Scale bars, 1 cm.

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at 2.8 Ma from Ledi-Geraru, Afar, EthiopiaHomoEarly

Ramon Arrowsmith and Kaye E. ReedBrian Villmoare, William H. Kimbel, Chalachew Seyoum, Christopher J. Campisano, Erin DiMaggio, John Rowan, David R. Braun, J.

published online March 4, 2015

ARTICLE TOOLS http://science.sciencemag.org/content/early/2015/03/03/science.aaa1343

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2015/03/03/science.aaa1343.DC1

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