Abstract Projectile weapons (i.e. those delivered from a distance) enhanced prehistoric hunting efficiency by enabling higher impact delivery and hunting of a broader range of animals while reducing confrontations with dangerous prey species. Projectiles therefore provided a significant advantage over thrusting spears. Composite projectile technologies are considered indicative of complex behavior and pivotal to the successful spread of Homo sapiens. Direct evidence for such projectiles is thus far unknown from >80,000 years ago. Data from velocity-dependent microfracture features, diagnostic damage patterns, and artifact shape reported here indicate that pointed stone artifacts from Ethiopia were used as projectile weapons (in the form of hafted javelin tips) as early as >279,000 years ago. In combination with the existing archaeological, fossil and genetic evidence, these data isolate eastern Africa as a source of modern cultures and biology.

Citation: Sahle Y, Hutchings WK, Braun DR, Sealy JC, Morgan LE, Negash A, et al. (2013) Earliest Stone-Tipped Projectiles from the Ethiopian Rift Date to >279,000 Years Ago. PLoS ONE 8(11): e78092. https://doi.org/10.1371/journal.pone.0078092 Editor: Michael D. Petraglia, University of Oxford, United Kingdom Received: June 28, 2013; Accepted: September 6, 2013; Published: November 13, 2013 Copyright: © 2013 Sahle et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: University of Cape Town’s Center for African Origins initiative and the Paleontological Scientific Trust (Scatterlings of Africa Program) financially supported research. Wenner-Gren Foundation supported YS’s doctoral study. The Marie Curie Fellowship program supported LEM’s post-doctoral study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction A key component in prehistoric subsistence strategies, the invention of projectile weapons was a decisive advance over the thrusting spear [1]–[3]. The ability to wound or kill dangerous animals or enemies at a distance is considered one of the most significant adaptive advantages for Paleolithic hunters, reducing the likelihood of injury and increasing prey breadth [1]–[3]. In the Late Pleistocene, complex projectiles such as the bow-and-arrow probably contributed to the technological advantage enabling Homo sapiens to expand out of Africa and outcompete Neanderthals [3]. At Kathu Pan, in South Africa, Middle Pleistocene hominins made hafted stone-tipped hunting spears ∼500 thousand years ago (ka); these were, however, not projectiles but hand-delivered thrusting weapons [4]. In addition, the stratigraphic placement of the studied artefacts from Kathu Pan relative to the dated layers remains as yet controversial. Pointed wooden spears from Schöningen, Germany, dating to ∼400 ka were likely used in hunting large game [5]. These were initially described as ranged weapons, but it has not been possible to definitively identify their mode of delivery [1], [2]. The identification of prehistoric projectile weaponry has been largely inferential. Paleolithic archaeologists suggest that mechanically-projected weapons, such as the bow-and-arrow, originated among modern humans in Africa ca. 100-50 ka [2], [3], [6], [7]. These inferences rely mostly on hafting mechanisms, morphometrics and weight thresholds of ethnographic specimens as a guide to identify prehistoric pointed artifacts suitable for use as projectile weapon tips. While such approaches have yielded useful insights, they are obviously constrained by the limitations of direct analogy with the ethnographic record. Micro- and macroscopic approaches, such as the analyses of hafting traces and macrofracture damage patterns, provide powerful independent means of inferring whether a pointed stone artifact was hafted and used [8], [9]. However, even these lines of evidence cannot confidently inform on the weapon delivery mechanism. Here we report on multiple independent lines of evidence that strongly indicate that pointed obsidian artifacts recovered from sites in the Gademotta Formation (Fm.) of the Main Ethiopian Rift (Figure 1) represent composite projectile weapons that were incorporated into the hunting repertoire of Homo as early as >279 ka. Specifically, we apply the velocity-dependent microfracture approach (alongside the more commonly used ballistic methods of studying macrofracture damage patterns and Tip Cross-Sectional Area and -Perimeter) to Middle- and Late Pleistocene pointed artifacts to reliably identify the mode(s) of weapon delivery. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 1. Location and stratigraphy of the Gademotta Fm. (A) A map showing the Gademotta ridge and major archaeological localities: the Gademotta type-site and the Kulkuletti area. Inset map shows the relative location of key later Middle Pleistocene sites in the Ethiopian rift, namely (1) Herto, (2) Gademotta, and (3) Omo Kibish. (B) A revised composite stratigraphic section of the Gademotta Fm. and the placement of major archaeological sites. https://doi.org/10.1371/journal.pone.0078092.g001 Microfracture features, such as fracture wings (one form of primary Wallner lines), are velocity-dependent ripple marks created when a crack force encounters intrinsic imperfections (such as bubble pores) in materials like obsidian [10], [11]. The analysis of these velocity-dependent fracture features provides a direct and objective tool for determining the velocity of fracture formation on stone artifacts. Specifically, it uses the geometry of the features along a crack plane of an impact-fractured stone artifact and the physical properties of the stone raw material to calculate the velocity of fracture formation [10], [11]. Details about the impact responsible for initiating the particular fracture, such as its type, can then be inferred from the velocity of the progressing fracture plane [11].

Discussion The evidence for the earliest composite projectile weaponry at Gademotta >279 ka is significant because it provides direct evidence for a highly advantageous, complex technology that pre-dates the emergence of H. sapiens [35]. Complex behaviors are therefore found amongst more than one hominin species, and are not unique to H. sapiens [36]–[38]. The evolution of our lineage was shaped by complex interactions between biology, environment and culture [39]. Throwing of composite stone-tipped javelins was one stage in a long process with much deeper evolutionary roots. Roach and colleagues [40] have recently shown that hominins’ ability to throw effectively depends upon a cluster of features in the anatomy of the shoulder, and that this first appeared in H. erectus about 2 million years ago. They argue that throwing – leading to an increase in hunting success – helped to shape the evolutionary trajectory of Homo. Prominent models for modern human origins propose that alterations in the costs and benefits of adaptation, triggered by paleoenvironmental changes that created stable ecological settings, contributed to the emergence of new adaptive strategies between 350 and 250 ka among populations ancestral to H. sapiens [41], [42]. The appearance and maintenance of these strategies reflects an overall pattern of increased reliance on social and technological information [43], [44]. This may have provided the evolutionary advantages that later allowed our species to expand to every known terrestrial habitat and outcompete other hominin species [3]. There is indirect/circumstantial evidence for the presence of projectile technologies prior to 200 ka [5], [8], [45], [46], but conclusive evidence has not, thus far, come from the period predating ∼80 ka. Our study shows that it was present by at least 280 ka. We suggest that the manufacture and use of multi-component stone-tipped throwing spears not only conferred significant advantages during the hunt, but also demonstrates complex behaviors not previously securely associated with this period. In recent years, studies of the behavior of Middle Pleistocene hominins, including early H. sapiens, have moved away from a list of traits considered ‘behaviorally modern’ to focus instead on evidence of complex behaviors [37], [47]. The onset of complex technological adaptations is probably linked to a series of factors that allow for more efficient processing of information and extraction of resources from a broader niche [39], [44], rather than sudden neural alterations among H. sapiens [38]. Among such adaptations are advances in hunting technologies including hafting and projectile weapon use [3], [9], [37], [43], [48]. The making of retouched pointed pieces, their hafting to a shaft and use as projectile hunting weapons is among innovations that have cognitive implications, even in the absence of direct evidence for transformative technologies, such as the use of irreversibly-altered adhesives [37], [49]. Sites in the Gademotta Fm. represent the earliest instance of a clearly MSA occupation from a securely dated context [12], [13]. Even the oldest (>279 ka) sites of ETH-72-8B and GDM7 in this Formation contain retouched tools (including unifacial and bifacial points) that compare in their technological variability with assemblages from younger sites [12], [13], [50]. The present direct evidence for the earliest stone-tipped projectile weapons is consistent with the relatively early occurrences of retouched points [13], [14] and hafting [33] at these sites. The late Middle- and earlier Late Pleistocene archaeological record from northeastern Africa particularly emphasizes the greater antiquity of behaviors considered complex [35], [37]. This evidence has not, however, been prominent in discussions of this topic; to date, early complex behaviors are most extensively documented from other regions of the continent and later time periods, e.g. [7], [9], [37], [43], [51], [52]. Earlier studies [14], [33] have suggested that the Middle Pleistocene record of the Gademotta Fm. attests to unique technological stability across much of the length of the MSA. This may be attributable to the presence of a nearby obsidian source, and the location of the site-complex in a near-lake and ecotonal environment [53]. These would have supported successful and continuous habitation in the region and created demographic and environmental settings conducive to the emergence of such innovative technologies [44]. Currently, fossil evidence for the world’s earliest H. sapiens derives from sites in the Ethiopian rift, namely Omo Kibish and Herto [15], [54], [55] (Figure 1A). At Herto (Figure 1A), all three early H. sapiens crania were carefully de-fleshed. These specimens were not associated with post-cranial skeletal remains; this is unlikely to be due to preservation, since conditions were good enough to preserve the relatively delicate skull of a juvenile individual [55]. This find therefore provides strong indications of ritual practice through post-mortem manipulation and curation of human remains by 154 ka [54], [55]. Recent geochemical provenancing supports the exploitation of obsidian raw material by these early modern human populations from as far as 289 km away, in addition to several nearby sources [56]. The Herto early humans therefore had a working knowledge of such distant resources on the ancient landscape and/or embraced complex social practices involving trade/exchange/gift-giving. The behavioral capacity of Middle Pleistocene hominin populations from the Kibish Fm. of the Ethiopian Omo basin (Figure 1A) is inferred to have been essentially similar to Late Pleistocene inhabitants of the same region [47], [50]. Several additional lines of evidence substantiate that the broader region supported stable adaptations across a long period of time. By 125 ka, humans in this region had occupied coastal areas of the Eritrean Red Sea and were probably exploiting marine resources [57]. Population expansion and/or contact across the early Late Pleistocene is evidenced in the wider region from the presence of the region-specific Nubian techno-complex at Gademotta by >105 ka [14], as well as elsewhere in the Ethiopian and Eritrean rifts (Kone [58]; Aduma [59]; Asfet [60]) and across the Red Sea in southern Arabia by ∼106 ka [61]. Current notions that Middle Pleistocene hominins were behaviorally dramatically different from their Late Pleistocene descendants [38] seem to be an artifact of our patchy knowledge of the archaeological record and data from disparate fields of study [39], [47], [62]. Accumulating genetic, fossil, archaeological and other lines of evidence from eastern Africa, especially the fact that modern humans who successfully spread beyond Africa all descend from the L3 genetic lineage prominent in eastern (but not southern) Africa, [15], [55]–[57], [63] point to the region as a significant source for modern humans.

Supporting Information Figure S1. Graphs of (A) relative probability and (B) inverse isochron of single crystal total fusion analyses for sanidines for samples T1S1, T1S2 (in red), and combined results from both samples. Xenocrysts are shown in pink on A, and are excluded from age calculations; they are not included on B. https://doi.org/10.1371/journal.pone.0078092.s001 (TIF) Figure S2. Pictures showing (A) a fracture surface containing FWs sampled for analysis from a locus at 34.7% of the crack length; (B) a photomicrograph of plane FWs; and (C) the measurement of angle of divergence of a prominent FW. https://doi.org/10.1371/journal.pone.0078092.s002 (TIF) Table S1. Material properties of the Kulkuletti/Worja obsidian. Data were collected via the pulse method. The ultrasonic transducer was set to 1 pulse per second for all measurements. The average E value was converted to Newton/m2, yielding a value of 8.9425e+10 N/m2, cf. [11]. Density of the Kulkuletti/Worja obsidian was determined via the immersion method and is 2.394 g/cm3. https://doi.org/10.1371/journal.pone.0078092.s003 (PDF) Data S1. Full Ar data for Trench1step1 and Trench1step2 samples. https://doi.org/10.1371/journal.pone.0078092.s004 (XLS)

Acknowledgments All necessary permits were obtained for the described study, which complied with all relevant regulations. All artifacts and raw material samples (Table 1 and Table S1) are housed in the National Museum of Ethiopia, Addis Ababa. We thank the Authority for Research and Conservation of Cultural Heritage of Ethiopia for research permits and laboratory facilities. We also thank CoMSIRU of the Department of Civil Engineering, University of Cape Town, for analysis of material density. YS thanks G Suwa for assistance in microscopic investigations; JJ Shea, S Wurz for raw metric data on lithic assemblages; F Abadi, TD White, J Parkington, Y Beyene, B Asfaw for discussions; T Gossa, KU Braun for assistance during fieldwork.

Author Contributions Conceived and designed the experiments: YS. Performed the experiments: YS WKH DRB. Analyzed the data: YS WKH DRB JCS LEM AN BA. Contributed reagents/materials/analysis tools: WKH. Wrote the paper: YS WKH DRB JCS LEM AN BA. Conducted fieldwork and recovered data: YS DRB LEM AN BA.