Taphonomic analysis of the early Pleistocene (2.4 Ma) faunal assemblage from A.L. 894 (Hadar, Ethiopia)

Journal of Human Evolution 62 (2012) 315–327

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Taphonomic analysis of the early Pleistocene (2.4 Ma) faunal assemblage from A.L. 894 (Hadar, Ethiopia) Manuel Domı´nguez-Rodrigo a, b, *, Bienvenido Martı´nez-Navarro c ´ frica), Museo de los Orı´genes, Plaza de San Andre´s 2, 28005 Madrid, Spain ´n en A IDEA (Instituto de Evolucio Department of Prehistory, Complutense University, c/Prof. Aranguren s/n, 28040 Madrid, Spain c ICREA, IPHES, Universitat Rovira i Virgili, Plaça Imperial Tarraco, 1, 43005, Tarragona, Spain a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 June 2009 Accepted 10 January 2010 Available online 6 March 2010

The A.L. 894 site (Hadar, Ethiopia) is, together with OGS 7 (Gona, Ethiopia), one of the oldest archaeological sites documenting a spatial association of stone tools and bones retrieved from an in situ excavation. In contrast with OGS 7, the better preservation of the bone assemblage at A.L. 894 allows the identification of taphonomic processes of bone breakage, thanks to abundant green bone fractures. The presence of tooth marks and the lack of hominin-produced bone modifications together argue against hominins as the responsible agents for bone accumulation and modification. This taphonomic study of A.L. 894 shows lack of evidence for functional associations between stone tools and bones, a pattern documented in several other early Pleistocene sites. Such a pattern underscores the complex phenomena involved in site formation processes, especially in the earliest archaeological assemblages Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Taphonomy Tooth marks Pliocene archaeology Stone tools Bone breakage Butchery

Introduction Currently, two very early archaeological occurrences, A.L. 666 and A.L. 894, have been found in the Maka’amitalu Basin (Hadar, Ethiopia). Both sites are situated stratigraphically in the Busidima Formation (Campisano and Feibel, 2008) and are dated to 2.36 Ma but may be slightly older than this (Kimbel et al., 1996). At each locality, there are spatial associations between stone artefacts and bone remains (Kimbel et al., 1996, 1997; Hovers et al., 2002). The nature of the functional association of lithics and bones in A.L. 894, in which a relatively large faunal assemblage was found (in terms of bone specimens but not of number of elements), is the scope of the present research. The goal is to discern if hominins were responsible for the faunal accumulation at the site and, if so, to study the strategies that they used to obtain animal resources. Earliest co-occurrences of fauna and lithics Traditionally, the emergence of stone tools has been explained as the result of the prominent role of meat in human evolution * Corresponding author. E-mail addresses: [email protected] (M. Domı´nguez-Rodrigo), [email protected] (B. Martı´nez-Navarro). 1 Recently, the International Commission on Stratigraphy (ICS) has elected to formally define the base of the Quaternary (and the Pleistocene) at 2.6 Ma. This resolution has now been ratified by the executive committee of the International Union of Geological Sciences (IUGS). 0047-2484/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2010.01.010

during the early Pleistocene1. This core idea received support from the discovery of 2.5 Ma cut-marked bones at both Bouri (de Heinzelin et al., 1999) and Gona (Domı´nguez-Rodrigo et al., 2005), with the oldest spatial association of bones and stone tools found at OGS 7 (Semaw et al., 2003). However, sites where fauna and stone tools occur together –and where a functional link of this association exists in the form of cut-marked and percussed boneare far less abundant than sites composed of stone tools alone. The 2.5 Ma archaeological sites (EG 10, EG 12, WG) reported by Semaw et al. (1997) are in low-energy contexts (clays and silts), showing excellent preservation of lithic remains, but they are practically devoid of fauna. Since 1997, a series of additional sites dating from 2.5 to 2.2 Ma have been found in Gona. Most of the sites are in undisturbed and primary position and lack faunal remains (Domı´nguez-Rodrigo, 2009). The absence of fauna at these sites is attributed to either a taphonomic bias or to a behavioural cause (Domı´nguez-Rodrigo et al., 2005). These alternatives apply to most of the early Pleistocene archaeological record prior to 2 Ma. Archaeological sites in member F in Omo (Ethiopia) (2.3 Ma) seem to be mostly secondary deposits (in a derived position), with the exception of two localities (FtJi 2 and Omo 123) (Merrick, 1976; Merrick and Merrick, 1976; Chavaillon, 1976). Polished and abraded artefacts from most of these sites reveal hydraulic transport from their catchment areas. Spatial associations among objects at these sites are, therefore, uncertain. Given that several channels in member F contain abundant faunal remains, the association of

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bones and stone tools in some of these sites may well be fortuitous. Both of the primary context sites, FtJi 2 and Omo 123, contain lithic artefacts but lack associated bones (Merrick, 1976; Merrick and Merrick, 1976; Chavaillon, 1976). A couple of presumably older sites have been found at Omo (Omo 71 and Omo 84) in member E. While both localities contain lithics and fauna, they are in uncertain stratigraphic positions, and one of them (Omo 71) consists of surface finds (Howell et al., 1987). In fact, both of these localities may not be anthropogenic (Torre, 2004). The Lokalalei 1 Site (GaJh 5, LA 1) at West Turkana (Kenya), dated at around 2.3–2.1 Ma, yielded an association of bones and stone tools (Kibunjia, 1994). Unfortunately, poor preservation of the fauna at the site makes it difficult to provide clear evidence of functional associations of bones and stone tools. The same holds true for Fejej (Ethiopia). This early Pleistocene site, dated around 2.2 Ma, also shows a spatial association of stone tools and bones, but no traces of hominin-imparted damage were observed on any bone surfaces. Claims of the anthropogenic origin of the association are exclusively based on the presence of very few long limb bone fragments (most of them from an impala) showing green fractures (Echassoux et al., 2004). No percussion marks were discovered on any bone specimen, but some carnivore-gnawed remains were discovered nearby. For this reason, the green fractures documented could also be due to the action of bone-breaking carnivores and not hominins. The only probable evidence of an intensive bone accumulation bearing hominin-imparted modifications ca. 2 Ma comes from Kanjera (Kenya). Artefacts found in association with diverse faunal remains, were discovered in Excavation 1 (Plummer et al., 1999; Plummer, 2004), some of them bearing traces of butchery (Ferraro, 2007). Overall, several factors cast doubts on the functional associations between bones and stone tools at many of these sites, spanning a time of about 500 k yr. Specifically, the uncertain depositional history of some of these sites, poor preservation of cortical bone in some sites where stone tools and bones occur in the same space, an abundant number of sites in undisturbed or minimally disturbed depositional contexts without faunal remains, and the fact that a total of only 15 cut-marked bones (as complete elements) have been retrieved, create uncertainties about the functional association of stone tools and bones at many of the earliest sites (see more extended review in Domı´nguez-Rodrigo, 2009). In light of the ambiguity of associations at such sites, the present analysis of A.L. 894 provides an opportunity to explore the complexity of early site formation processes. We should note that this is a relatively small, but extremely important faunal collection requiring detailed scrutiny. Our analysis of A.L. 894 underscores the need to understand many of these sites as palimpsests, given that the role played by hominins in the accumulation of faunal remains at many of these sites remains elusive and fails to receive taphonomic support. Materials and methods The faunal collection from the early Pleistocene archaeological site of A.L. 894 is composed of 330 bone specimens of macrovertebrates, most of which are very fragmented; only a few are identifiable to skeletal part. Additionally, there are 118 microvertebrate specimens (most of them rodents, but also Aves), and several eggshell fragments. This analysis focuses exclusively on large vertebrates, a sample that includes relatively few identified specimens. Gazella sp. is represented by a right hemimandible (A.L. 894– 4059). The hemimandible preserves the diastema, the roots of D2 and D3, the mesial root of D4, the 2nd and 3rd lobes of D4, the M1

and the mesial alveolus of the M2. Additional Gazella specimens include A.L. 894–2799a (calcaneum), A.L. 894–2799b (naviculocuboid), A.L. 894–2800 (proximal end of metatarsal), A.L. 894–2801 (talus) and A.L. 894–2803 (distal end of tibia). These likely correspond to the left hind limb of a single individual. We identified Tragelaphus sp. (small-medium size) through left and right hemimandible fragments. These correspond to the same juvenile individual (A.L. 894–222 and A.L. 894–2416, respectively). The left hemimandible shows the germens of the deciduous teeth -D2, D3, D4-and anterior lobe of M1, and the right hemimandible has the D3–D4-and anterior lobe of M1. Sivatherium cf. maurasium is disclosed by the presence of a left upper M2 (A.L. 894–4085), and another upper molar fragment (probably the upper left M1) and a palate fragment with a partial alveolus (A.L. 894–4084). Crocodilus sp., was identified based on trabecular structure of the internal surface and is represented by two bone fragments (A.L. 894–1465 and A.L. 894–2735). Several fragments were very difficult to identify, including a large size ungulate represented by several bone fragments corresponding to a right humeral diaphysis, a right and left radiusulna, a metatarsal, and a metacarpal. These specimens could correspond to Sivatherium. Lastly, a small carnivore (gen. et sp. indet.)(A.L. 894–3861a) is represented by the presence of only one distal end of a humerus. The minimum numbers of individuals of these taxa do not exceed one (Table 1). Bone specimens were classified as belonging to two animal size categories: small carcasses (sizes 1 and 2) and large carcasses (sizes 4 to 6) as defined by Bunn (1982). Anatomical sections were divided into cranial (cranium, mandible and loose teeth), axial (ribs, vertebrae) and appendicular sections. For taphonomic purposes, pelves and scapulae were treated together (when specified) with axial bones, given their similar overall cancellous texture. Long limb bones were classified as belonging to upper (humerus and femur), intermediate (radius and tibia), or lower (metapodials) limb bones, as indicated by Domı´nguez-Rodrigo (1997). Skeletal element representation was reconstructed by using minimum number of identifiable specimens (NISP) and minimum number of elements (MNE). We carried out element identification using shaft fragments according to carcass size and seeking the existence of overlaps (see Yravedra and Domı´nguez-Rodrigo, 2009). In order to detect hominin traces of modification on these bones, the study also focused on cortical preservation, bone surface modifications and bone breakage patterns. Frequencies of different types of shaft circumference sections of long bones were also tallied. Bunn (1982) noted that bone breakage (marrow extraction by humans and carnivores) produced a specific pattern of shaft fragment representation in terms of the preserved cross-section. Generally, the articular ends of broken limb bones preserve an attached section of shaft that is complete in circumference (Bunn’s [1982] ‘‘Type 3’’), while the isolated shaft fragments preserve either more than half (‘‘Type 2’’) or less than half (‘‘Type 1’’) of the sections of the original circumference. Bunn (1982) found that in broken assemblages the ratio of Types 3 and 2 (complete and more than Table 1 List of minimum number of individuals documented at AL 984. MNI Gazella sp. Tragelaphus sp. Sivatherium cf. maurasium Crocodilus sp. small carnivore large ungulate* * Could correspond to Sivatherium.

1 1 1 1 1 1

M. Domı´nguez-Rodrigo, B. Martı´nez-Navarro / Journal of Human Evolution 62 (2012) 315–327

half circumference, respectively) to Type 1 (less than half circumference) ranges from 0.44 to 0.10; that is, shaft specimens preserving less than half the circumference outnumber the other types. In the present study, an analysis of specimen size distribution was carried out to detect any preservation biases introduced by physical agents. Assemblages biased by post-depositional processes such as hydraulic jumbling or by selective recovery will show a biased preservation of smaller specimens, especially from long limb shaft sections. The presence of abrasion and polishing due to water transport was also taken into account. An evaluation of the cortical surfaces was made, followed by an analysis of bone surface modifications, namely, cut marks, tooth marks, percussion marks and natural marks (i.e., biochemical and abrasion marks). Tooth and percussion marks were identified using hand lenses under strong direct light following the methodological and diagnostic criteria specified in Blumenschine (1988, 1995) and Blumenschine and Selvaggio (1988). Cut marks were identified following protocols by Bunn (1981,1983) and Fisher (1995). In this analysis, the presence of cut and percussion marks, irrespective of their frequencies, are direct evidence of hominin modification of bones. The presence of tooth marks is evidence of non-hominin carnivore modification of bones. We are aware that, even though tooth marks may also have been caused by hominins, the more common consensus in taphonomic protocols is to assume that tooth marks are the sole responsibility of quadruped carnivores. Most of the sample (> 80%) was poorly preserved and showed very leached and exfoliated cortical surfaces, with a predominance of chemical alterations on bone surfaces over any other bone modifying process. Bone breakage was studied by differentiating two processes: green/dry fractures and agent of breakage in the ‘‘green’’ broken sub-assemblage. Villa and Mahieu’s (1991) criteria were used for identification of green and dry (including diagenetic) breakages. Dry breaks result in abundant breakage planes that run longitudinal and transverse to the axis of the bone, with the angle between the cortical and medullary surfaces close to 90 degrees, and an uneven breakage plane surface. In addition, micro-step fractures and rough uneven texture occur, contrasting with the smoother surface of green-broken specimens. Bone breakage patterns in the ‘‘green-broken’’ sub-assemblage can be analysed by measuring angles in breakage planes (Alca´ntara et al., 2006; Domı´nguez-Rodrigo et al., 2007a) to estimate whether dynamic (hammerstone) or static (carnivore) loading caused bone breakage. Capaldo and Blumenschine (1994) quantified the frequency and morphology of notches produced by carnivore gnawing and hammerstone breakage. They showed that percussion notches are more abundant and broader and shallower in cortical view than carnivore notches. The flakes removed from percussion notches also show a more obtuse angle, because they result from dynamic loading, which differs from the static loading created by carnivores. In the bone collection from A.L. 894, not a single complete notch was documented and oblique breakage planes from specimens larger than 4 cm -suggested by experimental work to be more useful for discriminating bone loading types (Pickering et al., 2005; Alca´ntara et al., 2006) - occurred in very few fragments. This precluded a meaningful comparison with experimental assemblages because the archaeological sample size was too small. For this reason, the agency of green bone breakage was inferred indirectly from the surviving bone surface modifications resulting from the action of the bone breaking agent(s). The relevance of bone surface modifications for interpreting the origin of the faunal accumulation required the statistical treatment of a small sample of modern assemblages to compare to the modifications identified in the A.L. 894 assemblage. Given the presence of tooth marks on some of the bone specimens, data on

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tooth mark frequencies were obtained from several hyena den studies: Maasai Mara (Kerbis, 1990; Egeland et al., in prep.), KNHD2 (Eyasi hyena den) (Prendergast and Domı´nguez-Rodrigo, 2008), Amboseli (Faith, 2007), Mashatu1, Mashatu 2, Mashatu 3, Mashatu 4, Gobadeb, Namib-Naukuft (Kuhn et al., 2009a, b) and Syokimau (Egeland et al., 2008). This referential assemblage set was reduced because it comprises the few examples of carnivore assemblages where tooth-mark frequencies have been tallied according to specimen size range. To overcome limitations of statistical approaches that require larger samples, we bootstrapped the mean values of tooth mark frequencies on mid-shafts from long bones in the hyena den assemblages selected both for the total frequencies of toothmarked shafts (irrespective of specimen size) and total frequency of tooth-marked shaft specimens smaller than 4 cm (see raw data in Table 4). The reference sample was bootstrapped 1000 times and then robust confidence intervals were obtained by applying a Monte Carlo technique on quantiles and the sampling error estimator. The two-tail sampling error was obtained from a nested bootstrap procedure (Wilcox, 2005). In this procedure, the original vector of 1000 bootstrapped cases estimates the 0.025 and 0.975 quantiles, and then 100 bootstrapped nested samples estimate the sampling error. The total number of bootstrapped samples used to create a 95% confidence interval is 10,000 times the original sample size. This allows more confidence in the interpretation of tooth mark frequencies from A.L. 894 when considering their proportion according to specimen size distribution and the differential representation of specimen sizes in the fossil sample. The statistical analysis was performed in R by using the ‘‘Robust’’ library (http:// www.r-project.org). Results Assemblage composition The small number of macro-faunal specimens identified to skeletal element (NISP ¼ 69), makes up only 21% of the total assemblage (Table 2). A high degree of bone fragmentation at the site and the poor preservation of most of the specimens (because of chemical leaching and concretion) limited identifications. Only 20 elements could be confidently identified (Table 3); most of these probably belong to only two individuals. Although tentative, our analyses suggest that the presence of different bones of similar size

Table 2 Number of identified specimens (NISP) for each skeletal part at AL 894.

Skull Mandible Teeth Vertebrae Ribs Scapula Pelvis Humerus Radius-ulna Carpals Metacarpal Femur Tibia Tarsals Metatarsals Phalanges Upper limb bone shafts Limb shafts Limb ends Indet.

Small carcass

Large carcass

Indeterminate

2 5 4

1 1 2

1

8

11

3

5

2 1 1 2 5 3 2 1 3 45 4 72

3 3 3

1 35 1 43

57

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318

Table 3 Minimum number of elements (MNE) identified at AL 894.

Skull Mandible Teeth Vertebrae Ribs Scapula Pelvis Humerus Radius-ulna Carpals Metacarpal Femur Tibia Tarsals Metatarsals Phalanges

Small carcass

Large carcass

1 2 2

1 1 2

1

1

1 1 1 1 1 3 1 1

1 1 1

can be parsimoniously interpreted as belonging to the same individual. We are aware that they could belong to more than one individual, but using the minimum number of individuals (MNI) minimizes the potential error in overestimating the actual amount. This conservative identification is supported by the presence of a sequence of correlative elements belonging to the same anatomical section. Thus, one individual could be a small bovid because most of the post-cranial elements identifiable to taxon (metatarsal, tibia, calcaneum, talus, cubo-navicular of a left hind limb) indicate that it is a gazelle rather than the small tragelaphini also present in the assemblage (see above). Also, the small tragelaphini is a yearling, with non-erupted germinal milk teeth, whereas the gazelle is an older juvenile, with worn DP4 and M1 and erupted M2, documented by its alveolus. For this specimen, the identifiable bones show no traces of the periosteal texture that characterizes younger individuals. Several bones, from all anatomical areas (skull, axial and limbs), represent this purported single gazelle. We infer that it may have been deposited at or introduced to the site in a complete state. If it was introduced, then the most remarkable feature is the lack of vertebrae, given the presence of parts of the axial skeleton (rib fragments). The other individual, a large ungulate, is represented postcranially mostly by a fragmented front limb. The large size of this ungulate is similar to the Sivatherium identified by teeth, so this front limb may correspond to the same individual. Unfortunately, the lack of epiphyses (only a small fraction of the proximal end of the metacarpal survived) renders the identification to species of the postcranial elements difficult. However, based on a comparison with fossils of Sivatherium stored at the National Museum in Addis, the size and shape of the sections of a radial shaft and a metacarpal shaft perfectly match the identified fossils in the collection. The virtual lack of most of the skeleton of this large animal contrasts with the more complete state of the small bovid. In both cases, most of the assemblage of identifiable specimens corresponds to long limb bone shafts (only five epiphyseal fragments have survived). Axial bones are scarcer and many non-identifiable specimens could correspond to those more fragile elements (namely, ribs) that break more easily diagenetically and by weathering. The presence of several long limb bones from both carcass sizes and the under-representation of long bone ends (less than a third of the expected number of ends has survived, based on the total MNE reconstructed using shaft specimens) are suggestive of carnivore ravaging of the assemblage. This could also explain the scarce representation of axial elements (see review of this topic in Domı´nguez-Rodrigo et al., 2007a). Diagenesis does not produce this biased long bone portion profile. Bone macrostructure shows the

differences in mechanical properties between cancellous ends and dense cortical shafts. However, diagenetic processes caused by soil chemistry affect the bone microstructure, not the macrostructure (Stiner et al., 2005a, b). Rates of dissolution seem to operate similarly in both types of bone because chemical processes take place at the molecular level and depend on microporosity. Cancellous bone has conspicuous porosity, but dense bone has abundant microscopic channels (e.g., micropores, Harvesian systems) where the diagenetic chemistry operates at the same pace as in cancellous bone (Nielsen-Marsh and Hedges, 2000a, b). The microstructure of bone is similar across the skeleton, so dissolution affects all bone tissues equally. Stiner et al., (2005a:40) indicate: ‘‘Compact bone may be less vulnerable to chemical dissolution than spongy bone, but the microporosity of compact and spongy bone, and the rates of loss of the two macrostructure classes in acidic environments, may be more similar than many faunal analysts realize’’. Furthermore, the chemical conditions necessary to modify bone chemistry, generally in acidic soils, would also affect volcanic stone tools. Lava and basalt can be weathered by soil conditions (pH, water table) creating chemical modification of the external surfaces, which, if they were so extreme as to cause bone deletion, would also create exfoliation and dissolution of the exposed external areas of the artefacts (Domı´nguez-Rodrigo et al., 2009a). At A.L. 894, the preservation of the stone tools is very good (GoldmanNeuman and Hovers, 2009) and does not indicate that the soil underwent the chemical processes that would account for bone chemical deletion. Specimen size representation We recorded one abraded specimen with a polished surface, suggesting that its incorporation into the assemblage could have been a consequence of hydraulic processes. The remainder of the assemblage lacked any indication of transportation. Specimen size distribution clearly shows that most of the specimens preserved are small. Seventy-five percent of the bone assemblage specimens are smaller than 4 cm, whereas 60% are smaller than 3 cm (Fig. 1a). This clearly contrasts with experimental assemblages of human-broken bones, where the frequencies of bones >4 cm are higher than that documented at this site (Blumenschine, 1995; Capaldo, 1995; Domı´nguez-Rodrigo, 1999) (Fig. 1b). This distribution, with the overwhelming presence of small specimens, is not consistent with physical agents of accumulation such as water transport. However, if the bone assemblage is divided according to carcass sizes, it becomes evident that specimen size distributions follow different patterns for the two carcass size classes (Fig. 1a). Whereas bones from small-sized animals are represented mostly by specimens <4 cm, bones from larger carcasses are clearly under-represented in the size fraction< 4 cm when compared to both small carcasses and to experimental scenarios. Larger animals should indeed produce larger bone fragments, but when broken under experimental conditions, specimens <5 cm are more abundant than specimens >5 cm (Blumenschine, 1995; Capaldo, 1995; Domı´nguez-Rodrigo, 1999). This fraction of small specimens is missing in the large ungulate sample at A.L. 894, even though substantial on-site diagenetic breakage (see below) should have artificially increased the number of small fragments. Diagenetic and recent dry breakage may bias specimen size representation. Axial elements and other cancellous elements are more severely affected by such processes, because they break more easily. This bias can be rectified by observing specimen size distribution of dense bones alone, such as long limb shafts. This also enables comparisons with referential frameworks, because bone breakage experiments have focused on long bones (Blumenschine, 1988, 1995; Capaldo, 1995; Alca´ntara et al., 2006; Gala´n et al.,

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2009). When plotting size distribution of shafts in modern and fossil assemblages, it becomes obvious that the differences in the representation of small specimens is more marked between carcass sizes when the small fragments of non-limb bones generated though diagenesis are removed (Fig. 1b). In addition to the different bone representation, the marked contrasts in fragment size distribution according to carcass size suggest that at A.L. 894, the two carcass sizes were deposited during different time intervals. The differential representation of bone specimen sizes for both carcass types, especially the underrepresentation of small fragments for the large ungulate bones, supports the hypothesis that the small bovid remains underwent minimal post-depositional disturbance. The large ungulate remains, on the other hand, may have experienced some disturbance prior or during sedimentation. Bone fragmentation

Figure 1. A. Distribution of specimen size ranges, according to each carcass size separately and with all carcass sizes together, including all skeletal elements. B. Distribution of specimen size ranges according to carcass size in A.L. 894 and three experimental assemblages: two hyena dens (Maasai Mara [Kerbis, 1990] and Eyasi [Prendergast and Domı´nguez-Rodrigo, 2008]) and a set of hammerstone-carnivore experiments stored at the Prehistory Department of the Complutense University.

Bone fragmentation processes (green versus dry) are documented in only 157 bone specimens (47.6% of the assemblage) (Fig. 2), given the intense carbonating of bones at the site. Green fractures are the most widely documented and occur on 64.3% of the bones with documented fractures (n ¼ 101). Dry fractures are also abundant and occur on 56.6% of these bones (n ¼ 89). The presence of clear green fractures (Fig. 3) implies that biotic agents broke bones at the site either through dynamic or static loading processes. Given the presence of elements with cancellous tissue, diagenetic breakage may account for the large portion of dry fractures in these elements, since this kind of breakage was also documented in the dense long limb bone shafts and on the stone tools (Hovers, 2003). When documenting bone breakage on the most abundant bone type identified (limbs), green fractures are even more abundant than when all elements are pooled. The fact that 80% of fractures on limb bone specimens are green emphasizes the role played by biotic agents in bone breakage at the site. However, not a single measurable complete notch has been documented, although there are several specimens with negative flake-scars on the medullary surface suggestive of green breakage

Figure 2. Distribution of frequencies of specimens bearing green and dry bone breakage patterns. All the bars (except the first one on the left) show percentages according to the fraction of the assemblage where dry/green breakage could be determined. Green and dry fractures in the different categories shown are established according to their frequencies with respect to the fraction of the assemblage with enough preservation to determine breakage type.

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Figure 3. Large overlapping notch showing two impact points (arrows) and the negative scar (stressed by a black line) of green fracture on the medullary surface of the large ungulate radius shaft. Scale ¼ 2 cm. Photograph by E. Hovers.

(Fig. 3). Few shaft specimens larger than 4 cm were observed whose breakage planes could be measured. Given the small number of measurements taken, no meaningful statistical comparison can be made with experimental assemblages. Therefore, the use of both notches and the angles from the breakage planes cannot be used to discern the bone breaking agent. Despite the inferred post-depositional disturbance for the larger ungulate remains, the introduced bias is probably small because several refits were possible. The larger shafts from both the radiusulna and the humerus of the large ungulate could be refitted (Fig. 4), suggesting that the purported post-depositional disturbance affected mainly the smallest fraction of the assemblage of bones from the large ungulate portion of the assemblage. The high fragmentation of the assemblage is reflected in the frequencies of the different shaft circumference types (Fig. 5). Most of the assemblage (97%) is composed of Type I shafts, that is shafts that preserve less than 50% of the section. Types II and III are less represented than at experimental assemblages, probably because of diagenetic breakage and also because the ends have been virtually removed from the site. Bone surface modifications: identifying the agents of bone accumulation Bone surface preservation is fairly poor in most of the assemblage. About 81% (n ¼ 268) of specimens show poor preservation in the form of chemical alteration of the bone surfaces or complete covering of the surface with carbonate and concretion (see Fig. 6).

This prevents the identification of element types, fracture types, and bone surface modifications. Thirty-two specimens (9.6%) show good cortical preservation with the original cortex intact and covering the entire surface. Another 31 specimens (9.3%) show moderately good cortex, defined as the preservation of good cortex on more than two-thirds of the surface, which was estimated to be enough to document possible marks. Both types of specimens (19%) were used together to establish the frequency of bone surface modifications. It is not possible to ascertain whether or not they are representative of the remainder of the poorly preserved sample. Weathering from subaerial exposure cannot be observed in 80% of the sample because of the intensive diagenetic processes chemically and physically altering the bone surfaces. Judging from the remaining sample with good to moderate cortical surface, we find no evidence of significant weathering caused by bone exposure. No remains show cracking of the surface or traces of modifications by chemical decomposition of the surface caused by the soil substratum (bone matrix potential, water dissolution, soil pH). Traces of chemical dissolution normally take the form of rugged irregular surfaces with a fibrous aspect (with a wavy shape at the micro level) alternating with deep cracking in the absence of exfoliation or with very limited exfoliation and presence of dissolution pitting. In subaerial weathering, the morphologically similar pattern (considering the depth of the cracked lines) occurs in conjunction with extensive exfoliation and the shape at the micro level (e.g., over less than 10 mm) shows a step-fracture pattern. Cracking associated with chemical micro-pitting occurred on two specimens from the large ungulate. This is common in bones affected by chemical weathering. However, most of the ungulate bone remains of both the large and small carcasses that had exposed surfaces (i.e., not covered by carbonate or affected chemically) showed no traces of subaerial exposure. This indicates that the bones were buried in sediments shortly after their deposition at the site. At least seven tooth-marked specimens (two from the smaller carcass size portion and five from the larger one) were documented. This suggests that at some point, carnivores, very likely hyenas, broke bones from both carcass sizes at the site. This could explain the total deletion of the axial skeleton of the gazelle (assuming it had been deposited at the site as a complete carcass) and the deletion of most long limb bone ends from both carcasses. Therefore, carnivores may have been an important agent of bone

Figure 4. Large ungulate humerus shaft showing multiple green fractures (arrows) and refitted pieces (smaller figures). Photograph by E. Hovers.

M. Domı´nguez-Rodrigo, B. Martı´nez-Navarro / Journal of Human Evolution 62 (2012) 315–327

Figure 5. Distribution (in percentages) of the different types of bovid long bone circumference types (Bunn, 1982) in experimental assemblages and at A.L. 894. Data for experimental assemblages are from Marean and Spencer (1991) and Marean et al. (2004).

breakage at the site. The small fraction of tooth-marked bone in both carcass sizes in the well-preserved bone sample (seven specimens out of 65 ¼ 10.7%) is similar to that reported for hominin-carnivore experimental models (Blumenschine, 1995). However, diagenetic breakage may have affected these frequencies by artificially increasing fragmentation. If the total number of fairly well preserved specimens and the frequency of 20% dryfragmented shafts are considered, the sample of well-preserved bone could be better estimated by reducing the original number by 20%, given that dry-fragmentation has produced an artificial 20% increase in shaft specimens than originally represented after green breakage (see Pickering et al. [2008] for a more detailed explanation of this correction formula). Even if that were the case, and the number of well-preserved bones were reduced from 65 to 52, the frequency of tooth-marked bone in this case would be ca. 13.5%, well within the range for hominin-carnivore models and very far from the high frequencies (>75%, Blumenschine, 1988; 50%, Domı´nguez-Rodrigo et al., 2007a) reported for mid-shafts in hyenafirst assemblages. It could be argued that hominins had primary access to the carcass remains at A.L. 894 by exclusion of hyenas (the

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other bone-breaking agent capable of generating the fractures documented in the large-ungulate portion of the assemblage) given the low tooth mark frequencies. It could alternatively be argued that the intense concretions and chemical processes affecting bones at A.L. 894 (micro-pits created by dissolution, etching and biochemical marking created by multiple rootlets) may have deleted several marks. Although the same could be argued of cut marks, this process may have affected tooth marks more intensively because long bones broken by hyenas bear tooth marks in frequencies higher than those reported for cut marks (Blumenschine, 1995; Capaldo, 1995; see however Domı´nguezRodrigo, 1997 for high estimates of cut marks on shafts). However, it is doubtful that preservation alone is responsible for the low tooth mark percentages reported at the site. Humancreated assemblages show frequencies of percussion marks ranging from 15–30% of shafts. In some hominin assemblages they are even higher than 30% (Domı´nguez-Rodrigo et al., 2009b), and cut mark frequencies on shafts generally range between 5%–30% (see review in Domı´nguez-Rodrigo et al., 2007a) but can be higher than 50% of specimens (Domı´nguez-Rodrigo, 1997). If we take into account that percussion and cut marks do not necessarily overlap on the same specimen, the presence of hominin-modified bones can be as frequent as to expect at least several specimens preserved in an assemblage the size of A.L. 894. If cortical preservation was the only argument provided, then a well-supported explanation for carnivore and hominin behaviours at the site could not be provided, so consideration of the combination of variables that follow is of utmost importance. The comparison of the A.L. 894 ‘‘good cortical preservation’’ sample with experimental assemblages is not completely justified because the most frequent size range in the latter (> 4 cm) is only minimally represented at A.L. 894 (see Fig. 1 and discussion above). Therefore, the A.L. 894 sample will show a much lower tooth mark frequency as it is composed of very small specimens and the smaller the fragment, the less likely it is to bear any bone surface modification (Blumenschine, 1995; Faith, 2007). If we observe the proportion of tooth-marked specimens in experimental assemblages, the fraction of specimens <4 cm bearing tooth marks is very low in carnivore-first(only) scenarios (< 25%, Egeland et al., in prep; Prendergast and Domı´nguez-Rodrigo, 2008) (Table 4) (Fig. 7a). Although the tooth mark frequency at A.L. 894 is similar to that reported for hammerstone-carnivore experimental scenarios for the specimen size ranges represented (see Blumenschine, 1995), it also falls within the range of variation of carnivore-first(only) experimental scenarios (Fig. 7b). This is statistically supported by Table 4 Distribution of the proportion of long bone specimens smaller than 4 cm with respect to total specimen size distribution, the percentage of those specimens smaller than four cm bearing tooth marks, and frequencies of total long bone toothmarked specimens in different hyena dens.

Figure 6. Specimen showing carbonate coating, the most common form of obliteration of bone surfaces at A.L. 894. Photograph by E. Hovers.

Hyena den

Proportion <4 cm

TM% <4 cm

Total TM%

References

Eyasi (KNHD2)

26%

14%

54%

Maasai Mara Amboseli Mashatu 1 Mashatu 2 Mashatu 3 Mashatu 4 Gobabeb NN1-2 all Mashatu & Namib-Naukuft Syokimau AL894

22% –

29% 45.9%

37% 71% 32.2% 53.5% 42% 39.1% 29%

Prendergast and Domı´nguez-Rodrigo (2008) Egeland et al. (in prep.) Faith (2007) Kuhn et al. (2009b) Kuhn et al. (2009b) Kuhn et al. (2009b) Kuhn et al. (2009b) Kuhn et al. (2009b) Kuhn et al. (2009a)

36.5% 45% 82%

*corrected for dry breakage.

15.6% 11.5%*

34.8% 13.5%*

Egeland et al. (2008) This work

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Figure 7. A. Tooth mark frequencies according to specimen size range. There is an increasing frequency of tooth-marking in larger specimens. Notice the differences in tooth-marking rates in the hyena dens reported here and the carnivore-only scenario documented by Blumenschine (1995: 40), which can be used as a cautionary note regarding experimental scenarios and the application of referential frameworks. B. Bootstrapped 95% C.I. analysis of the frequency of tooth-marked long bone midshafts from modern spotted hyena dens and A.L. 894 assemblages, tallying frequencies on the specimen size range <4 cm and on all specimen sizes separately. The confidence intervals were calculated using the t distribution, where p < 0$025 is the critical value of t with n  1 degrees of freedom. Data for the hyena assemblages are from Table 4. Data for the felid sample are from Domı´nguez-Rodrigo et al. (2007b).

a robust analysis of a bootstrapped sample (10,000 iterations) of the proportion of tooth-marked limb bones <4 cm in different hyena dens (Table 4). The bootstrapped results show that a 95% confidence interval embodies a variation that ranges from 5.49% to 49.49% of tooth-marked specimens for this size range (Fig. 7b). The same statistical analysis applied to complete long limb bone samples shows that within such a confidence interval toothmarked fragments range between 34.6% and 63.2% of all specimens. The A.L. 894 small sample fits within the range of variation found in modern hyena dens for the predominant specimen size range in the fossil assemblage. Some authors argue that carnivore consumption of carcasses should yield high frequencies of tooth-marked long bones (Blumenschine, 1995; Capaldo, 1995). However, the limited number

of carnivore-first(only) experiments carried out with hyenas fail to reproduce the range of tooth marking produced by hyena consumption of bone remains documented in dens of the spotted hyena (Crocuta crocuta). Specifically, the percentage of toothmarked bones in natural dens is usually substantially lower than reported experimentally (Table 4 and references therein) (Domı´nguez-Rodrigo and Pickering, 2010). Shafts from hyena dens are tooth-marked on average about 50% less than is reported in carnivore-first(only) experiments. There are even spotted hyena den assemblages with as low as 29% tooth-marked specimens (in contrast with >70% reported in experiments) (Kuhn et al., 2009b). In addition, it should be stressed that the use of ‘‘carnivore’’ models that do not pay attention to the type of bone-modifying agent are heuristically obsolete, since tooth mark frequencies depend on the type of carnivore involved in carcass consumption (Brain, 1981; see discussion in Domı´nguez-Rodrigo and Pickering, 2010). For instance, suids can produce similar frequencies of tooth-marked bone to that of hyenas both in primary (carnivore-first) or secondary (carnivore-last) experimental models (Domı´nguezSolera and Domı´nguez-Rodrigo, 2009). Furthermore, in contrast to the moderate to high range of tooth marks produced by hyenas, felids have been shown to produce very low frequencies of toothmarked bone. Long bones from carcasses consumed by felids and subsequently broken either by them or by hammerstones produce average frequencies of tooth-marked shafts under 12% (Domı´nguez-Rodrigo et al., 2007b). Therefore, it should be stressed that there are ‘‘carnivore’’ (felid) patterns of bone modification that produce very low tooth mark frequencies, irrespective of the specimen size range distribution discussed above. These data do not exclude the possibility that felids could have been responsible for the modification of some of the tooth-marked bones at A.L. 894. However, only hyenas are capable of breaking the large ungulate bones. For all the reasons described above, tooth mark frequencies alone are not enough to determine the order or type of agents breaking the bones at A.L. 894. The location of tooth pits on two specimens from the large ungulate carcass, right on the edge of green breakage planes (Fig. 8), suggests that a large carnivore was responsible for the breakage of the bones of this animal. This process could explain the green fractures observed on the humerus (see Fig. 4), given that one of the tooth-marked specimens bearing tooth pits on the edge of the breakage plane also was a humerus fragment (Fig. 8). This agent could also be responsible for the

Figure 8. Humerus shaft fragment from a large ungulate (A.L. 750) showing a green breakage plane (A) and a tooth score (arrow). Scale ¼ 1 cm.

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breakage of the large ungulate radius (see Fig. 3), since a double overlapping semi-notch is documented and the impact points are located just by the base of the ulna, where a hammerstone would not efficiently break the bone from a carcass of this size. The only direct evidence of hominin authorship in bone processing is the presence of cut marks and percussion marks. These were initially suspected to have occurred on four specimens. Macroscopic and microscopic examination with hand lenses (20), binocular lenses (35) and a Scanning Electronic Microscope refuted the interpretation that these might be hominin-imparted marks (Fig. 9). A gazelle mandible specimen showed some parallel oblique marks on the inferior lingual side of the body that were suggestive of possible cut marks. However, microscopic documentation of these marks proved that they were created through exfoliation of the cortical surface following its modification by dissolution, rather than through any object (tooth or stone tool) crushing through cortical layers. The marks do not show the diagnostic features of either tooth marks (U shape and crushed or polished internal surface) or cut marks (V shape and microstriations). The features identified in these marks are similar to those documented in marks created by root etching or fungi associated with mycorrhizae plants (Domı´nguez-Rodrigo and Barba, 2006). The main mark (Fig. 9A) showed a discontinuous trajectory and a substantial amount of the original cortical bone survived within its ‘‘groove.’’ Some parallel marks were also documented on one of the large ungulate humeral fragments, but under microscopic scrutiny it became evident that they were grooves for capillary vessels (Fig. 10). The lack of hominin-imparted marks on the A.L. 894 bones also casts doubts on the few ‘‘impact flakes’’ found. Two specimens that show the same morphology as the impact flakes generated through experimental percussion (cortical platform, medullary dorsal and

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ventral surfaces, including a little bulb) (Gala´n et al., 2009) have been found at A.L. 894 (Fig. 11). However, similar flakes from both small and large carcasses can also be documented in hyena dens (Fig. 11). There is currently no way of determining whether an impact flake is due to hammerstone percussion or to a hyena’s static loading on bone. For this reason, these impact flakes have to be excluded from the discussion of who broke the bones. Discussion The mere spatial association of stone tools and bones at archaeological sites is not necessarily the result of a functional link between both types of materials (e.g., Binford, 1981; Domı´nguezRodrigo et al., 2007a). As a matter of fact, most of the bone-plusstone tool associations documented and published so far in early Pleistocene sites prior to 1.5 Ma are palimpsests resulting from accidental associations of lithics and naturally discarded or accumulated bone in the landscape with only minimal contributions from functionally-related episodes of butchery in conjunction with naturally-formed bone accumulations (Domı´nguez-Rodrigo, 2009). This means that no claims of butchering behaviour can be made from the taphonomic evidence found at some of the oldest archaeological sites, such as OGS 7, where the oldest spatial association of stone tools and lithics has been documented (Semaw et al., 2003). The evidence for hominin butchery at sites older than 2 Ma, such as Gona and Bouri, is likewise scanty. The taphonomic study of A.L. 894 suggests that a large amount of green bone fracture occurred at the site. The only identified agents of this taphonomic process were carnivores that left tooth marks (including a tooth score associated with a fracture edge). Those were very likely hyenas, given the large size of the ungulate bones broken. The deletion of axial elements, and especially of

Figure 9. Mandible of gazelle showing oblique parallel marks (1), some of which show discontinuous trajectory (A). These marks, when observed under the microscope show broad and shallow grooves created through exfoliation as shown in the S.E.M. images 4 and 5. These marks are probably created by root etching - as shown in image 3 on a modern mandible as an example - and are fairly common in the archaeological record. Image 2 shows a specimen from FLK Zinj at Olduvai (Tanzania) where similarly structured root casts have been preserved overlying the bone and biochemically modifying it in conjunction with fungi.

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Figure 10. Capillary grooves documented on a humeral fragment of the large ungulate, showing broad and symmetrical U-shaped outline with completely polished surfaces and lacking microstriations. One of the grooves bifurcates into double parallel grooves (a).

most of the ends from the long limb bones documented at the site, is suggestive of intensive ravaging in the absence of alternative taphonomic processes. Diagenesis may have contributed significantly to bone breakage, as documented in the amount of drybroken bone. However, diagenesis does not preferentially delete

cancellous over dense bone, and under no circumstances do epiphyses disappear over long bone shafts as a result of soil chemistry (Stiner et al., 2005a, b). Alternatively, it could be argued that hydraulic disturbance could account for the underrepresentation of ends over shafts. Voorhies (1969) showed that cancellous elements tend to be dispersed more easily than limb bones. However, at present, no experimental study has shown that ends may be transported differently from shafts in hammerstonebroken bones. Even if it is common sense to think that this might be the case, hydraulic disturbance has not affected the integrity of the A.L. 894 assemblage, as shown by the virtual lack of abraded and polished bone, by the overwhelming dominance of small bone fragments in the faunal assemblage, by the abundance of small lithic fragments and by the abundance of refits among bones and, especially, among stone tools. The lithological context (silty clay) also indicates that no high-energy sedimentary processes took place at the site. Therefore, carnivores (given their positive signature in the form of tooth marks) were most likely responsible for bone breakage in the A.L. 894 faunal assemblage. Percussion and cut marks were not documented at the site. Therefore, even if hominins might have been an active agent in bone transport, accumulation, and bone breaking and discard, this is not evident in the small sub-sample of well-preserved bones out of the total assemblage at A.L. 894. Given that, the most parsimonious explanation for the taphonomic situation at A.L. 894 is that carnivores (probably, hyenas) broke the bones present at the site, irrespective of whether these animals were also the agents of transport, or the bones naturally accumulated at the site without any transport (by either hominins or hyenas). The evidence supporting this interpretation can be summed up as follows:

Figure 11. The two ‘‘impact’’ flakes from A.L. 894 (left) and two examples from the Maasai Mara hyena den (right). Notice how similarly-shaped the upper flakes are. Both show similar length and breadth, with asymmetric sides. The lower flakes are also similar showing a longer length than breadth. Cortical platforms (arrows) and ventral sides (V) are indicated.

M. Domı´nguez-Rodrigo, B. Martı´nez-Navarro / Journal of Human Evolution 62 (2012) 315–327

1) No hominin-imparted mark has been documented at the site. 2) Only tooth marks have been identified on the bones of small and large carcasses. 3) Tooth marks have been identified right at the edge of a breakage plane (Fig. 8), showing a cause and its effect (carnivores breaking the bone). 4) Double notches located on the caudal side of the large ungulate radius, at the very bottom of the ulna, cannot have been produced by hammerstone impact (without affecting the ulna) and are more frequently documented among hyena-broken bones (Domı´nguez-Rodrigo et al., 2007a). They occur on the same large ungulate remains where tooth marks have been identified. If the large ungulate humerus was broken by a carnivore, as indicated by the presence of tooth marks on the edge of the breakage plane, it is heuristically-supported to assume that the double notches were also made by carnivores and that, therefore, the radius may also have been broken by them. 5) The low proportion of tooth marks can be explained by a combination of two variables: the small number of wellpreserved bone specimens and, more importantly, an underrepresentation of bone specimens with good preservation larger than 4 cm (Blumenschine, 1988, 1995; Faith, 2007). Robust statistical analyses show that the A.L. 894 tooth-marked sample fits within the ranges of variation of modern hyena tooth-marked assemblages for the specimen size range represented. The A.L. 894 data also fit with the tooth mark frequencies reported in felid-modified assemblages. These frequencies also support hominin-carnivore experimental scenarios but these are discarded due to the absolute lack of positive evidence (in the form of hominin-imparted marks) that they played any role in the accumulation and modification of the faunal assemblage. 6) The abundance of the least tooth-marked portion (fragments< 4 cm) of the specimen size distribution is also responsible for the low tooth mark estimates. Table 4 shows that A.L. 894 contains between twice and almost four times more small specimens than several assemblages reported from spotted hyena dens. It is obvious that this over-abundance of small fragments deflates the percentages of tooth marks and renders the archaeological sample not directly comparable with experimental or modern ethological faunal assemblages where the presence of these small specimens is substantially lower. None of the following arguments is conclusive in the interpretation of the authorship of the faunal assemblage at A.L. 894, but they can be used as further supporting arguments to interpret a non-anthropogenic origin. They can also be judged against the growing body of evidence from other very early sites. For example, over 5000 artefacts were also found at the site, most of them flakes (Goldman-Neuman and Hovers, 2009). The number of potential cutting-tools present at the site exceeds the minimum number of flakes required to butcher a gazelle carcass and a large ungulate limb (even a few dozen of complete carcasses) by a magnitude of several hundred. The butchery of the carcasses represented at the site by postcrania could be successfully accomplished with less than 30 flakes (see discussion in Domı´nguez-Rodrigo et al., 2007a). The virtual lack of anvils and hammerstones, necessary for the breaking of the bones, also decreases the probability that the stone tools and bones at the site were functionally related. It could be argued that if the two types of archaeological remains (stone artefacts and bones) were not functionally related, the faunal remains should exist also in sediments unrelated to hominin activity, such as in sediments where there are no stone tools. At A.L. 894, the number of bones is highest in the

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stratigraphic zone where the number of lithics is highest, and neither type occurs below or above the occupation horizon (Hovers, pers. comm.). This spatial association criterion has guided hominin authorship of assemblages in Paleolithic sites for decades. Spatial association is easily obtained when more than one agent overlaps in the use of the same spot. Riverine environments seem to have undergone an intense overlap in spatial use by hominins and carnivores during the early Pleistocene (Domı´nguez-Rodrigo et al., 2007a). The functional association of lithics and bones has to be demonstrated instead of assumed, and the best way to document such an association is with the presence of hominid-imparted marks (percussion and cut marks). The excavation of A.L. 894 is not big enough to make such a claim that where lithics appear bones do or they do not appear in functional association. First, there is an important vertical distribution over 60 cm (Hovers, pers. comm.) even if most of the materials cluster in about 30 cm (Goldman-Neuman and Hovers, 2009), and that implies time-averaging; things were deposited in different moments along the vertical section. Second, the excavation embodies 21.5 m2 (Goldman-Neuman and Hovers, 2009) of an area whose features (i.e., presence or absence of trees and bushes, erosion) are not known, which could justify why both types of materials occur together. Because there are over 5000 lithics in this small space, virtually everything else occurring there will be spatially associated. There is no room for archaeological items not to be spatially dissociated. For example, the microfaunal remains show perfect spatial association with lithics too. To claim that bones are functionally associated with lithics because there are no bones where there are no lithics, it would be necessary to go off site and show that there are sections along the nearby landscape where both materials occur separately. This should be done by random sampling of the paleolandscape and not targeting specific localities where, for instance, conglomerates may have existed (Goldman-Neuman and Hovers, 2009). Even so, the occurrence of specific landscape features attractive to more than one type of agent (for example, a spring) can independently draw multiple agents whose activities will have overlapped in the same space and the resulting material accumulations of such activities will contrast with the surrounding landscape. The overwhelming taphonomic evidence of carnivore accumulation and consumption of carcasses at some of the well-preserved Olduvai sites in absence of any involvement of hominins (why would bone modifications created by carnivores, but not hominins, preserve on bones?) indicates that independent overlap in the use of the space by both agents occurred frequently (Domı´nguez-Rodrigo et al., 2007a). In those sites, stone tools were also accumulated by the hundreds. This means that certain landscape spots were attractive for more than one agent and each of them left their imprint, forming excellent palimpsests. This suggests that in the absence of direct evidence for a functional link between tools and bones, mere spatial association is no longer a tenable argument from an epistemic point of view (as initially suggested by Binford, 1981). In the apparent absence of taphonomic evidence of hominin involvement with the fauna at A.L. 894, this overlap by hominins and carnivores in the same space could have resulted in the independent accumulation of bones and stone artefacts. The scanty remains of some taxa (for example, crocodile or bushbuck) further show that the site was in an area where background bone scatters occurred without the need of either hominins or carnivores to accumulate them. The present study does not exclude any hominin butchery activity at the site, but indicates that such evidence can only be produced in future excavations of the site, because it cannot be sustained with the available evidence from the excavated bone assemblage.

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Conclusions Butchery remains an elusive topic in early Pleistocene archaeology before 2 Ma. There is no doubt that it was practised by hominins at some sites (Domı´nguez-Rodrigo et al., 2005; de Heinzelin et al., 1999). However, it is not a behavioural feature that can be widely documented, given the number of sites of this age that are currently available for taphonomic study. A preservation bias in the faunal assemblages at some sites could be partly (or mostly) responsible for that. The relative abundance of archaeological sites dated to 2.6-2.0 Ma (particularly in Gona/Hadar) with optimal preservation of stone tools – especially considering that the basaltic materials of which these tools were made are highly vulnerable to weathering – also argues in favour of contemplating other activities that may have been equally (or maybe even more) important than butchery, including plant processing. This broader perspective may help us to understand the emergence of the earliest stone tools and the key role that they played in the human evolutionary process. There is strong evidence that the sample of bones at A.L. 894, although small, shows that a large carnivore (probably a hyena) broke some of the bones from the small- and large-sized carcass postcranial elements at the site. This implies that hominins did not extract the marrow from these particular specimens. The frequency of tooth-marked shafts at the site is similarly low compared to numbers reported for modern hyena den assemblages when specimen size distribution is considered in the same proportion as that documented for A.L.894 (where most of the bones are <4 cm). There is a possibility that the hominin signal in the assemblage has been obliterated and is not taphonomically perceptible due to the small sample size. However, scientific interpretations must be parsimonious and based on positive empirical evidence rather than negative evidence. This means that the null hypothesis to be tested by future research at the site is that carnivores were responsible for the exploitation of the faunal assemblage at A.L. 894. This, by no means, excludes the possibility of hominins having some authorship in the assemblage or part of it. However, this possibility remains elusive at present, because it cannot be scientifically supported. Acknowledgements Fieldwork at A.L. 894 was supported by NSF grant BCS-0080378 to W. H. Kimbel and NGS grant #7352-02 to Erella Hovers. Laboratory work at the National Museum of Ethiopia, Addis Ababa, was supported by the L. S. B. Leakey Foundation (grants awarded to Erella Hovers and BMN). BMN also thanks the Spanish Ministry of Science and Education (projects CGL2006-13808-CO2-01 and CGL2009-08827). We would like to thank the Authority for Research and Conservation of Cultural Heritage (ARCCH) of the Ministry of Youth, Culture and Sports Affairs of Ethiopia for research permit. We appreciate the help of the National Museums of Ethiopia, where the study of the A.L. 894 faunal assemblage was conducted, for their assistance. We are thankful to Julian Kerbis for allowing us to have access to the Maasai Mara hyena den bone assemblage [explain more in Methods]. MDR is also thankful to Mathayo Mchawi from ICIPE (International Centre of Insect Physiology and Ecology) and Geoffrey Otieno from National Museums of Kenya for their help in preparing the samples and for their study in the scanning electronic microscope at ICIPE. MDR also wishes to acknowledge the assistance by E. Mbua, P. Kyara and A. Grossman while the analysis of the samples collected was finished at the National Museums of Kenya. MDR and BMN thank EH for the invitation to study the osteological collection from A.L. 894. MDR thanks M. Prendergast for her very useful suggestions and editorial

help. We also thank S. Leigh for his very useful scientific and editorial comments and N. Uhl for her editorial assistance.

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