Raw material quality and Oldowan hominin toolstone preferences: evidence from Kanjera South, Kenya

Journal of Archaeological Science 36 (2009) 1605–1614

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Raw material quality and Oldowan hominin toolstone preferences: evidence from Kanjera South, Kenya David R. Braun a, *, Thomas Plummer b, Joseph V. Ferraro c, Peter Ditchfield d, Laura C. Bishop e a

Department of Archaeology, University of Cape Town, Rondebosch 7701, Western Cape, South Africa Department of Anthropology, Queens College, CUNY & NYCEP, 65-30 Kissena Boulevard, Flushing, NY 11367, USA c Department of Anthropology, Baylor University, One Bear Place #97173, Waco, TX 76798-7173, USA d Research Laboratory for Archaeology and the History of Art, University of Oxford, Oxford OX1 3QY, UK e Research Centre in Evolutionary Anthropology and Palaeoecology, School of Natural Sciences and Psychology, Liverpool John Moores University, Liverpool L3 3AF, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 November 2008 Received in revised form 12 March 2009 Accepted 13 March 2009

The role of raw material quality in Oldowan technology has not been fully explored. There are numerous studies suggesting Oldowan hominins preferred certain types of stone for artifact manufacture. Previous studies of the artifact assemblage from the early Pliocene Oldowan locality of Kanjera South (South Rachuonyo District, Kenya) show that raw material selection and transport was an important aspect of Late Pliocene hominin adaptations. Yet the exact properties of stones that hominins were selecting remain enigmatic. Two potentially important features of artifact raw material are durability and fracture predictability. We investigate fracture predictability through mechanical tests of stone and investigations of the affect of stone properties on fracture patterns in archaeological collections. We investigate stone durability with actualistic studies of edge attrition combined with further mechanical tests of various lithologies. Oldowan hominins at Kanjera appear to have selected raw materials based on their durability. The ability for a stone to fracture consistently does not appear to be as important in hominin toolstone preference as previously assumed. Hominins that produced the assemblages at Kanjera South appear to have incorporated an extensive understanding of various attributes of raw material in the transport and production of stone artifacts. When combined with previous research on the transport patterns at Kanjera, the results of this study provide evidence for a more complex raw material acquisition strategy than has previously been suggested for Late Pliocene Oldowan hominins. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Oldowan Stone tools Transport Raw material quality Geoarchaeology Lithic technology

1. Introduction Early in the investigation of Oldowan hominin behavior, Isaac (1972) recognized that, despite the clear connection between the physical properties of stone and Oldowan hominin technology, there were surprisingly few formal systematic or experimental attempts to explain technological variation with this approach. More than 35 years later, the application of stone properties has yet to be fully explored in discussions of Oldowan technology (Braun, 2006; Harmand, 2004; Stout et al., 2005). Many archaeologists agree that raw material quality imposes serious technological constraints on artifact production and use (Andrefsky, 1994; Crabtree, 1967; Inizan et al., 1992; Kuhn, 1995, 1992; Luedtke, 1992; Roth and Dibble, 1998). It is assumed that the ability for a rock to fracture predictably will hamper the consistent

* Corresponding author. Tel.: þ27 21 650 2350; fax: þ27 21 650 2352. E-mail address: [email protected] (D.R. Braun). 0305-4403/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2009.03.025

production of large flake elements (Brantingham et al., 2000). In Middle and Late Pleistocene technological adaptations, raw material quality is a major factor in the organization of flaked stone industries (Andrefsky, 1994; Brantingham et al., 2000; Luedtke, 1992; Meignen and Bar-Yosef, 1988; Orton, 2008; Schiffer, 1979; Tavoso, 1984). Where significant variability in raw material quality exists, measures of the intensity of utilization (Geneste, 1985, 1988; Hiscock, 1981; Hiscock and Clarkson, 2005; Holdaway et al., 2008), and degree of raw material transport (Beck and Jones, 1990; Beck et al., 2002; Kuhn, 1991), track this variability. The affect of raw material quality variation on artifact morphology may be muted by the availability of suitable stone for artifact manufacture (Roth and Dibble, 1998). In contrast to the extensive literature on the interaction between raw material quality and technological strategies in the Middle and Late Pleistocene, little is known about how (or if) lithic material properties influenced Oldowan hominin toolstone preference (but see Stout et al., 2005). Although Oldowan technology is often seen as a ‘‘least effort solution’’ to producing sharp edges (Isaac and Harris, 1997), it is

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clear that the technological design of some Oldowan assemblages is more complex (de la Torre, 2004; de la Torre et al., 2003; Roche et al., 1999). Some aspects of Oldowan technology require the control of fracture to implement a specific reduction strategy (Delagnes and Roche, 2005; de la Torre, 2004). The Plio-Pleistocene hominins that produced the assemblage in the Koobi Fora Formation demonstrated an aversion to vesicular lavas and cobbles with weathering flaws when selecting raw materials for artifact manufacture (Toth, 1982). New research has shown that hominins at some Pliocene sites in West Turkana selected specific raw materials for the implementation of certain techniques because of the internal structure of these rocks (Harmand, 2007, 2005). Unfortunately, the majority of our understanding of the affects of raw material quality on technological organization relies on a poorly defined dichotomous distinctions (high vs. low raw material quality) (Andrefsky, 1994). If stone tool technology was a major component of Late Pliocene hominin adaptation, we would expect stone tool production, transport and discard behaviors to be sensitive to various measures of raw material quality. To test this hypothesis it is necessary to develop independent, ratio-scale measures of raw material quality that are both replicable and technologically relevant (Luedtke, 1992; Stout et al., 2005). Studies of physical properties of stone used in chipped stone industries have a long history (Goodman, 1944; Purdy and Brooks, 1971). Investigations of the physical properties of stone associated with stone artifact manufacture suggest that raw material quality is quantifiable, yet complex (Brantingham et al., 2000; Domanski and Webb, 1992; Domanski et al., 1994; Noll, 2000; Webb and Domanski, in press). Exactly which physical attributes of stone characterize high quality raw materials is not straightforward and may vary across time periods and even across space within a single timeframe (Domanski et al., 1994). In this study, we identify two properties of stone which may influence Oldowan artifact production: fracture predictability [the consistency with which a particular type of stone fractures, sometimes referred to as ‘‘flakability’’ (Domanski et al., 1994)]; and durability [the ability of an edge to resist degradation by a static or dynamic force]. The ability to predict the fracture pattern of a stone is obviously important when certain core reduction trajectories must be maintained over the use life of a tool (Bamforth, 1986; Roth et al., 1998; Shott, 1989, 1986; Tavoso, 1984). The durability of stone edges may have been important when tasks that increase raw material consumption were frequent in hominin activity patterns (Brantingham, 2003; Braun et al., 2008c; Dewbury and Russell, 2007). These different aspects of raw material quality can be investigated independently using techniques that isolate discrete aspects of rock physical properties. Previous studies have focused on different aspects of rock mechanical properties to address archaeological questions in younger assemblages (Brantingham et al., 2000; Cotterell and Kamminga, 1990; Lerner et al., 2007). However, few studies have quantified the physical properties of several raw materials relative to hominin toolstone preference (although see Domanski and Webb, 1992; Domanski et al., 1994; Webb and Domanski, in press). Controlled laboratory experiments are critical for understanding physical properties of rocks (Atkinson, 1987; Jaeger and Cook, 1979). However, it is necessary to understand the influence of physical properties of stone relative to the mechanics of stone tool use and manufacture. If we can identify which properties of stone influenced raw material selection by Oldowan hominins, we may be able to understand the importance of certain types of stone in Oldowan toolkits. In this study, we investigate raw material quality as it pertains to Oldowan hominin raw material selectivity at the Pliocene site of Kanjera South. We capitalize on the wide diversity of raw materials

at the Oldowan site of Kanjera South (Bishop et al., 2007; Braun et al., 2008b; Plummer et al., 1999) to gain insights into the relationship between Oldowan technology and ratio-scale (i.e. not ordinal) differences in raw material quality. This site is located in the South Rachuonyo District of western Kenya. Research at this archaeological site has been conducted by the Homa Peninsula Paleoanthropology Project over the last 20 years. Details of the geology (Behrensmeyer et al., 1995; Ditchfield et al., 1999); context (Bishop et al., 2007; Plummer, 2004; Plummer et al., 1999; Plummer and Potts, 1989) and artifacts (Braun, 2006; Braun et al., 2008a,b) can be found elsewhere. This assemblage is ideal for studying the influence of physical properties of raw materials on hominin selectivity at Kanjera South because of the extensive information already known about the availability of certain types of stone (Bishop et al., 2007; Braun et al., 2008b; Ditchfield et al., 1999; Plummer et al., 1999). We conduct a series of actualistic experiments designed to link the physical properties of stone to functional aspects of stone artifacts that are relevant to an investigation of Oldowan hominin behavior. Results show that Oldowan hominins selected specific raw materials for relatively long distance transport. Stones with high durability values were selected by hominins at a greater frequency than would be expected by random selection. This pattern suggests that Oldowan hominins based raw material selection decisions on the potential durability of stone edges. The importance of fracture predictability present in later time periods (Andrefsky, 1994; Brantingham et al., 2000; Dibble, 1991) is not as evident at Kanjera.

2. Background 2.1. Aspects of raw material quality The qualities of stone that affect selection by toolmakers are variable. When archaeologists refer to ‘‘high’’ quality raw materials they are usually referring to lithologies that are brittle, elastic, and isotropic (Cotterell and Kamminga, 1987; Cotterell et al., 1985). Materials that deform or undergo a reversible change are described as elastic. Brittle materials break at or near yield stress (Cotterell et al., 1990). Isotropic materials lack material properties that are direction-dependent, so that the propagation of fracture will follow the direction of applied force as opposed to the internal structure of the rock (Crabtree, 1967; Domanski et al., 1994). The predictability with which different raw materials break is extremely variable (Amick and Mauldin, 1997; Bradbury et al., 2008; Jones, 1979; Patterson, 1993), and severely limits the implementation of specific core production modes on some raw materials (Luedtke, 1992). Even common technical elements in later time periods, such as the Levallois technique, are restricted by access to high quality raw material (Dibble, 1991; Veyrier et al., 1951). Many investigations of raw material quality build upon the extensive knowledge of modern day flintknappers. Physical properties such as homogeneity or crystallinity have been emphasized by replicative flintknappers as important aspects of high quality raw material (Whittaker, 1994). These experimental studies isolate those properties which are significant for certain artifact production modes (Inizan et al., 1999). Callahan’s (1979) ranking system measures the ease of workability of stone based on his experience with artifact reproduction. This scale ranks materials from elastic (e.g. opal) through strong (e.g. coarse quartzite). Although skilled flintknappers have become very adept at assessing physical characteristics of stone, these evaluations are usually informal, and suffer from difficulties associated with applicability to statistical testing (Stout et al., 2005). Luedtke (1992) notes that although Callahan’s scale is based on the concept of ‘‘workability’’ actual

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mechanical measures of strength actually peak in the middle of his scale rather than either end. Other studies of raw material quality instead build upon the use of different raw materials in the archaeological record. Brantingham et al. (2000) found that percent crystallinity, average crystal size, crystal size range, and impurity abundance restricted the implementation of certain techniques in Asian Paleolithic industries. Raw material selectivity among Oldowan hominins at Gona was mediated by phenocryst size, percentages of phenocrysts on a fresh surface, and groundmass texture (Stout et al., 2005). Sometimes coarse-grained materials, considered to be of ‘‘low’’ quality by archaeologists, are actually preferred by toolmakers. Coarse-grained rocks are sometimes preferred because of the durability of their cutting edges (Hayden, 1977). Ethnographic evidence suggests that the ability of a stone to resist edge degradation is a significant factor in selection of raw material for stone manufacture (Hayden, 1979, 1977). Raw material selection based on the durability of edges has also been documented in some archaeological studies (Villa, 1983). Fortunately, fracture predictability (Doelman et al., 2001; Domanski et al., 1994; Luedtke, 1992; Webb et al., in press), is not directly correlated with the stone durability (Jaeger and Cook, 1979). These two physical properties can be investigated separately, so that their individual effect on raw material preference can be teased apart. Some rocks types, such as partially silicified carbonate rocks, may have high values in mechanical tests associated with fracture consistency (e.g. elasticity Inizan et al., 1999), but they can nevertheless easily be abraded (Luedtke, 1992). It is thus important to investigate both of these aspects of stone physical properties when trying to understand hominin raw material preferences. To distinguish the physical properties of stone most important in hominin raw material selection, it is necessary to accurately define these raw material properties, and link these to the archaeological record with experimental studies. Clearly the dichotomy between ‘‘high’’ and ‘‘low’’ quality raw material obfuscates more complex patterning in hominin selection of stone for artifact manufacture. 2.2. Fracture predictability Quantifying fracture predictability in a way that is relevant to stone tool manufacture is difficult because of the complexities of toolstone fracture mechanics (Pelcin, 1997a,b). Increased consistency of fracture is associated with longer flake removals, fewer step and hinge terminations, and the production of sharper edges (Bleed and Meier, 1980; Crabtree and Butler, 1964; Domanski et al., 1992). Toolkits made on materials that fracture predictably are thought to be easier to design because fracture is easier to control (Luedtke, 1992). Most often when a knapper initiates a crack in a rock the fracture propagates around grains in the rock (intergranular) rather than through grains (transgranular) (Webb et al., in press). Fractures usually initiate near microscopic cracks that exist in the rock structure (Cotterell et al., 1990; Griffith, 1921; Jaeger et al., 1979). Fractures are more likely to propagate at grain boundaries because impurities are concentrated there. Fractures are more likely to follow impurities in rocks with a larger grain size. Quite often these impurities and internal cracks, products of bonds between minerals, affect properties of stone such as elasticity (Luedtke, 1992). Stone materials that fracture predictably commonly possess little or no crystalline structure and few impurities that interfere with fracture propagation (Brantingham et al., 2000). In addition materials that fracture predictably often have an overall small average crystal size (Brantingham et al., 2000; Domanski et al., 1992; Murata and Norman, 1976). Modern flintknappers and engineering studies cite elasticity as an important feature in artifact

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manufacture (Domanski et al., 1992; Inizan et al., 1992). Studies of fracture propagation show that the stability of fracture propagation is directly related to rock strength and elasticity (Cotterell et al., 1990; Cotterell et al., 1985). This property of stone often correlates with rock density and grain size, both of which are linked to stone fracture predictability. Several studies have tested different aspects of the physical properties of raw materials in relation to stone tool manufacture (Bradbury et al., 2008; Domanski et al., 1994, 1992; Webb et al., in press). These mechanical properties are often associated with the frequency of microscopic cracks that are usually related to the grain size of the rock (Jaeger et al., 1979). In short, rocks with larger grain sizes absorb energy less effectively than finer-grained lithologies (Cotterell et al., 1990). Similarly, rocks with inhomogeneities and larger cracks tend to have lower overall strength because they deform more readily (Luedtke, 1992). The ability of rocks to resist strain is often calculated using Young’s modulus. In general, the higher the Young’s modulus score, the more resistant a rock is to strain and deformation (Luedtke, 1992). This measure is valuable in studies of fracture mechanics as it can be used to calculate the stability of the path of fracture through a rock (Cotterell et al., 1990). Previous studies have shown that non-destructive assessments of rebound hardness are directly correlated with the ability of a rock to withstand strain (Katz et al., 2000; Yilmaz and Sendir, 2002). Yasar and Erdogan (2004) have shown that rebound hardness values are correlated with other measures of rock strength and elasticity in different rock types. These studies have also shown that a lack of homogeneity in stone is a major factor leading to lower rebound hardness values. Measures of Young’s modulus, as well as rebound hardness values, are known to reflect the consistency with which rocks fracture (Domanski et al., 1994; Noll, 2000). In this study, we investigate the relationship between rebound hardness and fracture consistency associated with stone tool manufacture. Rebound hardness is most commonly measured with a Schmidt Hammer, a hand held device that uses a calibrated spring and steel plunger to record the ability of a stone mass to absorb energy (Goudie, 2006). The Schmidt hammer test is ideal for investigating the relationship between rock hardness and stone tool manufacture for several reasons. First, Schmidt hammer test produces consistent results that can be compared across a variety of rock types (Goudie, 2006). Second, the Schmidt hammer can test samples that are similar in size to stone artifacts (i.e. 100s– 1000s cm3). This is important because many measures of rock strength are directly correlated to physical sample size (Jaeger et al., 1979). In contrast, other mechanical tests often involve large volumes of rock under extreme pressures, a context that may not be relevant for archaeological studies (Luedtke, 1992). Finally, the Schmidt Hammer test can be done quickly and easily on a large number of rock samples, facilitating a greater understanding of variation in rock hardness within rock types. In this study we investigate rebound hardness values on a variety of rock types, and link this measure to inferences of fracture consistency measured from the Oldowan archaeological assemblages from Kanjera South. 2.3. Durability Ethnographic studies have shown that toolmakers preferred coarser-grained materials over finer-grained varieties because the former materials had tougher cutting edges and were less likely to dull (Hayden, 1977). Similar material preferences are invoked to explain typological composition of archaeological assemblages under the assumption of links between tool type and function (Kimura, 1999; Mosquera-Martinez, 1998). Durability of stone edges is known to be influenced by the size, hardness and angularity of the abrading particles, as well as the

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presence of fluids between abraded materials (Kamminga, 1979, 1982). Abrasion sometimes occurs in the form of auto-abrasion, which is caused by small pieces of the tool edge breaking off and lodging between the tool and material being worked (Cotterell et al., 1990). The ability of a rock to resist this edge degradation is linked with grain size, presence of water in the rock, and cleavage patterns (Luedtke, 1992). One of the most widely cited measures of rock durability is Moh’s hardness scale (Winkler, 1973). This measure is difficult to apply when addressing archaeological questions, as the steps in Moh’s hardness scale are not ordinal-scale (e.g. diamonds are more than three times harder than corundum but are only one step above corundum in Moh’s scale). Furthermore, the vast majority of rock types used for artifact manufacture fall within a limited range of the Moh’s hardness scale (Hayden, 1979). Thus, other measures of hardness may prove more valuable for interpreting archaeological data. Both Noll (2000) and Cotterell and Kamminga (1990) have investigated measures of hardness (e.g. Los Angeles Test, Taber Abrasion test) in reference to archaeological materials. The Taber abrasion test has proven particularly useful for understanding tool edge attrition because it simulates the resistance of a rock to displacement of particles by a tangential abrasion force (Cording and Alvarez, 1992). Here we use actualistic experiments to connect a similar measure of hardness (American Standard Abrasion Hardness test: ASTM C241) to the ability of a stone tool to resist edge degradation. 3. Methodology To test the applicability of engineering tests to questions surrounding Oldowan hominin raw material preferences, it was necessary to first use actualistic studies to isolate archaeologicallyrelevant measures of raw material quality that were replicable. We then determined the degree to which these measures are correlated with specific engineering tests. Here we outline the methodologies employed to investigate the link between engineering tests and actualistic/archaeological measures of raw material durability and fracture predictability. 3.1. Fracture predictability It is difficult to use modern flintknapping studies to quantify fracture predictability because most results will, at least partially, reflect flintknapper skill in circumventing flaws within the raw materials (Ahler, 1989; Brantingham et al., 2000; Stout, 2002). Previous studies have assessed the consistency of rock fracture by focusing on the mineralogical and crystalline structure of rocks in relation to raw material preference in artifact production (Brantingham et al., 2000; Stout et al., 2005). Brantingham et al. (2000) have quantified this relationship by measuring encounter rate of impurities in rocks. Higher encounter rates of impurities result in less predictable fracture patterns. The present study mirrors previous analyses that suggest rock defects play a role in fracture mechanics (Domanski et al., 1994). Brantingham et al. (2000) quantified this encounter rate by recording the number of impurities seen on an artifact divided by the number of flake scars on an artifact. They termed this as an ISCAR value (i.e. impurities per flake scar) a measure which has been shown to be a powerful tool for understanding fracture consistency. Here we investigate the impurity encounter rate on archaeological materials from the Kanjera South archaeological collection (Braun, 2006). This study focused on specific raw materials that were incorporated into previous geochemical provenance studies (Braun et al., 2008b). Rock types from southwestern Kenya in the experimental study include: Nyanzian Rhyolite; Nyanzian Chert; Bukoban Felsite; Bukoban Basalt; Bukoban Quartzite; fenetized

Andesite/Rhyolite; Phonolite and Carbonatite from the Homa Mountain; and Limestone from the southern shores of the Kavirondo Gulf. ISCAR values were collected on all complete flakes from these rock groups from all levels of Excavation 1 at Kanjera South (see Table 1). A wider series of rock types were included in the study of engineering properties of stone. Granodiorite from the Miriu region, Granite from the Oyugis region of southwestern Kenya, and Basalts from the Kendu fault region were excluded from the analysis because of small archaeological sample sizes. The Basalts from the Kendu fault region have undergone intense metamorphism and are generally referred to as ‘‘schistose rocks’’ (Braun, 2006; Braun et al., 2008b; Saggerson, 1952). Depending on the nature and extent of each outcrop exposure, at least 10 samples of each rock type were selected for study. A total of 183 separate rock outcrops were sampled, resulting in 366 analyses. All Schmidt Hammer analyses were conducted at the Kenya Ministry of Works, Materials Testing Laboratory. To insure the accuracy of the Schmidt Hammer tests, all specimens were analyzed measured at least 10 cm  10 cm  10 cm. All samples were checked for macroscopic evidence of internal defects, and only unweathered rock surfaces were analyzed (Xu et al., 1990). The Schmidt Hammer apparatus was used in an exactly vertical orientation on each specimen (Yasar and Erdogan, 2004). To counteract these potential sources of error (e.g. slippage during testing, cracking during testing), individual rock samples were tested 18 separate times. The resulting values for each individual rock sample were then ranked from highest to lowest, and an average taken of the middle ten values (Goudie, 2006).

3.2. Durability The measurement of durability has not had the same level of archaeological investigation as fracture predictability (although see Hayden, 1979). We conducted an extensive actualistic study, to understand this attribute of stone hardness relative to Oldowan tool use. We devised experiments that would create a standardized level of edge attrition, within a naturalistic context to make these experiments relevant to questions of Oldowan behavior (Braun et al., 2008c; Levins, 1966). Three whole flakes [detached piece with a complete platform and intact distal edge with a feather termination (as described in Braun et al., 2005)] were experimentally produced from each of the raw materials described above (except for Bukoban Basalt and Nyanzian Rhyolite which were not included in this part of the experimental study). Each flake was of a standardized size: 4–6 cm in length and had an average edge angle of 25–35 as measured with a goniometer. These dimensions were based on: (1) average dimensions found in the Kanjera South Oldowan assemblage and (2) edge angles described in the ethnographic literature that are associated with butchery activities (Gould et al., 1971). Furthermore, all edges selected for this experiment were straight because of the documented variation in edge attrition associated with edge shape (Collins, 2008). All experiments were conducted by an adult male from the Samburu tribe of the Laikipia region of central Kenya who had more than fifteen years experience with butchery activities. For each experiment, we drew, with an indelible marker, an area 2 cm long and 0.5 cm wide on the edge of each flake. We termed this circumscribed area of the flake the ‘experimental region’ (Fig. 1). The butcher was instructed to only use that area of the flake in the experiments. Each flake was used in one of three separate experiments designed to standardize levels of edge attrition. In the first experiment, the butcher was instructed to deflesh a size 2 bovid (Klein, 1976) carcass (domestic goat) for 1000 strokes, following Egeland (2003). In the second experiment, the butcher skinned a domestic goat for 1000 strokes.

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Table 1 Sample sizes and values for the various measures of fracture predictability incorporated into this study. Raw material

Artifacts analyzed for ISCAR values (n)

Median ISCAR value

Hr (n)

Median Hr value

Hr interquartile range

Nyanzian Chert Nyanzian Rhyolite Bukoban Felsite Bukoban Basalt Bukoban Quartzite Fenetized Nyanzian Homa Phonolite Homa Limestone Homa Carbonatite Total

64a 94 127 66 67 527 150 79 79a 1253

0.79 0.32 0.28 0.63 0.1 2.25 1.1 0.6 1.95

10 10 14 14 14 31 21 10 31 155

51.3 60.9 54.8 44.3 66.1 32.5 46.7 57 32.3

45.4–61.6 56.3–62.9 47.4–57.4 41.0–48.4 65.7–68.2 27.9–38.2 40.5–61.7 56.1–57.9 28.1–37.3

a Samples augmented with actualistically-produced artifacts; Nyanzian Chert: Archaeological sample (16), Actualistic sample (48); Homa Carbonatite: Archaeological sample (22) Actualistic sample (57); values for archaeological and actualistic samples do not differ significantly.

In the final experiment the butcher skinned a domestic goat for 500 strokes. We captured digital images of the experimental region on each flake before and after each experiment (Braun, 2005; Braun et al., 2008c; McPherron and Dibble, 1999). Following the methodology described in previous studies (Braun, 2005; Braun et al., 2008c) the area of the experimental region was calculated from a digital image using Image J 4.0.1 software before and after each experiment (Fig. 1). We made these calculations three separate times for each image to prevent error introduced by subtle changes in pixel color which could affect the visual determination of the experimental region. These values were then averaged to determine the area of the experimental region for each image. This calculation of the experimental region was conducted before and after the whole flake was used in the experiments. Although we attempted to standardize the area of the experimental region (1 cm2), the irregularity of some flake edges on a small scale (<1 mm) resulted in a range of initial area measurements. Subsequently, flake edge attrition was recorded as the percentage lost of the original size of the experimental region. Edge attrition thus represents the percent difference in the size of the experimental region before and after each experiment. To connect the results of these actualistic experiments of edge attrition to engineering properties of the different lithologies found

in the Kanjera South Oldowan assemblage, we conducted a series of abrasion hardness tests. Previous studies have shown the utility of the Taber abrasion hardness test for understanding hominin raw material selectivity (Noll, 2000). At each outcrop sampled for rebound hardness (see above), we collected two extra samples for abrasion hardness testing. We then conducted a modified form of the Taber abrasion test called the American standard abrasion hardness test (ASTM C241). For each test, we used a water-cooled lapidary saw to shape rock samples into a cube measuring 5 cm  4 cm  2.5 cm. Sharp edges were grounded off to prevent edge crumbling that may have affected test values. The cubes were then placed in a ventilated oven for 48 h at 140  F. During the 46th through the 48th h, each sample was repeatedly weighed to document that all relevant moisture has been removed from the specimen. We weighed the specimens to the nearest 100th of a gram on a digital scale. Sample densities were then determined by weighing each sample in water. The drying process was then repeated. Lastly, each sample was then placed in the abrasion apparatus with the largest surface exposed to a grinding lap. The grinding lap had a standard abrasive and each sample was exposed for 225 revolutions. The specimens were then weighed a second time. Abrasion hardness values were calculated using the following formula:

Ha ¼ 10  G  ð2000 þ Ws Þ=2000  Wa where G is the specific gravity of the sample (calculated prior to the analysis); Ws is the average mass of the sample (sum of original mass and final mass divided by 2); and Wa is the mass lost during the experiment (Alexander, 1985; ASTM, 2005). 4. Results To assess the sensitivity of hominin raw material selectivity to different physical properties of stone required a combination of actualistic and laboratory based investigations of lithologies used in the production of artifacts. Here we compare actualistic and archaeological measures of raw material quality with engineering tests of similar mechanical properties. This will provide the basis for comparing rock mechanical studies with measures of hominin selectivity. Estimates of hominin raw material selectivity used in this analysis are based on previous studies of hominin selection and transport at the Kanjera South Oldowan locality (Braun et al., 2008b). 4.1. Fracture predictability

Fig. 1. The calculation of edge attrition on experimental flakes. The experimental region is outlined and an example of edge attrition is depicted.

The rock types incorporated into this study exhibit a diversity of rebound hardness values (Hr) that range from 11.72 to 75.54. (Hr) values of rock samples from the same outcrop show remarkable

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Fig. 2. Regression between rebound Hardness (Hr) and the ISCAR value which is calculated as the number of impurities per flake scar (Brantingham et al., 2000).

consistency. Average differences in rebound hardness values between samples from the same sources are less than 3.18 (4.5% of the total range of values) and differences between raw materials are distinct (ANOVA; df ¼ 10; p < 0.001). ISCAR and Hr variables are highly correlated. Over 80% of the variation in Hr values can be explained by the variation in ISCAR values (Pearson r ¼ 0.89; p ¼ 0.001; Spearman’s rho ¼ 0.837; p ¼ 0.009; Fig. 2). The archaeological dataset reflects a similar range in raw material quality as reflected in ISCAR values. Extremely high values were recorded for the heavily fenitized Nyanzian rocks (2.25 impurities per flake scar). This is expected as this rock has undergone a major metasomatic reorganization. Very low ISCAR values are also to be expected from the very fine grained Nyanzian Rhyolites and Bukoban Quartzite (0.32–0.12 impurities per flake scar). 4.2. Durability The results of the actualistic experiments show that variation in the intensity of different tasks (i.e. number of strokes) results in higher degrees of edge attrition (Braun et al., 2008c). Raw materials in this study vary widely in their tolerances for edge abrasion and differences between raw materials are distinct (ANOVA; df ¼ 5; p < 0.001; Table 2). For each experiment there is a significant relationship between edge attrition and Ha (Abrasion hardness; Table 2). Although there is some variation in edge attrition that cannot be explained by variation in Ha, the general trend suggest we can use Ha as a proxy for the ability of a particular raw material to resist edge degradation during cutting activities. Analysis of the data suggests the relationship between Ha and edge attrition is continuous (i.e. not step-wise), though not linear. The relationship between edge attrition and Ha appears to be slightly different depending on different tasks (butchery vs. skinning) suggesting the relationship between Ha and the use life of flakes will depend on the variation in tool function (Shott, 2002; Shott and Sillitoe, 2004,

2001). However the general trend is that rock types with higher Ha values have increasingly smaller percentages of area lost during the different experiments (Fig. 3). Rock types with Ha values greater than 50 appear to be unaffected by even the most abrasive conditions. During the actualistic experiments it was possible to observe the result of these differences in tool utility. During the experiments 500 strokes with a Homa Limestone flake was not able to remove the skin from one limb of the domestic goat. The flake became dull after 200 strokes and the efficiency of cutting decreased rapidly thereafter. In contrast, 500 strokes with Bukoban quartzite flake were able to remove the skin from four limbs without significant change in the effectiveness of the tool edge. Artifacts made on rock types with high Ha values should have significantly longer potential use-lives because their edges will withstand greater degradation (Shott and Sillitoe, 2004, 2001; Shott and Sillitoe, 2005).

4.3. Application to the Oldowan: Kanjera Late Pliocene artifacts from Kanjera South are made on a wide variety of raw material types (Braun, 2006; Braun et al., 2008b). Previous studies have shown that hominins transported a large percentage of these materials to the site from secondary sources of raw material (conglomerates) located over 10 km away. Previous research has also shown that hominins appear to have been selective in their choice of raw material for transport to Kanjera (Braun et al., 2008b). Oldowan hominin selectivity in this study is measured by the difference between the availability of certain rock types in different river systems relative to their abundance (measured in frequency of the total assemblage falling into a specific raw material category) in the archaeological collection. We suggest that high frequencies of certain raw material types are the result of hominin selection based on the physical properties of these raw materials. Freeman–Tukey deviates were used as a measure of difference between the frequency of certain raw materials in conglomerates and their inclusion in the archaeological record (Braun et al., 2008a,b,c). A Freeman–Tukey deviate allows for an indication of the representative importance of a specific cell within a Chi-square test. The greater the positive value of a deviate the greater the frequency of that raw material type deviates from the expected values. Negative Freeman–Tukey deviates reflect raw materials that are underrepresented in the archaeological collection based on the expected values. The expected values are derived from the appearance of specific raw material types in the region around Kanjera (see Braun et al., 2008a,b,c for more details). The experimental and engineering tests conducted as part of this study show that it is possible to now determine which attributes of these rocks are influencing the decisions of hominin raw material selectivity.

Table 2 Details of the various measures of ‘‘durability’’ measured in this study. Details of the correlation between the various attributes are given at the base of the table. Raw material

Defleshing (1000 strokes) Skinning (500 strokes)

Skinning (1000 strokes)

Median Ha values (interquartile range) Ha sample number

Edge attrition (% area lost) Edge attrition (% area lost) Edge attrition (% area lost) Homa Limestone Homa Carbonatite Homa Phonolite Fenetized Nyanzian Nyanzian Chert Bukoban Quartzite

8.35 8.95 4.31 7.68 1.88 0.28

Correlation: % edge attrition and (0.96) [0.001] Ha (Spearman’s Rho) [p]

18.96 13.02 6.81 3.19 1.68 0.33

25.58 17.30 11.84 6.99 3.20 0.78

(0.89) [0.02]

(0.89) [0.02]

6.8 (6.1–7.8) 6.1 (3.5–8.3) 26.6 (21.4–34.9) 23.9 (20.1–33.1) 66.8 (57.5–74.1) 73.1 (59.5–78.4)

14 32 18 40 14 12

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Fig. 3. The relationship between abrasion hardness and experimentally-determined measures of edge durability at Kanjera South. Note the nature of the relationship between the two variables, suggesting that raw materials with abrasion hardness values over 50 are unlikely to exhibit any edge attrition in even extremely abrasive conditions.

Results show a relationship between Ha and the Freeman–Tukey deviates from previous analyses (Braun et al., 2008b). Rock types with high abrasion hardness values are found in the archaeological assemblage at higher frequencies than they are found in secondary sources of raw material (Spearman’s Rho ¼ 0.738; p ¼ 0.01; Fig. 4). The same pattern is not found in measures of Hr. The frequency of a rock type in the archaeological record is not necessarily explained by its rebound hardness value. (Spearman’s rho ¼ 0.59; p ¼ 0.06). 5. Discussion The experiments conducted as part of this study show that it is possible to measure attributes of raw material quality relevant to Oldowan hominin behavior. The mechanical properties measured in this study are easily reproduced and show statistically significant (although non-linear) relationships with the ability of Oldowan hominins to manufacture and use stone tools. By distinguishing between fracture predictability and durability it is possible to investigate the nature of hominin raw material selectivity further. The results of this study suggest that hominins at Kanjera South selected raw materials based primarily on their ability to resist abrasion, and not one their fracture predictability. Rock types that have relatively low Hr (rebound hardness) but high Ha (Abrasion hardness) values were frequently transported and utilized (e.g. Oyugis Granite). In other words, hominins tolerated raw materials with high impurity encounter rates if they maintained a sharp cutting edge. This finding is further bolstered by the fact that these materials were selected not on site, but 10 or more kilometers from their final discard location (Braun et al., 2008b). While fracture predictability may not have been the primary determinant in raw material selection, it may still have been appreciated. The raw materials selected at the highest frequency relative to their abundance in conglomerates are those that are both durable and fracture predictably (e.g. Nyanzian Quartzite). The results of our actualistic experiments, in concert with archaeological data from Kanjera South, suggest that hominins preferred rock types that resist abrasion. As shown in previous studies, skinning activities are exceptionally abrasive on tool edges (Braun et al., 2008c; Kamminga, 1982). In raw materials with Ha values less than 25 (e.g. Homa Limestone) the ability of tool edges to effectively skin or deflesh carcasses was seriously hampered after 200 strokes. This suggests that even simple butchery tasks would have exhausted the edges of tools made of these materials relatively quickly. As a result, hominins would have required a much larger quantity of flakes from these raw materials to process

Fig. 4. The relationship between two different measures of durability and fracture predictability and hominin raw material preferences at Kanjera South. Freeman–Tukey deviates describe the deviation of the archaeological assemblage from previously documented secondary sources of raw material. The two measures of rock hardness are Ha (abrasion hardness correlated with durability) and Hr (rebound hardness correlated with fracture predictability).

large mammal carcasses. The processing of large mammal carcasses in Oldowan contexts (de Heinzelin et al., 1999; Dominguez-Rodrigo et al., 2005) may have influenced hominins to preferentially select raw materials that could withstand the abrasive nature of these activities. The processing of tough plant resources would have also abraded tool edges (Gould et al., 1971). Recent analysis of residues confirms the importance of Early Stone Age toolkits in the processing of plant material (Dominguez-Rodrigo et al., 2001). Preference for raw materials with high abrasion hardness may help explain the high incidence of seemingly intractable raw materials at many Oldowan sites (e.g. quartz: Blumenschine et al., 2008; Chavaillon, 1976; de la Torre, 2004; Harris et al., 1987; Howell et al., 1987; Kimura, 1999; Merrick and Merrick, 1976). Previous assessments of hominin raw material selectivity have suggested that when hominins selected raw materials for artifact manufacture, they selected lithologies that fracture predictably (e.g. chert) (Stiles, 1998; Stout et al., 2005). Yet many of the raw materials that have been selected by hominins are rocks which have high silica content and are probably very durable. It is possible these materials were actually selected for their ability to resist abrasion. Another possibility is that the longer use life of these durable materials results in higher frequency of these materials (Brantingham, 2003). A further analysis of the technology associated with these different raw materials will investigate this question (Braun et al., 2008a). Hominins recognized differences in raw material physical properties and incorporated this information into their selection

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process. But how they assessed these differences is difficult to ascertain. Replicative flintknappers have suggested that certain subtle visual characters of stone attest to the rocks structural integrity (Inizan et al., 1999). Ethnographic evidence has suggested that flintknappers will use the sound of a rock when it is hit to measure its quality (Binford and O’Connell, 1984). The presence of tested cobbles may actually represent part of the evaluation process that hominins were incorporating into the acquisition and transport of raw materials (Ludwig and Harris, 1998). Whatever the evaluation process was, the integration of specific attributes of stone in the overall transport strategy at Kanjera South is suggestive of a more developed technological system than previously reported for Oldowan hominins. 6. Conclusion This study has demonstrated the relationship between the quantitative assessments of the mechanical properties of stone and Oldowan hominin raw material preferences. Several previous studies have suggested that Oldowan hominins preferentially selected high quality raw materials for the manufacture of certain tool forms (Harmand, 2006; Kimura, 2002, 1999; Stout et al., 2005; Texier et al., 2006). Although the concept of high quality raw material is rarely defined, it often assumed that certain types of stone were selected for the predictability with which they fractured. In the case of the Oldowan toolmakers at Kanjera South, we have explicitly tested different mechanical attributes of stone. This study shows that durability was probably more important than fracture predictability when hominins selected cobbles for artifact manufacture and transport. The wide variety of raw material types available on the Homa Peninsula, and the extensive raw material survey conducted as part of this study, facilitated a unique investigation of hominin toolstone preferences (Braun, 2006; Braun et al., 2008b; Plummer, 2004; Plummer et al., 1999). The particular aspects of raw material selection demonstrated here further emphasize the importance of tool mediated resources at Kanjera South. Combined with previous studies of transport, this work highlights the importance of stone tool technology in Oldowan hominin subsistence behavior (Ditchfield et al., 2001; Ferraro, 2007; Plummer, 2004; Plummer et al., 1999). Further research on the Kanjera Formation will investigate the interaction between the ecology of Oldowan hominins and variation in hominin technology. Acknowledgements This research on the Homa Peninsula and excavations at Kanjera South are part of a collaborative project of the National Museums of Kenya, Smithsonian Institution, and CUNY Queens College. We thank the Office of the President, Republic of Kenya, and the National Museums of Kenya for permission and support in conducting the field and laboratory studies described here. This research was conducted under the co-operative agreement between the National Museums of Kenya and the Smithsonian Institution. Logistical support was provided by the Smithsonian’s Human Origins Program, and we would like to thank Rick Potts, Director of the Program, for support during all phases of our research. Funding from the L.S.B. Leakey Foundation, the Leverhulme Trust, the National Geographic Society, the National Science Foundation, the Professional Staff Congress-City University of New York Research Award Program, the Wenner-Gren Foundation, the NSF International Research Fellowship Program (0602021), and the University of Cape Town Emerging Researchers Program for Kanjera field and laboratory research is gratefully acknowledged. We also acknowledge the continued support of the NMK Division of Archaeology, especially Dr. Purity Kiura who

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