Raw material selectivity of the earliest stone toolmakers at Gona, Afar, Ethiopia Dietrich Stout a, *, Jay Quade b , Sileshi Semaw a , Michael J. Rogers c , Naomi E. Levin d a Stone Age Institute and CRAFT Research Center, Indiana University, 1392 W. Dittemore Rd. Gosport, IN 47433, USA b Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Department of Anthropology, Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515-1355, USA d Department of Geology and Geophysics, University of Utah, 135 S. 1460 East, Salt Lake City, UT 84112-0111, USA Received 3 June 2004; accepted 31 October 2004 Abstract Published evidence of Oldowan stone exploitation generally supports the conclusion that patterns of raw material use were determined by local availability. This is contradicted by the results of systematic studies of raw material availability and use among the earliest known archaeological sites from Gona, Afar, Ethiopia. Artifact assemblages from six Pliocene archaeological sites were compared with six random cobble samples taken from associated conglomerates that record pene-contemporaneous raw material availability. Artifacts and cobbles were evaluated according to four variables intended to capture major elements of material quality: rock type, phenocryst percentage, average phenocryst size, and groundmass texture. Analyses of these variables provide evidence of hominid selectivity for raw material quality. These results demonstrate that raw material selectivity was a potential component of Oldowan technological organization from its earliest appearance and document a level of technological sophistication that is not always attributed to Pliocene hominids. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Gona; Oldowan; Raw materials; Lithic technology; Pliocene; Cognition * Corresponding author. Department of Anthropology, The George Washington University, 2110 G Street, NW, Washington DC 20052, USA. E-mail addresses: [email protected](D. Stout), [email protected](J. Quade), [email protected](S. Semaw), rogersm1@ southernct.edu (M.J. Rogers), [email protected](N.E. Levin). 0047-2484/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2004.10.006 Journal of Human Evolution 48 (2005) 365e380
Transcript
Journal of Human Evolution 48 (2005) 365e380
Raw material selectivity of the earliest stone toolmakersat Gona, Afar, Ethiopia
Dietrich Stouta,*, Jay Quadeb, Sileshi Semawa,Michael J. Rogersc, Naomi E. Levind
aStone Age Institute and CRAFT Research Center, Indiana University, 1392 W. Dittemore Rd. Gosport, IN 47433, USAbDepartment of Geosciences, University of Arizona, Tucson, AZ 85721, USA
cDepartment of Anthropology, Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515-1355, USAdDepartment of Geology and Geophysics, University of Utah, 135 S. 1460 East, Salt Lake City, UT 84112-0111, USA
Received 3 June 2004; accepted 31 October 2004
Abstract
Published evidence of Oldowan stone exploitation generally supports the conclusion that patterns of raw materialuse were determined by local availability. This is contradicted by the results of systematic studies of raw material
availability and use among the earliest known archaeological sites from Gona, Afar, Ethiopia. Artifact assemblagesfrom six Pliocene archaeological sites were compared with six random cobble samples taken from associatedconglomerates that record pene-contemporaneous raw material availability. Artifacts and cobbles were evaluated
according to four variables intended to capture major elements of material quality: rock type, phenocryst percentage,average phenocryst size, and groundmass texture. Analyses of these variables provide evidence of hominid selectivityfor raw material quality. These results demonstrate that raw material selectivity was a potential component of Oldowan
technological organization from its earliest appearance and document a level of technological sophistication that is notalways attributed to Pliocene hominids.� 2004 Elsevier Ltd. All rights reserved.
Keywords: Gona; Oldowan; Raw materials; Lithic technology; Pliocene; Cognition
* Corresponding author. Department of Anthropology, The George Washington University, 2110 G Street, NW, Washington DC
366 D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
Introduction
Thanks to the hard work of several generationsof researchers (Hay, 1976; Stiles, 1979; Jones, 1979,1994; Clark, 1980; Isaac, 1984; Schick and Toth,1994; Isaac et al., 1997; Sahnouni et al., 1997;Ludwig and Harris, 1998; Semaw, 2000), theimportance of considering lithic raw materials instudies of Early Stone Age (ESA) technologicalbehavior is now widely recognized. Raw materialcomposition has been identified as a major sourceof assemblage-level technological variation (Jones,1979; Isaac et al., 1981) as well as an importantindicator of hominid ranging and stone transportactivities (Stiles et al., 1974; Hay, 1976; Schick,1987; Stiles, 1998). A somewhat smaller body ofevidence has been accumulated regarding thedegree of selectivity shown by ESA toolmakersfor particular types or grades of rawmaterial (Toth,1985; Schick, 1987; Isaac et al., 1997). The researchpresented here contributes to this foundation byproviding a systematic assessment of raw materialselectivity in the earliest known archaeologicalassemblages (ca. 2.6 Myr) from Gona, Ethiopia.
For modern stone knappers, the location,identification and selection of suitable raw materi-als can be one of the more challenging aspects ofproduction (Whittaker, 1994: 65; Stout, 2002).Although Oldowan toolmakers certainly did notdisplay the high degree of raw material selectivityand transport seen later in prehistory (e.g. Feblot-Augustins, 1999), neither did they conform topatterns of resource exploitation and transportknown in extant apes (e.g. Boesch and Boesch,1984). Assessment of the technological and cogni-tive sophistication of Oldowan toolmakers re-quires that the details of their interactions with theenvironment be further specified.
Evidence from the East African sites of Olduvai(Hay, 1976), Koobi Fora (Isaac et al., 1997),Lokalalei (Kibunjia et al., 1992), Omo (Merrickand Merrick, 1976), Peninj (de la Torre et al.,2003), Kanjera South (Plummer et al., 1999) andGona (Semaw et al., 1997; Semaw, 2000) revealsthat Oldowan toolmakers typically used rawmaterials, including quartz, basalt, and otherfine-grained lavas, that were abundant in local(!4 km) stream channels. The best-documented
exceptions to this come from Olduvai Gorge,where phonolite, gneiss and quartzite were col-lected from bedrock outcrops and transported fordistances up to 13 km (Hay, 1976), and where theperiodic exposure of chert deposits occasioned theintensive exploitation of this material (Stiles et al.,1974). A portion of the artifacts (w15%) from theLate Pliocene KS1 and KS2 Beds at KanjeraSouth may also derive from non-local sources(Plummer et al., 1999).
In general, the stream channels and bedrockoutcrops which served as raw material sources inthe Oldowan would have been fairly obviousfeatures on the paleo-landscape, and their discov-ery and exploitation by early toolmakers is notparticularly surprising. More interesting are detailsregarding 1) the degree of technological acumenreflected in the selection of suitable materials fromthese sources, and 2) the extent of foresight andplanning revealed by subsequent transport pat-terns. This latter question has been well addressedby research on the formation of Oldowan sites atKoobi Fora and Olduvai (Schick, 1987; Potts,1991; Kroll, 1997). Technological analyses haveshown that there was a substantial flow of artifactsboth into and out of Oldowan sites (Toth, 1985;Schick, 1986, 1987), which are thought to repre-sent attractive locations of repeated hominidactivity on the landscape. Hominid activitiesleading to the formation of Oldowan sites clearlyinclude a scope and intensity of resource transportunknown in non-human primates.
In contrast, published evidence of raw materialselectivity among Oldowan hominids is moreequivocal. At Koobi Fora, the high frequency ofbasalt in local conglomerates (i.e. ancient streamchannels) is closely mirrored by its predominancein the artifact assemblages (Toth, 1985; Schick,1987; Isaac et al., 1997). Although quartz, chertand glassy volcanics are ‘‘reasonably easily avail-able’’ in the conglomerates, these material types donot appear to have been specifically selected for(Isaac et al., 1997: 268). On the other hand, Schickand Toth (e.g. 1993) do observe a degree ofselectivity for raw material quality, noting that theKoobi Fora toolmakers systematically avoidedvesicular lavas and cobbles with weathering flaws.Toth (1982: 121) concluded that ‘‘it is likely these
367D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
hominids were able to discriminate between easilyflaked, non-weathered material and less suitablerocks; however the actual selection of materials fortheir stone artifacts appears more opportunisticthan selective with regard to specific rock type.’’
At Olduvai, the situation seems more complex.In Bed I times (1.85 e 1.70 Myr), the archaeolog-ical assemblages are dominated by volcaniccobbles from local streambeds. These cobblesappear to have been selected for size andcomposition in much the same way as those fromKoobi Fora (Hay, 1976, Schick, 1987), and theoccurrence of minimally reduced cores of low-quality, vesicular lava at some Bed I sites (DK,FLK) further suggests that these undesirablematerials may have been tested and rejected byhominid toolmakers (Ludwig and Harris, 1998). InBed II (!1.7 Myr), on the other hand, assemb-lages show a clear tendency toward the increaseduse of quartz (Schick and Toth, 1994) and exoticvolcanic rocks (Hay, 1976). This trend, along witha temporary increase in the exploitation of chert inLower Bed II, lies at the heart of the technologicalvariation initially recognized by Mary Leakey(1971) in her typological distinction betweenOldowan and Developed Oldowan assemblages.
The increased use of chert at Olduvai around1.65 Myr was clearly occasioned by the temporaryexposure of rich sources of this material by theretreating waters of the paleo-lake (Hay, 1976).Hominid toolmakers at this time readily appreci-ated the superior flaking properties of chert, leadingto the formation of the earliest known special-purpose quarry site at MNK CFS (Stiles et al.,1974; Stiles, 1998). Causes underlying the increaseduse of quartz over time are less clear-cut, and mayinclude changes in hominid ranging patterns, tool-using behaviors and/or technological sophistica-tion (Schick and Toth, 1994; Ludwig and Harris,1998). The question of whether Bed II hominidsactually sought out quartz for its unique techno-logical properties, or simply adapted their technol-ogy to its expedient availability, remains open.
Quartz was also used by Late Pliocene tool-makers (ca. 2.3 Myr) in assemblages from OmoShungura Member F, where its frequency is moreclearly a reflection of local availability (Merrick andMerrick, 1976). Published accounts from the Late
Pliocene sites of Lokalalei LA1 (GaJh 5) and LA2Csimilarly suggest that raw material selection re-flected local availability (Kibunjia, 1994; Rocheet al., 1999) at these sites, although recent reportsfrom more systematic raw materials studies haveindicated a bias toward the exploitation of phono-lite (Harmand, 2004). Interestingly, Ludwig andHarris (Ludwig and Harris, 1998; Ludwig, 1999)have argued that ‘‘subtle raw material flaws’’account for a high incidence of step fracturesobserved on cores from LA1, raising the possibilitythat toolmakers at this site were less attentive tocobble quality than those at LA2C,Koobi Fora andOlduvai. Further studies of raw material exploita-tion at Lokalalei are sure to yield interesting results.
For the time being, however, evidence of rawmaterial selectivity among Oldowan toolmakersremains quite limited. This is especially true inLate Pliocene assemblages from Omo, Koobi Fora(KBS), and Bed I of Olduvai, where raw materialcomposition closely parallels local availability andthe only evidence of selection comes from quali-tative observations of cobble suitability. Reports ofexotic raw materials at Kanjera South (Plummeret al., 1999) may indicate some degree of selectivityand/or long range transport by Pliocene hominidsat that site, but more detailed data regarding rawmaterial availability in the region are needed fora definitive assessment. Taken together, evidencefrom these sites generally supports the parsimoni-ous conclusion that patterns of Oldowan rawmaterial exploitation were determined by localavailability rather than choice on the part of thetoolmakers.
In this context, recently described Pliocenearchaeological sites from the Gona study area inEthiopia (Semaw et al., 1997; Semaw, 2000;Semaw et al., 2003) provide an important counter-example. As at many of the sites discussedabove, the Gona toolmakers obtained their rawmaterials from local channel gravels that are nowpreserved as cobble conglomerates in the modernstudy area. The Gona gravels are, however,distinguished by their greater representation offelsic volcanic rocks (trachyte, rhyolite, latite) asopposed to basalt (e.g. Olduvai, Koobi Fora) orquartz (Omo). As documented by Semaw et al.(1997), the Gona toolmakers showed a clear
368 D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
preference for these felsic volcanic rocks, usingthem in much greater proportion than would beexpected from their representation in the gravels.Aphanitic (without phenocrysts) volcanic rocks,which are quite rare in the gravels, were alsoexploited in substantial numbers at Gona (Semaw,2000; Semaw et al., 2003).
The strong pattern of rawmaterial selection seenat Gona demonstrates that low levels of selectivityare not universal in the Oldowan. The fact that theassemblages fromGona are the oldest known in theworld also argues against an overarching temporaltrend toward increasing selectivity within the Old-owan. More detailed and specific explanations ofthe observed variation are thus needed, ideallybased on robust, quantitative analyses of rawmaterial availability and utilization at multiple sitesand study areas. The age anddistinctive character ofthe Gona assemblages makes them a particularlyimportant focus for such investigation.
Background
The Gona Paleoanthropological Research Pro-ject (GPRP) study area (Fig. 1) encompasses morethan 500 km2 in the Afar Depression of theEthiopian rift valley. Artifact and fossil-rich Plio-Pleistocene deposits of the Busidima Formation(Quade et al., 2004) lie exposed in a number ofmajordrainages dissecting the eastern portions of thestudy area. These include the Kada Gona, OundaGona, Dana Aoule, Busidima and Asbole drain-ages, which are ephemeral tributaries of the AwashRiver (Fig. 1). Surface scatters of artifacts were firstobserved along the east side of the Kada Gonaduring the 1970s (Corvinus and Roche, 1976;Corvinus, 1976; Roche and Tiercelin, 1977), andthe presence of in situ artifacts in sediments on bothsides of this drainage was subsequently demon-strated by systematic excavations during the 1980s(Harris, 1983; Harris and Semaw, 1989) and 1990s(Semaw, 1997; Semaw et al., 1997; Semaw, 2000).This latter work, including excavations at the highdensity sites of EG (East Gona) 10 and 12, firmlyestablished the great age (2.60 - 2.52 Myr) andtypical Oldowan affinities of the Kada Gonaartifacts.
The current phase of research at Gona, initiatedin 1999, has pursued systematic geological (Quadeet al., 2004), paleoenvironmental (Levin et al.,2004), paleontological and archaeological (Semawet al., 2003) studies of the entire GPRP study area,resulting in the identification of numerous newpaleonotological and archaeological localities.Among these are five Pliocene archaeological sitesfrom the Ounda Gona and Dana Aoule drainages,including Ounda Gona South (OGS) 6 and 7(Semaw et al., 2003), two unpublished excavationsfrom the Dana Aoule North area (DAN1 and 2d),and a controlled surface collection from DanaAoule South (DAS7), also unpublished. Dataregarding raw material composition and localavailability at these sites, as well as the previouslyidentified Kada Gona site EG13 (Semaw, 1997),are presented here.
Geological and paleogeographic context
The geological context of the Gona Oldowanhas recently been described by Quade et al. (2004)and the relevant features are summarized here.Sediments in the lower portion of the BusidimaFormation at Gona are characterized by finingupward sequences (Fig. 2) of basal conglomerates,coarse sands, bedded silts and vertic paleosols thatrepresent the channel, bank and floodplain depos-its of the ancestral Awash River. All of theOldowan sites known from Gona occur in siltsand paleosols of these fining upward sequences.These silts are over-bank flood deposits of theancestral Awash River and the associated paleo-sols represent pedogenesis that post-dates artifactdeposition. Following Walther’s law of correlationof facies (Walther, 1894), the vertical associationof silts and underlying gravels seen today impliesthat the Gona Oldowan sites were in close spatialproximity (within 10’s to 100’s of meters) to a riverchannel and exposed cobble bars (Fig. 3) fromwhich raw materials would have been readilyavailable. These gravels provide the only localsource for the large (O10 cm), well-roundedcobbles exploited by Pliocene toolmakers at Gona.Although it is not possible to identify exactsections of conglomerate that would have beenexposed during the formation of particular sites,
369D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
Fig. 1. The Gona Paleoanthropological Research Project study area is located in the Afar region of Ethiopia. The easternmost portion
of the study area is shown here, including the six Pliocene archaeological sites discussed in the text. After Semaw et al. (2003).
stratigraphically associated conglomerates do re-cord the overall character of paleo-Awash rivergravels in the region at the time of site formation.This situation provides an excellent opportunity toassess the raw material selectivity of the earlieststone toolmakers at Gona.
Methodology
In this study, hominid raw material selectivitywas assessed through a systematic survey of raw
material composition in six Pliocene archaeologicalassemblages and their associated conglomerates(Table 1).
The archaeological sample
The archaeological sample consists of artifactsrecovered in excavations and controlled, 100%surface collections conducted between 1999 and2003 at the sites of EG13, OGS6a, OGS7, DAN1,DAS7 and DAN2d. Five of these sites date to the
370 D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
period between 2.6 and 2.5 Myr while the sixth(DAN2d) is located stratigraphically above DAN1and DAS7 and dates to between 2.58 and
Fig. 3. The stratigraphic context of the Gona Oldowan sites
indicates that they were deposited on the proximal floodplain of
the paleo-Awash River. Raw materials from the paleo-Awash
gravels would have been available from point bars in the main
channel. After Quade et al. (2004).
Fig. 2. A representative fining upward sequence as seen at the
OGS-7 locality. This site is located at the contact between an
alluvial sand bank and overlying floodplain silts, stratigraph-
ically above an ancestral Awash River gravel. Data from
Semaw et al. (2003).
2.27 Myr. All surface and in situ artifacts fromthese sites that were of sufficient size for analysis(generally O20 mm) were evaluated in terms ofraw material type and quality (see below).
All of the sites presented here occur on steepmodern erosion surfaces in silts or paleosolsoverlying large-cobble conglomerates. Erosion isvery rapid in these deposits, and the surfacecollections reported here clearly derive from thesedimentary sequence where they were recovered.This conclusion is supported by multiple lines ofevidence, including: 1) the remarkable freshnessand locally high density of the artifacts, 2) thesteepness of the topography and the high rate ofartifact turnover observed from year to year, 3)refitting between surface and in situ materials (fourrefits between surface and in situ artifacts at OGS-7, including flake-on-flake and flake-on-core), and4) the fact that the overlying sedimentary sequen-ces at each of these sites have been mostly erodedaway, removing any possibility of admixture fromabove. Small but statistically significant differencesin material composition do exist between thesurface and in situ samples reported here. Howeverthese differences do not affect the overall results ofthe study (see below).
Excavated areas range in size from 2 m2 to10 m2 (Table 1). Although at least a portion ofeach site remains for future excavation, the lithicsamples so far collected are large enough toprovide high levels of statistical significance inthe current study (e.g. Table 5). The samplespresented here, though incomplete, are represen-tative of the various sites investigated and there isno reason to suppose that they are systematicallybiased in any direction. Consistent patterns in rawmaterial frequencies and characteristics acrossmultiple sites in this study (see below) corroboratethe representative nature of the samples, andreflect a lack of confounding intra-site variation.
The geological sample
Each of the sites included in this study isassociated with a coarse-grained cobble conglom-erate that contains the pene-contemporaneousmaterial composition of the paleo-Awashriver gravels. Random samples (nZw100) of
371D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
Table 1
Archeological and Geological Samples
Locality Age (Myr) Archaeological sample Excavated area # artifacts analyzed Geological sample
EG-13 2.6 e 2.5 1999 excavation,
2003 surf. coll.
2 m2 surface: 152
in situ: 27
Total: 179
2000 (nZ 100)
OGS-6a 2.6 e 2.5 2000 excavation 4 m2 surface: 48
in situ: 52
Total: 100
2000 (nZ 108)
OGS-7 2.6 e 2.5 2000 excavation 2.6 m2 surface: 65
in situ: 188
Total: 253
2001 (nZ 99)
DAN-1 2.6 e 2.5 2000 surf. coll.,
2001 excavation
2.8 m2 surface: 67
in situ: 45
Total: 112
2001 (nZ 100)
DAS-7 2.6 e 2.5 2001 surf. coll.,
2003 surf. Coll.
Surface only surface: 190
in situ: 0
Total: 190
compared with
DAN-1 sample
DAN-2d 2.58 e 2.27 2001 excavation 10 m2 surface: 24
in situ: 36
Total: 60
2000 (nZ 99)
2001 (nZ 99)
Total - - 21.4 m2 surface: 559
in situ: 335
Total: 894
Total: 605
approximately 10-20 cm diameter cobbles werecollected from these conglomerates in the imme-diate vicinity (!30 m) of each archaeological site,with the exception of DAS7. Although DAS7 isseparated from DAN1 by the modern Dana Aouledrainage, both sites appear to lie stratigraphicallyabove the same gravel and were compared witha single cobble sample taken near DAN1. Thegravel below the site of DAN2 was sampled twice,in 2000 and 2001, as a test of the samplingmethodology. In all cases, cobble samples con-sisted of the first 100 cobbles of appropriate sizecollected from a particular section (w1-2 m) ofconglomerate.
Raw material analysis
One major problem confronting the study ofhominid raw material selectivity is the difficulty ofobserving and quantifying technologically relevantmaterial characteristics. For example, the formalpetrologic classifications used by geologists (e.g.Travis, 1955; Philpotts, 1990; Klein and Hurlbut,
1993) have the advantage of being well establishedand highly replicable, but do not capture many ofthe variables that would have been relevant toancient toolmakers. Proper petrologic assessmentalso requires destructive laboratory analyses (thin-sectioning) which may not be appropriate forartifacts, cannot be conducted in the field, and aregenerally not practical for the evaluation ofvariation within large samples. In contrast, qual-itative assessments of lithic materials by experi-enced knappers deal directly with technologicalsuitability and are relatively easy to produce, butsuffer from being informal, difficult to replicate,and unsuitable for statistical analysis. In thisstudy, an attempt was made to bridge the gapbetween these approaches by focusing on fourspecific variables related to raw material qualityand composition: rock type, percentage of phe-nocrysts per unit area (of a fractured surface),average size of phenocrysts, and groundmasstexture (Table 2). This approach is similar to thatpreviously used by Brantingham et al. (2000) toassess raw material quality in a late Pleistocene
372 D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
Table 2
Variables used in Raw Material Analysis
Variable Values Data type Method of comparison
Rock Type Trachyte, Rhyolite,
Latite, Quartz
Latite, Aphanitic
lava, Basalt,
Vitreous volcanic, Other
nominal frequency distribution
Percentage of Phenocrysts 0, %5, %10, %15, %20, . ratio 2-tailed t-test
Average size
of phenocrysts (mm)
!1, 1, 2, 3, 4, 5, . ratio 2-tailed t-test
Texture Glassy, Smooth, Coarse, Chalky/Decayed ordinal* Mann-Whitney test
* when chalky/decayed (a small percentage of observations) is excluded.
assemblage from Tsagaan Agui cave, Mongolia.For reasons of practicality, all variables wereassessed through the visual inspection of fracturedsurfaces using a hand-lens.
Rock typeInitial cobble samples collected during the 2000
field season led to the identification of eight majorvolcanic rock types in the Gona study area:trachyte, rhyolite, latite, quartz latite, aphaniticvolcanic, basalt, vitreous volcanic and ‘‘other.’’These types are defined geologically by differencesin mineral composition, texture, and the occur-rence of phenocrysts (e.g. Travis, 1955; Klein andHurlbut, 1993). It should be noted that, for thepurposes of this study, the use of these categoriesdoes not reflect rigorous petrological analysis, butrather a pragmatic system of field identificationbased on visual inspection with a hand lens.
Basalt, for example, is a mafic rock that is easilyidentified in the Gona conglomerates by its finegrain, high density and dark color. Less dense, felsicrocks occur atGona in fourmain varieties (trachyte,rhyolite, latite and quartz latite) that are defined bydiffering abundances of quartz and by potassiumfeldspar/plagioclase proportions (Table 3) andwere
Table 3
Felsic Rock Classification System
Mineral composition Potassium Feldspar
to Plagioclase Ratio
R 2:3 2:3 e 1:3
O10% quartz Rhyolite Quartz Latite
!10% quartz Trachyte Latite
identified for this study through careful examina-tion of phenocryst mineral composition with a handlens. Felsic rocks without phenocrysts were classi-fied as either aphanitic volcanic or vitreous volcanic.‘‘Vitreous volcanic’’ is the term that we adopt todescribe high-quality glassy or cryptocrystallineaphanitic rocks with vitreous luster found in lowfrequencies in theGona conglomerates.Muchmorerarely occurringmaterials such as volcanic breccias,ignimbrites and obsidian were grouped togetherunder ‘‘other.’’ The resulting nominal rock typedata were used to construct frequency distributionsfor comparison across samples (Figs 4 and 6).
The eight-part classificatory system employedhere is a refinement over preliminary studies of theGona conglomerates (Semaw, 1997, 2000), whichgrouped all cobbles into four basic raw materialtypes (trachyte, basalt, rhyolite and ‘‘other’’). Theincreased number of types recognized in the newsystem provides more detailed information aboutthe composition of conglomerates and artifactassemblages, although it yields rock type distribu-tions that are not directly comparable withprevious results. Despite this methodologicaldifference, present and prior results both clearlyreflect the same underlying pattern of hominid rawmaterial selectivity at Gona.
The classificatory system used here, thoughsimplified, allows for the practical and objectivedescription of raw material composition amongthe Gona gravels and artifact assemblages. Thedescription of rock types also captures some basicelements of technological suitability, for examplein distinguishing vitreous volcanics from morefracture resistant basalt. On the other hand, there
373D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
is a large degree of variation in material qualitywithin categories, depending on factors such astexture, uniformity, weathering flaws, etc. Begin-ning in 2001, additional data on phenocryst andgroundmass characteristics were collected in orderto assess some of this variability. Future experi-mental research will be important in more pre-cisely defining the influence of rock type,phenocryst and groundmass variables on theflaking properties (e.g. evenness of fracture) andpractical utility (e.g. edge durability) of variousraw materials.
Size and frequency of phenocrystsThe felsic rocks found in the Gona gravels are
dominantly hypocrystalline, and range from apha-nitic, where few to no phenocrysts are visible, toporphyritic, where variable proportions of macro-scopic phenocrysts or spherulites are encased ina fine-grained groundmass. Depending on theirsize and frequency, such phenocrysts can createuneven fracture patterns that detract from a mate-rial’s technological suitability, although this is notalways the case. In this study, data on phenocrystsize and frequency were collected through the
Fig. 4. Distribution of rock types in the artifact and conglom-
erate samples. Types are organized from left to right in order of
their descending frequency in the conglomerates.
visual examination of hand specimens. The aver-age size of phenocrysts was estimated to thenearest millimeter, using a scale as a guide, andthe percentage of phenocrysts per unit area wasestimated to the nearest 5%. Because phenocrystscan be uneven in their distribution, percentageestimates were based on the entire exposed surface.These estimations provide a systematic, practicaland reasonably accurate means of assessing rawmaterial characteristics of potential relevance toknapping quality. Phenocryst size and percentagedata were compared across samples using Stu-dent’s t-tests. Consensus reached by multipleresearchers on rock type identifications (NL, JQand DS) and phenocryst percentages (JQ and DS)helped to ensure the consistency of results.
Groundmass textureAnother important factor influencing raw
material quality is the texture of the groundmasssurrounding the phenocrysts. Although mostrocks at Gona exhibit groundmasses that are ‘‘finegrained’’ in a geological sense (crystals !1 mm),there is nevertheless a substantial amount oftechnologically relevant variation. In this study,individual samples were placed into one of fourtexture categories: glassy, smooth, coarse, andchalky/decayed. ‘‘Glassy’’ included materials likevitreous volcanics with no discernable grain(but probably cryptocrystalline, not vitrophyric),whereas ‘‘smooth’’ was applied to materials withan even fracture and a fine but noticeable grain.Materials with a more irregular fracture andgroundmass crystals approaching 1 mm in sizewere termed ‘‘coarse,’’ and heavily weatheredsamples (usually artifacts) were classified as‘‘chalky/decayed.’’
Results
Comparison of the geological and archaeolog-ical samples presented here clearly demonstratesthe high degree of raw material selectivity exercisedby the Oldowan toolmakers at Gona. Pooled rocktype frequency distributions show marked differ-ences between the artifact assemblages and cobbleconglomerates, most notably in the avoidance of
374 D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
Table 4
Comparison of pooled phenocryst data (Student’s t-test)
All cobbles Artifacts Mean difference 2-tailed significance
Mean phenocryst size 2.8 mm nZ 249 all: 1.6 mm, nZ 618 1.2 mm p! 0.001
surface: 1.7 mm, nZ 425 1.1 mm p! 0.001
in situ: 1.4 mm, nZ 193 1.3 mm p! 0.001
basalt and preferential exploitation of trachyte,vitreous volcanics and other aphanitic clasts(Fig. 4). Material quality data similarly show thatcobbles selected by hominids had fewer, smallerphenocrysts (Table 4) and finer groundmasses(Fig. 5; Mann-Whitney asymptotic significance!0.001) than would be expected in a randomsample from the conglomerates. There are observ-able differences between the surface and in situartifact samples, but these are an order ofmagnitude smaller than the differences betweenartifacts and cobbles and do not affect overallresults. Two-tailed t-tests show that surface materi-als display an average of 1.4 % (tZ 3.38, dfZ 856,pZ 0.001) more phenocrysts than in situ artifacts,
Fig. 5. Distribution of rock textures in the conglomerate and
artifact samples.
and the mean size of these phenocrysts is 0.2 mmgreater (tZ 2.49, dfZ 616, pZ 0.013). Thesestatistical differences largely result from the factthat the surface sample includes 15% fewer apha-nitic artifacts, and lose significance (tZ 0.938, dfZ710, pZ 0.348; tZ 2.26, dfZ 609, pZ 0.024) ifaphanitic artifacts are removed from the analysis.
The causes behind the under-representation ofaphanitic artifacts in the surface sample are lessclear. Currently available metric data (from OGS7,OGS6a and DAN1) do not show any sizedifference between aphanitic and other artifactsin situ that might possibly lead to differentialwinnowing on the surface. Aphanitic artifacts dodisplay the highest percentage of ‘‘chalky/de-cayed’’ groundmasses (7%), suggesting thatweathering might differentially affect the survivaland/or identification of aphanitic artifacts on thesurface, but the extremely low incidence of suchdecay (1% overall) argues against this beinga major factor. The reasons underlying the un-der-representation of aphanitic artifacts in thesurface sample thus remain unclear. What is clear,however, is that this difference is of insufficientmagnitude to affect the overall results of the study,and that discarding the surface data would onlyserve to increase the contrast between artifact andcobble samples.
Examination of individual assemblage andcobble sample data further show that overalldifferences between the archaeological and geo-logical samples do not result from local anomaliesin raw material availability and exploitation. Infact, there is relatively little variation in phenocrystand groundmass attributes from site to site orfrom conglomerate to conglomerate (Table 5).Statistical comparisons (t-tests) of phenocryst size
375D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
1.1 3.0 1.5 not collected 1.4 not collected 2.8 2.5 1.7 2.9 1.9
% glassy groundmass 22.6 0.0 3.0 not collected 1.1 not collected 5.1 0.0 10.8 1.0 5.8
% smooth groundmass 75.8 11.6 90.0 not collected 93.8 not collected 71.2 21.1 63.1 29.0 68.8
% coarse groundmass 0.0 87.3 6.0 not collected 4.9 not collected 15.3 78.9 21.6 70.0 24.9
% chalky groundmass 1.6 1.1 1.0 not collected 0.2 not collected 8.5 0 5.5 0 0.5
* Two cobble samples were collected at DAN2, but only one (2001) included phenocryst size and groundmass texture data.
and percentage data between cobble samplesrevealed only three cases of significant (p% 0.05)difference (out of 18 comparisons): the OGS7cobbles have (1) a lower percentage and (2)a greater mean size of phenocrysts than theDAN2 2001 cobble sample, and (3) the OGS6cobbles have a higher percentage of phenocryststhan those from OGS7. In contrast, each localityshows the same characteristic pattern of differencebetween artifacts and cobbles (Table 5). The soleexception to this pattern is the unusually largemean size of phenocrysts in artifacts fromDAN2d, which actually exceeds that in theunderlying conglomerate.
Rock type frequencies observed at each localityshow greater variation (Fig. 6), but reveal a simi-larly characteristic pattern of difference betweenartifact assemblages and their associated conglom-erates. The close accord between separate cobblesamples taken at the DAN2 locality during 2000and 2001 also confirms the reproducibility of thesampling methods used in this study. Variations inlocal material availability do appear to affectassemblage composition in some cases, but this isalways relatively minor compared to the influencesof hominid preference. At OGS7, for example,unusually high concentrations of quartz latite inthe conglomerate may explain the above-averagerepresentation of this material in the assemblage.More striking, however, are the preferentialexploitation of locally scarce vitreous volcanicand aphanitic clasts, and the conspicuous avoid-ance of locally abundant basalt.
The avoidance of basalt is a common feature atevery locality where data were collected, and isespecially striking with respect to the basalt-rich
conglomerates at DAN2 and OGS6. Hominidpreferences are slightly more complex, in somecases being limited to vitreous volcanics and otheraphanitic clasts (OGS7, DAN2d), but usuallyincluding a substantial bias toward phenocryst-poor trachyte as well (DAN1, EG13, DAS7). Inany case, it is clear that hominid preferences werea major factor influencing raw material composi-tion in each of the assemblages presented here.
Discussion
These results make it clear that Pliocenetoolmakers at Gona were able to locate, identifyand preferentially select materials with particularattributes. Hominids’ evident preference for finer-grained, less porphyritic materials is most likelyrelated to the influence of these variables onfracture patterns and technological suitability, butthis hypothesis will need to be tested experimen-tally. The same may be said with respect to theselection of vitreous volcanics and other aphaniticclasts and the avoidance of basalt. Potentialreasons underlying a preference for trachyte areless immediately obvious, but are likely related tothe same factors. In fact, a comparison of thematerial attributes of the different felsic rock typesfound in the Gona conglomerates reveals thattrachyte cobbles have the lowest mean size andpercentage of phenocrysts, as well as a greaterfrequency of ‘‘smooth’’ groundmasses (Table 6).In sum, rock type, phenocryst and groundmassdata all converge to show a high degree of rawmaterial selectivity in the early Oldowan assemb-lages at Gona. The disproportionate representation
s are compared with the DAN1 cobble sample (see
Fig. 6. Distribution of rock types in the artifact and conglomerate samples from each locality. Note that the DAS7 artifact
text). Two cobble samples were collected at the DAN2 locality and both are depicted here.
377D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
Table 6
Attributes of Felsic Rocks in the Gona Conglomerates
* p values are for 2-tailed t-tests vs. trachyte data.
of certain rock types, and preference for finer-grained, phenocryst-poor clasts must arise from thepatterning of hominid technological activity, andcannot be attributed to any known taphonomicprocess.
Oldowan artifacts from Gona appear remark-ably refined, especially given their early date(Semaw et al., 1997; Semaw, 2000; Semaw et al.,2003). Inspection of the artifacts themselvessuggests that this apparent sophistication may belargely due to the quality of raw materials used.Not only is it easier to initiate and control fracturein fine-grained, isotropic rocks; such materials alsotend to preserve more of the technological traces(e.g. flake scars, retouch, ripple marks, percussionbulbs) used by archaeologists to evaluate artifacts.The Gona artifacts do indeed provide evidence ofwell-developed flaking skills (Semaw et al., 1997;Stout and Semaw, in press), but are probablymore remarkable for the materials from whichthey are made than for the specific techniques oftheir making. It is thus important to recognizethat the dynamics of raw material procurement,selection, transport and use may be as revealingof technological and cognitive sophistication asare knapping plans and acquired perceptual-motor skills (Stiles, 1998; Inizan et al., 1999;Stout, 2002).
There are two complementary patterns ofhominid activity which might plausibly havecontributed to the pattern of raw materialrepresentation seen in Oldowan sites at Gona: 1)preferential selection at raw material sources, and2) preferential transport to and/or discard atarchaeological sites. Selection of materials atcobble bars would most likely have involved
visual evaluation of cortex characteristics and atleast some test flaking. Personal observation hasshown that cortex characteristics such as color,texture, incipient fracture cones, and pits left byexfoliated phenocrysts can be good indicators ofmaterial quality. Any biases in subsequent pat-terns of material transport and discard on thelandscape would likely have been based on directexperience actually flaking and/or using particularcores and flakes, although visual identification ofdesirable characteristics may still have beenimportant if accumulated lithic scatters themselvesserved as secondary material sources (Schick,1987).
Further research will be needed in order todefine the relative importance of initial selectionvs. subsequent transport and use. However, it isclear that both reflect hominid awareness oftechnologically desirable material characteristics.Even in modern, language-bearing humans, suchpractical technical knowledge is generally acquiredthrough experience (Keller and Keller, 1996;Stout, 2002), and this would almost certainly havebeen the case with Oldowan hominids as well. Theattention paid to raw material characteristics bythe Gona toolmakers thus reflects the deliberateand well-practiced nature of their technologicalbehavior.
Conclusion
Evidence from the earliest known archaeolog-ical sites at Gona clearly demonstrates that, ina situation where an assortment of raw materialswas available, Pliocene toolmakers were capable of
378 D. Stout et al. / Journal of Human Evolution 48 (2005) 365e380
exercising a high degree of selectivity. The greatage of the Gona sites indicates that such selectivitydid not develop over time, but rather was a featureof Oldowan technological variation from its veryinception. This selectivity reflects a level oftechnological understanding and sophisticationamong Pliocene toolmakers that is not alwaysappreciated. Insofar as high-quality materialsgreatly facilitate knapping success and skill acqui-sition, it is even possible that the availability andselection of advantageous raw materials was animportant factor in the initial invention and spreadof Oldowan technology.
Acknowledgements
This research was made possible by thegenerous support of the L.S.B. Leakey Founda-tion. Additional funding came from the NationalScience Foundation (Award-0004103), theWenner-Gren Foundation and the National GeographicSociety. Essential permits and facilities wereprovided by the ARCCH of the Ministry ofYouth, Sports and Culture of Ethiopia, theNational Museum of Ethiopia and the AfarAdministration. The research was organized fromthe Stone Age Institute and CRAFT ResearchCenter at Indiana University, and we would liketo thank Kathy Schick and Nicholas Toth (theCo-Directors) for their support and comments ona draft of this paper. We would also like to thankBerhane Asfaw and Yonas Beyene for advice,Alemu Admasu and Menkir Bitew for generalassistance and facilitation, and our Afar col-leagues at Eloha for friendship and work in thefield. DS thanks the CRAFT Research Centerand the Office of the Vice President of Research(formerly RUGS) at Indiana University forgraduate research fellowships which made hisparticipation possible. SS thanks the CRAFTResearch Center and Stone Age Institute forproviding a Research Associate position andother institutional support during this research,and Mr. Rogers and Friends for their support.MR thanks the Fulbright Program for supportduring the 2000 field season. The field participa-tion of Manuel Dominguez-Rodrigo, Travis
Pickering, Melanie Everett and Leslie Harlackeris acknowledged. We also thank three anonymousreviewers for their comments and suggestion.
References
Boesch, C., Boesch, H., 1984. Mental map in wild chimpanzees:
an analysis of hammer transports for nut cracking. Primates
