Current events Sally McBrearty Variability in traces of Middle Department of Anthropology, Pleistocene hominid behavior in the University of Connecticut, Storrs, Connecticut 06269, U.S.A. Kapthurin Formation, Baringo, Kenya Laura Bishop Department of Human Anatomy and Cell Biology, University of Liverpool, Liverpool L69 3BX, U.K. John Kingston Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, U.S.A. Journal of Human Evolution (1996) 30, 563–580 Introduction and background New research in the Kapthurin Formation, Kenya, suggests a greater degree of variability in hominid behavior during the late Middle Pleistocene than has previously been suspected. The Middle Pleistocene commences with the shift from reversed to normal magnetic polarity at about 780 ka, and ends at the beginning of the last interglacial, at approximately 130 ka (Butzer & Isaac, 1975; Imbrie & Imbrie, 1980; Berger et al., 1984; Martinson et al., 1987; Cande & Kent, 1992; Baksi et al., 1992). Homo erectus was replaced in Africa around 500 ka by hominids termed archaic Homo sapiens. Anatomically modern H. sapiens is represented by fossil evidence dating to ca. 130 ka from East Africa and by fossils dating to at least 90 ka from South Africa (Day & Stringer, 1982; Stringer & Andrews, 1988; Grün & Stringer, 1991; Foley & Lahr, 1992; Deacon, 1993). Statistical principles suggest that known age range is in part a function of sample size, and it has been empirically demonstrated that expanded sample sizes can affect first and last appearance data for both vertebrate and invertebrate fossil taxa, including hominoids (Strauss & Sadler, 1989; Marshall, 1990; Springer, 1990; Hill, 1987; Flynn et al., 1995). Due to the extremely small size of the early anatomically modern H. sapiens sample, it is unlikely that the dates of 130 ka or 90 ka represent the true first appearance of modern humans in Africa. Evidence for the processes leading to the last major event in human evolution, the origin of modern humans, must therefore be sought in the late Middle Pleistocene. A conspicuous late Middle Pleistocene archaeological event in Africa is the end of the Earlier Stone Age (ESA) and the beginning of the Middle Stone Age (MSA). The ESA Acheulian industry disappeared, giving way to more diverse MSA traditions. K–Ar dates at the site of Gaddemotte, Ethiopia, indicating that the MSA occurrence there may be as old as 180 ka (Wendorf et al., 1975) have been widely cited, although it is possible that the MSA has even greater antiquity (Evernden & Curtis, 1965). The Kapthurin Formation itself provides one of the few reliable datum points for the age of the terminal Acheulian. Here late Acheulian artefacts are capped by a tuff dated by K–Ar to 240–250 ka (Tallon, 1978). Because stone tools are the products of hominid behavior, examining the shift from Acheulian to MSA technology is useful in showing how hominid adaptations changed 0047–2484/96/060563+17 $18.00/0 ? 1996 Academic Press Limited 564 . ET AL. coincident with the replacement of H. erectus by H. sapiens, and perhaps ultimately in clarifying the nature of the speciation event itself. The Acheulian contains large bifaces, while the MSA is characterized by smaller tools, often made on flakes struck from prepared cores. Because both early H. erectus (H. ergaster of Wood, 1992) and the Acheulian industry appeared in Africa between 1·7 and 1·8 Ma (Feibel et al., 1989; Roche et al., 1994), it is tempting to see the two events as linked. An examination of the later record, however, shows no clear association of hominid taxon with archaeological industry. Archaic H. sapiens fossils have been found associated with Acheulian, Sangoan, and MSA artefacts at various sites (Mturi, 1976; Mehlman, 1984, 1987); MSA artefacts are also known to occur with the remains of anatomically modern H. sapiens (Singer & Wymer, 1982). At some sites, industrial variants such as the Sangoan, characterized by heavy duty tools, or the ‘‘Fauresmith’’ containing small bifaces, appear to follow the Acheulian proper and to precede the MSA (Volman, 1984; McBrearty, 1987). These industrial variants have been linked to broad physiographic regions of Africa and their implied paleohabitats. Thus the Sangoan has long been considered a forest or woodland adaptation, while the ‘‘Fauresmith’’, has been thought confined to savanna zones (Clark, 1964, 1965, 1972, 1982, 1988), although the possible role of lithic raw material in determining artefact form has long been noted (e.g., Humphreys, 1979). An expansion of geographic range in the MSA and accompanying regional diversification has been thought to illustrate the improved adaptive abilities of early H. sapiens over those of H. erectus (Clark, 1970, 1988, 1992). Two assumptions are central to the prevailing view of African hominid behavioral change during the late Middle Pleistocene: (1) the Acheulian is fairly uniform, typologically and technologically, throughout its spatial and temporal range, in contrast to the MSA which presents a variety of artefact types and manufacturing techniques. (2) Different terminal Acheulian and MSA industries are found in distinct widely separated geographic regions (Clark, 1970, 1988, 1992). Our work in the Kapthurin Formation leads us to question these assumptions. Having relocated and reexamined 13 previously reported sites and discovered 28 new sites, we find that: (1) there is unequivocal variability among roughly contemporary assemblages throughout the sequence, whether they be termed ESA or MSA. (2) Assemblages that can be assigned to industries formerly thought to be confined to distinct biogeographic zones are found at sites no more than 4 km apart. The Kapthurin Formation also provides evidence for possible signs of modern behavior at an early date. Despite the presence in Africa of anatomically modern human fossils before 100 ka, undisputed evidence of modern human behavior at this time depth has thus far been sparse (Klein, 1989, 1992; Clark, 1992), although convincing signs of precocious behavioral modernity have recently been described for MSA sites in Zaire (Brooks et al., 1995; Yellen et al., 1995). In Europe, the blade-based Aurignacian industry accompanies the arrival of modern humans ca. 45 ka (White, 1982; Klein, 1989; Mellars, 1992). Because of the simultaneous appearance of blades and modern humans in the later Pleistocene record of Europe, and because blade production is a more efficient use of lithic raw material than flake production, blades are often treated in the literature as a unique modern human innovation. In fact, the origins of blade production appear more complex. Although most European Middle Paleolithic industries are flake-based, it is now realized that assemblages from a number of European Middle Paleolithic sites, dating to between ca. 90 ka and ca. 115 ka, have a significant blade component (Révillion & Tuffreau, 1994; Conard, 1990). Blades are also known from the pre-Augignacian industry of coastal North Africa (McBurney, 1967), which 565 has been thought to date to about 80–60 ka (Clark, 1992), but is probably substantially older. The Amudian industry of the Levant contains both blades and bifaces (Garrod & Bate, 1937; Jelinek, 1982). Electron spin resonance (ESR) dates from Tabun (Grün & Stringer, 1991) indicate that the Tabun Amudian may date to as much as 200 ka, whereas TL dates recently announced by Mercier et al. (1995) suggest a possible antiquity for the Amudian of as much as 300 ka. Elongated flakes present at the Acheulian site of Latamne in Syria are thought by Clark (1967) to presage the laminar technology of the Amudian. In Africa, blades are clearly present very early. Although the frequency of blades in the Howiesonspoort industry of South Africa has attracted much attention (e.g., Ambrose & Lorenz, 1990; Parkington, 1990), it is often overlooked that the makers of South African MSA industries routinely manufactured blades from a variety of core types (Sampson, 1972, 1974; Volman, 1984). The South African MSA, as a whole, is poorly dated; the earliest MSA levels at Klasies River Mouth probably date to 100–130 ka (Deacon, 1993), but South African MSA technology no doubt has its beginnings substantially earlier (Volman, 1984). Our work confirms the observations of Leakey et al. (1969) that blades occur with Acheulian bifaces in the Kapthurin Formation and demonstrates that the roots of African blade production penetrate deep into the Middle Pleistocene. In the Kapthurin Formation, prismatic blades are securely dated by K–Ar to >240 ka (Tallon, 1978). The Kapthurin Formation The Kapthurin Formation forms part of the sedimentary sequence in the Tugen Hills, a complex tilted fault block about 75 km long, lying on a roughly N–S axis in the floor of the Kenya Rift west of Lake Baringo (Figure 1). The Kapthurin Formation is exposed over an area of about 150 km2, and the best known sections are found in drainages of the Ndau, Kapthurin, and Chemeron Rivers. It was divided by Martyn (1969) into five major units (K1 through K5; Figure 2), and the stratigraphy further refined by Tallon (1976, 1978). Kapthurin sediments range from coarsely bedded boulder conglomerates to fine grained volcanic tuffs. Both fluvial and lacustrine facies are represented, and paleosols imply a series of intermittently stable landsurfaces. Artefacts and fossil fauna and flora are found throughout the sequence. With estimated thicknesses of 125 and >150 m (Tallon, 1978; Martyn, 1969) the Kapthurin Formation represents a substantial span of geologic time. The Ndau trachymugearite, dated at 1·57 Ma (Hill et al., 1986), lies near the top of the underlying Chemeron Formation, and provides a maximum age for Kapthurin rocks. Tallon (1978) and Cornelissen et al. (1990) report conventional K–Ar dates for the pumice tuff (K2), a thick conspicuous marker horizon near the base of the middle part of the formation. These dates, corrected for new constants using formulae provided by Ness et al. (1980, range from 620&6to890&260 ka.