Diachronous dawn of Africa’s Middle Stone Age: New 40Ar/39Ar ages from the Ethiopian Rift Leah E. Morgan1, Paul R. Renne1,2 1Department of Earth and Planetary Science, University of California–Berkeley, Berkeley, California 94720, USA 2Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA ABSTRACT The Middle Stone Age (MSA) of Africa, like the Middle Paleolithic of Europe, is thought to represent a time period wherein toolmakers acquired signifi cant increases in cognitive abili- ties and physical dexterity. Existing data fail to resolve whether the MSA emerged gradually, abruptly, or discontinuously, and whether this industry refl ects the activity of Homo sapiens. Here we present new 40Ar/39Ar geochronological data revealing that advanced MSA archaeol- ogy at two sites in the main Ethiopian Rift is older than 276 ka, much older than technologi- cally comparable MSA archaeology from elsewhere. An age of 183 ka for a unit farther upsec- tion, along with the technological stasis observed throughout the section, indicates that similar technology was used here for ~93 ka. These results suggest that MSA technology evolved asyn- chronously in different places, and challenge the notion of a distinct time line for either the appearance of the MSA or the disappearance of the earlier Acheulean. These and other recent results indicate that the oldest known MSA consistently predates fossil evidence for the earliest Homo sapiens. ARCHAEOLOGY AND crystal 40Ar/39Ar dating indicates that it is older the Signor-Lipps effect (Signor and Lipps, 1982) PALEONTOLOGY IN THE than 284 ± 24 ka (Deino and McBrearty, 2002). would tend to make the archaeological record of LATE-MIDDLE PLEISTOCENE (Uncertainties are given at the 2σ level here and H. sapiens appear to predate its fossil record. The nature and timing of the transition from throughout, with the possible exception of K-Ar With mounting geochronological support the Acheulean period of the Early Stone Age to ages published by Laury and Albritton [1975] for MSA industries older than ca. 250 ka, it is the Middle Stone Age (MSA) is not well under- and Wendorf et al. [1994], for which confi dence increasingly clear that evidence for this techno- stood. Generally, a shift was made from larger levels were not reported.) In contrast, hominids logical revolution precedes the evidence for the Acheulean hand axes, picks, and polyhedra, to at Herto in Ethiopia utilized what appears to be a appearance of anatomically modern people in smaller, more diverse MSA artifacts (McBrearty technology transitional between the Acheulean Africa, but not necessarily of archaic H. sapiens and Brooks, 2000). The tools of the MSA are and MSA as recently as 160 ka (Clark et al., or other closely related members of the genus characterized by points, blades, and other tools 2003). The appearance of MSA-typical tech- Homo. Understanding the nature of these tech- that are often made by the Levallois technique, nology may indicate a shift in tool-making nological and biological changes, particularly in which utilizes prepared cores and thus seems to abilities, whereas the persistence of Acheulean light of the global climatic changes evident in the require “more complex cognitive abilities” (Bar- technology might be expected to be spatially late-middle Pleistocene, will require intensifi ed Yosef and Dibble, 1995 p. x) than those seen in hetero geneous, depending on such variables as paleoanthropological efforts and explorations. the Acheulean. However, in recent years it has available source materials and cultural circum- become apparent that this transition was quite stances. Understanding the relationships and GADEMOTTA FORMATION complex, as hand axes and MSA tools have been intertwining of these two stone tool traditions Gademotta and Kulkuletti are located on the found together at some sites (Clark et al., 2003) in Africa will require precise chronometry often fl anks of a collapsed caldera ~2 km apart and and Levallois technology has been recorded lacking for key sites across the continent. ~5 km west of Lake Ziway in the Main Ethio- at localities otherwise considered Acheulean The fossil record of Homo sapiens and other pian Rift Valley (Fig. 1A; GSA Data Repository (McBrearty, 2003). Most well-dated and docu- closely related species during the late-middle Fig. DR11). The Gademotta Formation is found mented MSA sites postdate 130 ka. However, Pleistocene is similarly unresolved. Debate at the type locality of Gademotta (Fig. 1B) a few sites hint at a much earlier origin of the continues over the species assignments of some and elsewhere on Gademotta Ridge, including MSA. These include Malewa Gorge, Kenya, fossils (i.e., Omo 2), the relationships between Kulkuletti (Fig. 1C). It was deposited on the dated to before ca. 240 ka by K-Ar (Evernden various species, and even the existence of some Kulkuletti Volcanics, some of which were dated and Curtis, 1965); Twin Rivers, Zambia, dated species (Rightmire, 2008; Tattersall and Schwartz, by Vogel et al. (2006) to ca. 1.3 Ma. The forma- to before 265 ka by U-series on speleothems 2008). The apparently oldest fossils assigned to (Barham and Smart, 1996); and Cartwright’s H. sapiens are from the Kibish Formation in the 1GSA Data Repository item 2008241, analytical site at the Kinangop Plateau, Kenya, also dated Omo, bracketed by 40Ar/39Ar dates to between methods, results from unit 5 at Gademotta, Figures by K-Ar to before 439 ka (Evernden and Curtis, 104 ± 14 ka and 196 ± 4 ka, with an inferred DR1 (location map), DR2 (age probability diagrams 1965). Although the old age from Cartwright’s age of 195 ± 10 ka (McDougall et al., 2005). and inverse isochron for sample K2), DR3 (age spec- 40 39 trum and inverse isochron for sample G3), DR4 (age site is intriguing, its accuracy is questionable, as H. sapiens from Herto are dated by Ar/ Ar to probability diagrams for sample K1), DR5 (age prob- stratigraphic relationships at this locality are not between 160 ± 4 and 154 ± 14 ka (Clark et al., ability diagrams for samples G1 and G2) and DR6 clear (Evernden and Curtis, 1965) and the K-Ar 2003). Existing geochronological evidence seems (age probability diagram and inverse isochron for method is highly susceptible to contamination to indicate that the earliest MSA predates the fi rst Malewa Gorge sample KA963), and complete Ar isotopic and geochemical data, is available online at by older grains. The oldest MSA archaeology appearance of H. sapiens, but it is to be expected www.geosociety .org/pubs/ft2008.htm, or on request whose age is well documented occurs in the that fossils are more poorly preserved and ini- from [email protected] or Documents Secre- Kapthurin Formation in Kenya, where single tially less abundant than lithic artifacts. Thus, tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, December December 2008; 2008 v. 36; no. 12; p. 967–970; doi: 10.1130/G25213A.1; 3 fi gures; Data Repository item 2008241. 967 14°N B 19 A 0.34 A Site 18 Addis Ababa ETH 72-5 17 0.32 Gademotta/Kulkuletti 16 Unit 15 15 0.30 Site 14 Samples 45°E Figure 1. Geographic and ETH 72-6 13 G1, G2 (wt%) 2 0.28 stratigraphic placement D TiO of archaeology at Gade- Unit 10 (G3) Site C 0.26 motta and Kulkuletti. Unit 10 (K2) ETH 72-7B B A: Map showing location A 1m Lower unit 15 (G1) 0.24 Upper unit 15 (G2) of Gademotta and Kul- 11 Unit 10 Unit D (K1 sampled 1970s) kuletti within Ethiopia. Unit D (K1 sampled 2007) B: Stratigraphic section Sample G3 0.22 Site 276 ± 4 ka 8.50 8.70 8.90 9.10 9.30 9.50 Al O (wt%) of Gademotta Formation ETH 72-8B 9 2 3 at type locality. C: Cross Unit 10, Sample G3 B section of Gademotta Site Age = 276 ± 4 ka Formation at Kulkuletti. ETH 73-2 Site C ETH 72-1 MSWD = 0.9 For B and C, unit numbers Unit D Site P = 0.57 are shown to right of sec- Sample K1 ETH 72-9 tion. Sampled tuffaceous 183 ± 10 ka units are shaded in gray. Artifacts are represented with black ovals. Adapted Relative probability from Laury and Albritton (1975). Reproduced with Unit 10 permission. Sample K2 240 280 320 280 ± 8 ka Age (ka) Unit D, Sample K1 C 0.0028 Age = 183 ± 10 ka ~12× vertical 0.0024 40Ar/36Ar Intercept = 330 ± 40 South exaggeration North MSWD = 0.91, P = 0.64, n = 46 0.0020 Ar 40 0.0016 Ar/ 36 0.0012 tion consists largely of volcaniclastic sediments not preceded by assemblages with Sangoan or and paleosols that can be divided into two por- Fauresmith characteristics, regarded by some 0.0008 tions: a lower section consisting mainly of lahar workers as transitional to the Acheulean. 0.0004 deposits and lacking artifacts, and a paleosol- The great age attributed to Gademotta and 0 0 0.2 0.4 0.6 0.8 1.0 1.2 rich upper section containing abundant arti- Kulkuletti has played a key role in establishing 39Ar/40Ar facts. The upper section begins with Laury and the antiquity of the MSA industry. Based on Albritton’s (1975) unit 9, wherein small Final early K-Ar dating, Gademotta and Kulkuletti Figure 2. Geochemical and geochronological Acheulean hand axes are found at the base of the were believed until recently to contain the oldest results. A: Glass shard geochemistry from sampled units at Gademotta and Kulkuletti. unit (Wendorf et al., 1975, 1994). The upper part known MSA artifacts with an age of 235 ± 5 ka Filled squares were sampled in the 1970s by of unit 9 and overlying units in the Gademotta for unit 10 (Wendorf et al., 1994).