Olduvai's oldest Oldowan
Introduction
The Oldowan stone tool industry dominates the Early Stone Age archaeological record from its first appearance at 2.6 Ma (Braun et al., 2019) up until the advent of Acheulean technology at around 1.76 Ma (Semaw, 2006; Lepre et al., 2011). Tools and applied methods of lithic reduction provide an important record of the technological adaptations and behavior of Pleistocene hominins (Braun et al., 2008). Although commonly viewed as being characterized by rather simple core and flake methods of production (e.g., Leakey, 1971), some studies debate variation stemming from raw material constraints (Toth, 1985) and more complex technological strategies of Plio-Pleistocene hominins than was previously thought (e.g., Semaw et al., 1997; Roche et al., 1999; Brown et al., 2008; Reti, 2016; Toth and Schick, 2018). Oldowan tools also have a broad geographic range across much of Africa, Asia, the Middle East, and Europe. They are best known from East Africa (Plummer, 2004), and only a few sites in the 2.0–2.6 Ma range have been found outside of this region. The Oldowan has long been regarded as the oldest known tradition of toolmaking, often associated with early Homo. However, it has recently been argued that tools from the Lomekwi 3 site in West Turkana, Kenya, dating to 3.3 Ma are an even earlier stone tool culture distinct from the Oldowan (Harmand et al., 2015, 2019; Lewis and Harmand, 2016), although such interpretation has been contested (e.g., Domínguez-Rodrigo and Alcalá, 2016, 2019; Pickford, 2018).
The Oldowan industry was named after the site where the culture was first defined, in Olduvai Gorge, NE Tanzania, itself (Fig. 1A). Olduvai Gorge has previously provided some of the largest and best preserved assemblages of Oldowan tools from anywhere in the world, the oldest of which known so far is from the Upper Bed I DK site (Fig. 1B; Leakey, 1971), just older than Tuff IB, which dates at 1.848 ± 0.003 Ma (Deino, 2012). We report here on a new Oldowan assemblage from the yet underexplored western Olduvai Basin that predates all others in the region, time bracketed by the Coarse Feldspar Crystal Tuff (CFCT) and Tuff IA, dated at 2.015 ± 0.006 Ma (Deino, 2012) and 1.976 ± 0.004 Ma (Walter et al., 1992), respectively (Fig. 2A). Bracketing dates are tested and generally confirmed by comparison with orbitally defined timescales (Stanistreet et al., 2020a: Fig. 10) and preliminary results of ongoing magnetostratigraphic analysis and 40Ar/39Ar dating ( Deino et al., 2020 ). This section is also important because it is missing in the only time-equivalent exposure in the eastern Olduvai Basin, where it is represented by a pronounced unconformity beneath the Bed I Basalt (Stanistreet et al., 2020b).
Agewise, the new Trench 168 assemblage is thus almost contemporaneous with, e.g., Kanjera South, Kenya (Plummer and Bishop, 2016), Renzidong, China (Hou and Zhao, 2010), and probably Pabbi Hills, Pakistan (Dennell, 2004, 2015), and Riwat, Pakistan (Dennell et al., 1988, 2006). In the context of other radiometrically dated Oldowan archaeological sites (Fig. 2B), the archaeological occurrence is younger than Lomekwi 3, Kenya (Harmand et al., 2015), Gona, Ethiopia (Semaw et al., 2003), Hadar, Ethiopia (Campisano, 2012), Lokalalei, Kenya (Roche et al., 1999), Omo, Ethiopia (Feibel et al., 1989; de la Torre, 2004), Ain Boucherit, Algeria (Sahnouni et al., 2018), Swartkrans, South Africa (Gibbon et al., 2014), and Sterkfontein, South Africa (Granger et al., 2015). Outside of Africa, other claims of sites older than Trench 168 include Gongwangling (Zhu et al., 2018) and Longgupo (Han et al., 2017) in China and Masol in India (Gaillard et al., 2016). However, the assemblage is also older than many other Oldowan occurrences, including all the other Oldowan sites within Olduvai beds I–II, Fejej, Kenya (De Lumley et al., 2004), Area 123, Koobi Fora, Kenya (Gathogo and Brown, 2006), Dmanisi in the Republic of Georgia (Ferring et al., 2011), and the Nihewan Basin Site in China (Liu et al., 2014). Both Australopithecus (Paranthropus) boisei and fossils assigned to early Homo are known in East Africa from the time of this archaeological occurrence. Worldwide, there is a dearth of archaeological sites that bear stone technologies older than 2 Ma (Fig. 2B). Our new discovery not only adds to our knowledge of the technological changes of Homo worldwide but also extends the age range of hominin activities at Olduvai to ∼2 Ma. Furthermore, it extends the age range of the classic Oldowan culture at Olduvai further back in time by about 75% from previously a span of ∼200 kyr (Tuff IB to Tuff IIA/IIB) to now a span of ∼350 kyr (CFCT to Tuff IIA/IIB).
The Olduvai Gorge (Fig. 1A) is located to the northwest of the Ngorongoro Volcanic Highlands (NVHs) that flank the western side of the Eastern Branch of the African Rift System in NE Tanzania (Hay, 1976; Ashley and Hay, 2002). The gorge is deeply incised into the southern Serengeti plain and naturally exposes a ∼100 m-thick Plio-Pleistocene section of fluviolacustrine sediments interbedded with volcaniclastics and lavas of the Olduvai Basin. The basin fill has been subdivided into seven beds that contain a variety of tephrostratigraphic marker units (Fig. 2A), enabling reliable correlation and dating of the rich paleoanthropological and archaeological finds (Hay, 1976) that have made the Olduvai Gorge a world-famous site. Since 2010, the site was inscribed along with the Ngorongoro Conservation Area and Laetoli footprints site as a World Heritage Mixed Property by the United Nations Educational, Scientific and Cultural Organization. Bed I is the thickest of the stratigraphic intervals identified and is the principal object of this contribution. The Bed I stratigraphic interval is exceptionally important from a paleoanthropological perspective because it records a rich sequence of hominin land use associated with Oldowan technology (Leakey, 1971; Blumenschine et al., 2012a, 2012b; Domínguez-Rodrigo et al., 2017).
According to Hay (1976), Bed I extends from the top of the Naabi Ignimbrite up to the top of Tuff IF; McHenry et al. (2008) then included the Naabi Ignimbrite in the Bed I succession. Subdivisions of Bed I have been used differently in geological and archaeological contexts (Fig. 2A): Hay (1971) chose the basaltic lava flows to distinguish a Lower Member of Bed I below the lavas from an Upper Bed I Member above them; the lavas themselves he labeled as Basalt Member. Leakey (1971) then further subdivided the Bed I Upper Member sensu Hay (1971) into Lower, Middle, and Upper units to define detailed stratigraphic positions of excavated sites and lithic industries on the basis of Bed I marker tuffs. Here, we follow Hay's (1971) geological nomenclature, recently refined by geochemical fingerprinting of tephrostratigraphic markers (McHenry, 2012; McHenry et al., 2020), chemostratigraphy (Habermann et al., 2016a), and sequence stratigraphy (Stanistreet, 2012; Stanistreet et al., 2018a) and use the term ‘Lower Bed I’ to address strata below the Bed I Basalt. Lower Bed I is only exposed in the western gorge, apart from a small exposure at Locality 9A, immediately below the basalt (Hay, 1976). Lower Bed I exposed in the Western Basin dates back to the Naabi Ignimbrite at 2.038 ± 0.005 Ma (Deino, 2012) and predates the basalt lava effusion at 1.956 ± 0.031 Ma (Ar/Ar date Olduvai Gorge Coring Project [OGCP] Core2A: Deino et al., 2020), which is within the error of ∼1.94 Ma (astronomically derived date: Stanistreet et al., 2020a) and includes the Naabi Ignimbrite, the CFCT, and Tuff IA as important marker horizons (Fig. 2A). The new Olduvai Basin stratigraphy proposed by Stanistreet et al. (2020b), however, defines the top of the CFCT and its subsequent or occasionally laterally equivalent reworked units (also termed the CFCT zone; Habermann et al., 2016a) as the base of Bed I; consequently, the Trench 168 assemblage would be located in the top part of the Ngorongoro Volcanic Formation, just beneath the overlying Bed I Formation (Fig. 2A).
During Bed I times, the Olduvai Basin was mostly occupied by a saline-alkaline lake, named Paleolake Olduvai (Hay, 1976; Hay and Kyser, 2001), suggesting that the basin was mostly hydrologically closed. Short-term and voluminous sediment input to the Olduvai Basin was provided by explosive eruptions and erosion of nearby NVH volcanic centers, the products of which are registered by both primary and secondary (reworked) pyroclastic units, previously all encompassed by the term ‘Tuff’ (capital T) in archaeological excavations (Leakey, 1971; Hay, 1976). Large mixed braided river and debris flow-dominated volcaniclastic fans fed by the NVH prograded into the lake from the southeast (Habermann et al., 2016b; Stanistreet et al., 2020b). Reworking of tephra and admixture of nonpyroclastic material led to the deposition of volcaniclastic sandstones, gravels, and diamictites between successive volcanic events. Dating of tuffs, most significantly by Deino (2012), and analysis of their potential volcanic sources in the NVH (Mollel et al., 2008), integrated with mineralogical and geochemical studies, led McHenry et al. (2008), Mollel and Swisher (2012), and Habermann et al. (2016a) to identify Ngorongoro Volcano as the main source for the largely rhyolitic, trachytic to basaltic tuffs of Lower Bed I (sensu Hay, 1971), whereas the majority of the Upper Bed I tuffs are trachytic and derived from Olmoti Volcano (Fig. 1).
In outcrop, the first direct evidence for the existence of Paleolake Olduvai from the geological record was found just above Tuff IA (Hay, 1976), although Habermann et al. (2016b) recognized lacustrine claystones immediately below Tuff IA and reported the presence of sedge phytoliths and diatoms from levels just above the CFCT, suggesting the proximity of freshwater sources at that time or earlier. Drilling results recently proved an even earlier existence of pre–Bed I lacustrine environments, dating the existence of Paleolake Olduvai back to at least 2.24 Ma (Deino et al., 2020). During Bed I, the principal lake-related settings comprise a central lake body, to the east of Richard Hay Cliff (RHC) and beyond the Fifth Fault (Fig. 1B), which is fringed by alternately flooded and dried lake-marginal flats, including grasslands (Bamford et al., 2008) and vegetated freshwater wetlands (Liutkus and Ashley, 2003; Blumenschine et al., 2009, 2012a; Albert et al., 2018).
Driven by wet/dry climatic fluctuations, the lake repeatedly expanded and contracted, including almost complete drying out, for example, at the time of Tuff IF (Stollhofen et al., 2008). However, flooding of large areas of the basin also occurred, e.g., during periods of enhanced precipitation (Hay, 1976; Ashley and Hay, 2002) and/or lake damming through faulting, lava flow, or pyroclastic flow barriers. For Paleolake Olduvai, Hay and Kyser (2001) were the first to infer wet and dry periods during Upper Bed I/Lower Bed II from alternating periods of lake expansion and contraction, which Ashley (2007) and Deino (2012) subsequently used to conclude the existence of precession-driven (∼19 kyr) climate cycles. Magill et al. (2013a, 2013b) and Colcord et al. (2018) deduced from carbon isotopic composition of organic matter that grassland and woodland proportions covary with orbital precession. Using the geochemistry of authigenic clays as a paleosalinity proxy, Deocampo (2015) identified lake ‘freshening events,’ which coincide with calculated orbital precessional episodes. Freshening events correlate best to Southern Hemisphere insolation (20°S) precessional cycles for December, a timing that today is associated with the short southern monsoon (Deocampo et al., 2017). Thus, a number of studies found evidence of precessional forcing of climate from ∼1.7 to 1.9 Ma, but apart from Stanistreet et al. (2020a), no dated lake sequences from eastern Africa, including Olduvai, are published yet that record orbital forcing for the period from 1.9 to 2.0 Ma.
It was the systematic and detailed description of Bed I and Bed II lithics associated with lake-margin deposits of Paleolake Olduvai (Leakey, 1971) that provided the basis for the definition of the Oldowan industry, depending on their inferred use, and its separation from the overlying Acheulean industry. Oldowan tools are based on a simple technology (Leakey, 1971; Reti, 2016), considered a ‘least effort solution’ (Isaac and Harris, 1997), and essentially consist of assemblages of stone core forms (choppers, polyhedrons, etc.) made from rounded river cobbles or angular chunks from bedrock outcrops, from which flakes were struck, and the resulting flaking debris (flakes and flake fragments), as well as smaller numbers of retouched flakes (scrapers, etc.) and battered hammerstones, subspheroids, and spheroids (Toth, 1982, 1985). Such Oldowan lithic assemblages were subsequently used as a standard for classifying other archaeological sites of similar age in Africa. At Olduvai, Bed I deposits of the FLK, FLK N, and FLK NN sites (Fig. 1) yielded not only the most extensive assemblages of the Oldowan stone tool industry but also the first discoveries of Paranthropus boisei overlain by Tuff IC (Leakey, 1971) with an interpolated age of 1.832 Ma (Deino, 2012) and of Homo habilis slightly lower (Leakey et al., 1964).
Although the Olduvai Gorge is the type area, the oldest currently known Oldowan tools are from Ledi-Geraru, Ethiopia, dated at 2.61–2.58 Ma (Braun et al., 2019); from the Siwaliks of Northwest India (Dambricourt Malassé et al., 2016; Gaillard et al., 2016), dated at ∼2.6–2.5 Ma; from Gona, Ethiopia, dated at 2.58–2.55 Ma (Semaw et al., 2003); from Longgupo, China, dated at ∼2.48 Ma (Han et al., 2017); from Hadar, Ethiopa, dated at 2.36 ± 0.07 Ma (Campisano, 2012); and from Lokalalei 2C, W Turkana, Kenya (Roche et al., 1999) and Omo, Ethiopia (Feibel et al., 1989), both dated at ∼2.34 Ma. At Olduvai, Oldowan tools occur throughout Beds I and II (Fig. 2A). MNK Skull site, between marker Tuffs IIA (∼1.71 Ma; Curtis and Hay, 1972) and IIB (not dated, but older than Tuff FLKWb: 1.664 ± 0.019 Ma; Diez-Martín et al., 2015), is considered the youngest Oldowan locality at Olduvai, whereas artifacts and faunal remains at the DK site (Geological Locality 13; Archaeological Site 22) in the eastern Olduvai Basin are commonly regarded as representing the oldest assemblage of ‘classic’ Oldowan lithics there (Leakey, 1971). The DK site is time-bracketed by Tuff IB (1.848 ± 0.003 Ma; Deino, 2012) and the Bed I basalt lava, originally dated in Eastern Basin exposures at 1.877 ± 0.013 Ma (Deino, 2012), but has now been 40Ar/39Ar dated in OGCP Core 2A at 1.956 ± 0.031 Ma (Deino et al., 2020), which is within the error of ∼1.94 Ma (Stanistreet et al., 2020a: astronomically derived date). At stratigraphically equivalent levels, other sub-Tuff IB Oldowan assemblages were discovered at Geological Localities 10 (LK), 11 (MK), 14 (JK), 34, and 61 (Hay, 1976). Notably, all except the latter site are located in the eastern Olduvai Basin between the lake margins of Paleolake Olduvai and the NVH-sourced fan system beyond. The only well-known Bed I site in the western Olduvai Basin is Olduvai Landscape Paleoanthropology Project (OLAPP) Trench 57/65 at Geological Locality 64 (Fig. 1B), where a dentally complete H. habilis maxilla with lower face (OH65) was discovered. With associated Oldowan stone artifacts, and butchery-marked bones, the find was positioned between Tuff IC and Tuff IF (Fig. 2A) in a unit geochemically fingerprinted as syn-Tuff ID (Blumenschine et al., 2003).
Inspired by the recent discovery of the first direct, in situ, and to date oldest evidence of living trees at Olduvai Gorge in Lower Bed I of the western Olduvai Basin (Habermann et al., 2016b), found together with a few scattered Oldowan stone artifacts at Locality 66e (Fig. 1B), a systematic search was undertaken for sub-Tuff IA bones and artifacts throughout the area. During a consequent remeasurement of Locality 68 (Hay, 1976), the assemblage of lithics and bone described here was discovered ∼2.7 m above the CFCT and ∼10.0 m below Tuff IA (Fig. 2A). Apart from recent discoveries at Localities 66e and 68, throughout the eastern and western gorges, Oldowan assemblages have only been recovered from sediments deposited subsequent to the outpouring of the Bed I Basalt lava. Thus, previously reported assemblages are almost restricted to strata designated Upper Bed I by Hay (1963, 1976) and redesignated so by McHenry et al. (2008), that include Leakey's (1971) Lower, Middle, and Upper Bed I units. Until now, no hominin-related assemblage has been reported from Lower Bed I from below the equivalent stratigraphic position of the Bed I Basalt.
Section snippets
Materials and methods
Figure 1 provides a map of the Olduvai Main and Side Gorges indicating faults, locations of outcrop areas, measured sections, and the OGCP boreholes 2A and 3A (Stanistreet et al., 2020b; Njau et al., 2021) referred to in the text. The term ‘Junction Area’ addresses the area next to FLK, where the Side Gorge joins the Main Gorge, and also provides the roughly defined boundary between the Olduvai western and eastern basin. Individual valleys (korongos [K], e.g., FLK, MNK, DK) and cliffs (C, e.g.,
Stratigraphy and sedimentary environments of Trench 168
The assemblage level has a total thickness of ∼0.6 m and is associated with four main facies whose characteristics and environmental interpretations are outlined in Figure 4. Dominant in the Trench 168 section (Fig. 3) are multiply stacked, reddish brown or greenish gray, matrix-supported pebble- and isolated boulder-bearing medium to coarse sand-grade units, which in the context of the assemblage are labeled as Levels 1, 4, and 6. Based on their grain size distributions, they are classified as
Paleoenvironments and paleoecology
The facies associations and their architecture show that the post-CFCT sediments were deposited on the toe of a fan-delta which, as per available ages (e.g., Deino, 2012), geochemical data (e.g., McHenry et al., 2008; Mollel and Swisher, 2012; Habermann et al., 2016a, 2016b), measured paleocurrent indicators, and natural clast assemblages, was fed dominantly from Ngorongoro while it was an active volcano. Additional material would have been provided by other, then extinct volcanoes, such as
Conclusions
Trench 168 in the western Paleolake Olduvai Basin records a widespread Lower Bed I sub–Tuff IA paleolandsurface associated with an Oldowan artifact and bone assemblage dated at ∼2.0 Ma. This predates the DK assemblage in the eastern Olduvai Basin, just older than overlying Tuff IB, by ∼150 kyr. Trench 168 thus exposes the oldest assemblage yet found at the Oldowan type locality of the Olduvai Gorge.
The artifact assemblage comprises dominantly debitage (88.7%), besides much less frequent simple
Declaration of competing interest
The authors declare there is no conflict of interest.
Acknowledgments
Research reported here was supported by the German Science Foundation (DFG STO 275/9) to H.S., by the U.S. National Science Foundation (NSF) BCS 1623873 to J.K.N., by the Spanish Ministry of Economy and Competitivity to R.M.A. (HAR2016-75216), and by a grant of the Spanish Ministry of Economy and Competitivity to A.R.-C. (BES-2014-067985), which is gratefully acknowledged. The Stone Age Institute organized and funded the Olduvai Gorge Coring Project (OGCP) with grants from the Kaman Foundation,
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