Saharan green corridors and Middle Pleistocene hominin dispersals across the Eastern Desert, Sudan

Abstract

The Sahara Desert episodically became a space available for hominins in the Pleistocene. Mostly, desert conditions prevailed during the interpluvial periods, which were only periodically interrupted by enhanced precipitation during pluvial or interglacial periods. Responding to Quaternary climatic changes, hominin dispersal was channeled through vegetated corridors. This manuscript introduces a recently discovered group of Acheulean and Middle Stone Age sites far from the Nile Valley in the Eastern Desert (Sudan), referred to as Eastern Desert Atbara River (EDAR). The ∼5 m stratigraphy of the area is divided into three units (Units I–III) bounded by erosion surfaces. Each contains archaeological horizons. The EDAR area has rich surface sites with Acheulean horizons under the surface, singular finds of hand-axes within stratigraphic context in exposures, and large Acheulean sites partly exposed and destroyed by the gold mining activity. Optically stimulated luminescence (OSL) dating of Acheulean and MSA horizons from the EDAR 135 site indicates that the sedimentary deposits with stone artifacts were formed during the Middle Pleistocene between Marine Isotope Stages (MIS) 7 (pluvial) and 6 (interpluvial). Based on the OSL dating from the top of Unit IB, Acheulean artifact-bearing sedimentary deposits from overlying Unit IIA are younger than ca. 231 ka. Unit IA is the oldest Acheulean horizon in the EDAR area, not yet dated but definitively older than ca. 231 ka. An MSA horizon found in fluvial sediment was dated to be between 156 and 181 ka by OSL. The EDAR Pleistocene archaeological sites provide evidence for the presence of additional corridor(s) across Nubia, which connects the early hominin dispersals from the Nile and Atbara River systems to the Red Sea coast.

Introduction

According to current evidence, Pleistocene hominins left Africa to disperse into Eurasia during at least two dispersal events. Out-of-Africa I refers to the early hominin dispersal prior to Homo sapiens (e.g., Homo erectus/Homo ergaster; Bar-Yosef and Belfer-Cohen, 2001, Mirazón Lahr, 2010). Modern humans’ dispersal to Eurasia (Out-of-Africa II), with its very early phase (∼200 ka) into the Levant (Hershkovitz et al., 2018) and a later phase (∼85 ka) into Arabia (Groucutt et al., 2018), took place long after the appearance and evolution of H. sapiens in Africa (Mirazón Lahr and Foley, 2016, Soares et al., 2016, Hoffecker, 2017). Different routes have been suggested for this migration of hominins out of Africa, including the Mediterranean coast of North Africa, the Sahara, the Nile Valley, the Red Sea coast, and the Bab-el-Mandab Strait (Bailey, 2009, Petraglia, 2011). At least one of these possible routes crossed Sudanese Nubia. The roles of Nubia and the Nile Valley in this process have already been an element of the general debate of the migration from Africa (Van Peer, 1998, Vermeersch, 2001, Rose, 2004, Mirazón Lahr, 2010, Abbate and Sagri, 2012, Garcea, 2012, Wurz and Van Peer, 2012).

The role of NE Africa in the hominin dispersal from the African continent is insufficiently recognized. This region is archaeologically understudied (Masojć, in press), and is limited to several Middle and Late Pleistocene sites from the Nile and Atbara valleys (e.g., Wendorf, 1968, Chmielewski, 1987, Van Peer, 1998, Van Peer, 2016, Vermeersch, 2000) or the Egyptian oases (e.g., Caton-Thompson, 1952, Schild and Wendorf, 1981, Wendorf et al., 1993). Only two sites from Sudan have been previously dated to Marine Isotope Stages (MIS 7–4) using optically stimulated luminescence (OSL; Van Peer et al., 2003, Masojć et al., 2017). Recent gold mining operations in the Eastern Desert of Sudan reveals multiple formerly buried Acheulean and Middle Stone Age (MSA) sites. This is a new area that could potentially provide new chronological data and lithic inventories within a geological context. Moreover, it could help to understand human dispersals in the Pleistocene through previously unknown routes, as well as the demise of the Acheulean tradition.

We report here the latest discovery of an agglomeration of localities from the Acheulean Industrial Complex (de la Torre, 2016, Wynn and Gowlett, 2018) in the Eastern Desert of Sudan far from the Nile Valley, named EDAR (Eastern Desert Atbara River; Figure 1A; see Nassr and Masojć, 2018), suggesting the presence of yet another migration corridor(s) across Nubia connecting the Nile and Atbara River system to the Red Sea coast (Fig. 1B). Both Acheulean and MSA sites are recognized. A complex of EDAR sites is located in the exposed areas of gold mines/shafts (Fig. 1C, D) in the lower reaches of the Atbara River, about 70 km from the town of Atbara. Placed within the large Wadi el Arab stretching from the Red Sea Mountains to the Atbara River, EDAR sites are found in their original stratigraphic contexts. Preliminary results from the study of the EDAR sites indicate that these are the latest African Acheulean sites in the Eastern Desert. The evidence of occupation is preserved in the fluvial environment of several braided river channels from the upper Middle Pleistocene, between MIS 7 (pluvial) and MIS 6 (interpluvial).

The EDAR sites are distributed mainly on the basement complex of the Arabian-Nubian Shield. The basement complex is composed of Precambrian to Paleozoic schists and granitic or granodioritic gneisses with associated metasedimentary and metavolcanic rocks which were assumed to be formed by plutonism and metamorphism before the Mesozoic (Sefelnasr et al., 2015). Upstream in the Atbara River valley, Neogene basalts are overlain by Early to Middle Pleistocene deposits like the Butana Bridge Synthem (Abbate et al., 2010). Toward the downstream section of the Atbara River, the fluvial deposits, including the Khashm El Girba (Fig. 1) subsynthems, are prevalent on the gneiss. This progressively transitions to Late Pleistocene fluviolacustrine deposits, particularly in the south-western part of the Red Sea Hills (Harrell, in press).

The Acheulean is present across much of Africa. Hand-axes, cleavers and other large cutting tools represent typical Acheulean products (i.e., Mode 2 of Clark, 1969; compare also with the modified technotypological characterization for Sahara of Clark's ‘Mode’ nomenclature by Cancellieri et al., 2016). A recently proposed division of the Acheulean divides it into three phases: Early (1.75–1.0 Ma), Middle (∼1.0–0.6 Ma), and Late (∼0.6–0.3 Ma), although the latest date for the African Acheulean ranges between 0.27 and 0.17 Ma (Sahnouni et al., 2013, Gilbert et al., 2016). In Africa, the Acheulean overlaps in time with the presence of H. erectus/H. ergaster, Homo heidelbergensis and post-H. erectus Middle Pleistocene hominins (Klein, 2009, Stringer, 2016, Sahle et al., 2018).

In its early phase, as exemplified by several sites in the Konso Formation (Ethiopia), the Acheulean toolkit made on large flake blanks or cobbles contains both large cutting tools (LCT) such as crude hand-axes or cleavers and heavy-duty tools such as picks and core-axes that are bifacially and unifacially shaped (Beyene et al., 2013). Cores and small and medium-sized flaked debitage similar to those known in earlier Oldowan sites are also present in early Acheulean sites (de la Torre and Mora, 2018). A simple core preparation for large flake production, including the systematic preparation of striking platforms and some degree of predetermined morphology, could also be considered as a characteristic feature of Early Acheulean industries (Semaw et al., 2009). After about 1 Ma, Acheulean assemblages depict the technological evolution of hand-axes into more standardized forms, such as triangular, oval and other forms, with advanced thinning and symmetry (Sharon, 2007). This Middle Acheulean phase with predetermined, sophisticated technologies for large flake production of hand-axes and frequent cleavers on flakes was proposed to be named ‘Large Flake Acheulean’ (Sharon, 2010). The late Acheulean in Africa, including the youngest sites, is distinguished by fewer cleavers, soft hammer usage for hand-axe production, the appearance of ovate and pointed hand-axes, and the introduction of the Levallois technology (Tryon and McBrearty, 2002, Gilbert et al., 2016).

By approximately 300 ka, MSA technology was already distributed across a large part of Africa and was associated with the earliest dated occurrences of H. sapiens (Richter et al., 2017). The lowermost MSA horizon from the EDAR area belongs to the Nubian Complex of the MSA. The Nubian Complex represents a specific variant of NE African MSA (Van Peer, 2016). It stretches along the Nile river and the surrounding deserts from Egypt and Libya to the North to Ethiopia and the Horn of Africa to the South. The eastern border of the Nubian MSA appears to be the Sinai and southeastern Arabia (Rose et al., 2011:Fig. 1). The Nubian Complex is a distinctive type of Levallois reduction strategy that produces Nubian points from triangular Nubian cores of type 1 and type 2 (Marks, 1968, Van Peer, 1991, Van Peer, 1992). Although the evidence remains insufficient, the early phase of the Nubian Complex appears to feature leaf-shaped foliates and Nubian end-scrapers while, in the later phase, the Nubian cores of type 1 are more frequent than the cores of type 2 for the production of Nubian points. Near the EDAR area, in the neighboring Bayuda desert in Sudan, a well-dated Nubian MSA site with a large lithic inventory has been recently investigated - BP177 (Goat Mountain; Masojć, 2018).

The formation of the Atbara River contributed considerably to the dispersion of the Acheulean tradition towards the north during Early and Middle Pleistocene. The Acheulean sites from the region of Khashm el Girba in the upper reaches of the river were previously mentioned by Arkell (1949). Their stratigraphic context was presented by Chmielewski (1987) and later by Abbate et al. (2010). During the late Early Pleistocene to early Middle Pleistocene, the Atbara area was characterized by coarse-grained braided rivers. Paleolake Atbara formed in the lower reaches of the present-day Atbara River, overlying late Middle Pleistocene to Late Pleistocene deposits. At the beginning of the Holocene after the Last Glacial Maximum arid period, due to increased rainfall in the Nile and Atbara catchments and the melting of the ice cap of Semien Mt. (Ethiopia), a rise in the water level caused the overflow of the Atbara paleolake. Consequently, its northward outlet cut the Nubian massif along the north-south Wadi Gabgaba fault system and reached the Nile system (Abbate et al., 2010). Abbate et al.’s (2010) research shows that a 50 m thick Pleistocene fluvial succession is extensively exposed in the area along the Atbara River from Khashm el Girba to Halfa al Jadida. The chronology of the succession relies on fossil assemblages, artifacts, paleomagnetic, and radiometric data (U/Th dating). Vertebrate remains and several Acheulean artifacts were found in the sediments from late Early to early Middle Pleistocene (before MIS 7) as well as in Middle and Late Pleistocene deposits. Site 047 on the W bank of the Atbara River yielded hand-axes, retouched choppers, flakes and blanks. The occurrence of Elephas recki recki, a large elephant extinct ca. 900 ka (Todd, 2005) and Hippopotamus cf. gorgops, a large hippopotamus extinct ca. 700 ka (Boisserie, 2005), along with associated Acheulean artifacts, point to an Early Pleistocene date. The Atbara Acheulean artifacts are similar to those found in Buia in Eritrea (Martini et al., 2004) and could be associated with African H. erectus/H. ergaster. However, the younger lithic industries could have been manufactured by H. sapiens. It is possible that throughout the Pleistocene, both hunter species were using the Atbara area and its northward prolongation along the Nile valley as a wide settlement territory and a bridgehead towards Eurasia.