Ancient Watercourses and Biogeography of the Sahara Explain the Peopling of the Desert
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Ancient watercourses and biogeography of the Sahara explain the peopling of the desert Nick A. Drakea,1, Roger M. Blenchb, Simon J. Armitagec, Charlie S. Bristowd, and Kevin H. Whitee aDepartment of Geography, King’s College London, Strand, London WC2R 2LS, United Kingdom; bKay Williamson Educational Foundation, 8 Guest Road, Cambridge CB1 2AL, United Kingdom; cDepartment of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom; dSchool of Earth Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, United Kingdom; and eDepartment of Geography, University of Reading, Whiteknights, Reading RG6 6AB, United Kingdom Edited by Ofer Bar-Yosef, Harvard University, Cambridge, MA, and approved November 22, 2010 (received for review August 23, 2010) Evidence increasingly suggests that sub-Saharan Africa is at the region could have been habitable during “green” periods of great- center of human evolution and understanding routes of dispersal er humidity (9,10). This humid period was first suggested by “out of Africa” is thus becoming increasingly important. The Sahara Duveyrier (11) to explain the existence of the Nile crocodiles Desert is considered by many to be an obstacle to these dispersals in isolated Saharan oases and has periodically been promoted and a Nile corridor route has been proposed to cross it. Here we since, notably by Dumont (12) who noted the trans-Saharan dis- provide evidence that the Sahara was not an effective barrier tribution of numerous aquatic animal species. Recent evidence and indicate how both animals and humans populated it during for this view is provided by isotopic data from a Mediterranean past humid phases. Analysis of the zoogeography of the Sahara marine core and aquatic snails that suggest a river corridor across shows that more animals crossed via this route than used the Nile the Sahara during marine isotope stage (MIS) 5e (13) and varia- corridor. Furthermore, many of these species are aquatic. This dis- tions in dust concentrations in a North Atlantic marine core that persal was possible because during the Holocene humid period the indicates humid conditions during the last two interglacials (14). region contained a series of linked lakes, rivers, and inland deltas The idea of the Sahara as a barrier to faunal dispersal is comprising a large interlinked waterway, channeling water and an- becoming increasingly important because of its role in the out-of- imals into and across the Sahara, thus facilitating these dispersals. Africa debate. Here we use numerous lines of evidence to eval- This system was last active in the early Holocene when many spe- uate the barrier hypothesis, and use this evidence to propose cies appear to have occupied the entire Sahara. However, species models for the peopling of the Sahara during past humid phases. that require deep water did not reach northern regions because of In the next section, we map the palaeohydrology of the Sahara weak hydrological connections. Human dispersals were influenced and show that during humid periods it contains an extensive in- by this distribution; Nilo-Saharan speakers hunting aquatic fauna terconnected hydrological system that animals appear to have with barbed bone points occupied the southern Sahara, while peo- used to migrate across the green Sahara. The following section ple hunting Savannah fauna with the bow and arrow spread south- compares the spatial distribution of archaeology and languages ward. The dating of lacustrine sediments show that the “green to that of the fauna and palaeohydrology to develop theories Sahara” also existed during the last interglacial (∼125 ka) and pro- of the peopling of the green Sahara during its last humid phase. vided green corridors that could have formed dispersal routes at a Finally, we evaluate dates of palaeolake sediments from the likely time for the migration of modern humans out of Africa. previous humid phase and show that it corresponds with the age of the first permanent modern human occupation of the North biogeography ∣ human migration ∣ animal migration ∣ palaeoclimate ∣ African coast and the Levant, suggesting a crossing of the Sahara Saharan palaeohydrology at this time. raditionally, it has been assumed that the Nile was the primary African Humid Period Biogeography and Palaeohydrology Tcorridor for the dispersal of hominins across the Sahara desert Reanalysis of the Saharan zoogeography (SI Appendix, Section 1 and then out of Africa (1). However, this route has proven hard to and Table S1) suggests that many animals, including water-depen- substantiate for three reasons. First, considerable research efforts dant creatures such as fish and amphibians, dispersed across the have failed to uncover evidence for its consistent use and the Sahara recently. For example, 25 North African animal species movement of its inhabitants into the Levant (2). Second, the have a spatial distribution with population centers both north and chronology of the evolution of the modern Nile drainage, which south of the Sahara and small relict populations in central re- is essential to create a functional corridor, is controversial (3, 4). gions. This distribution suggests a trans-Saharan dispersal in the Finally, although many ancient fluvial deposits have been recog- past, with subsequent local isolation of central Saharan popula- nized in the Nile Valley, some with lithics that confirm the pre- tions during the more recent arid phase. If a diverse range of spe- sence of hominins, their stratigraphic relationships and ages are cies (including fish) can cross the Sahara, it is impossible to poorly understood (5). envisage the Sahara functioning as barrier to hominin dispersal. The need for a Nile migration corridor arises from the view The zoogeography of the Nile suggests that it was a much less that a belt of aridity has always separated the more humid regions effective corridor (SI Appendix, Table S1). Only nine animal spe- north and south of the Sahara, yet the evidence for this arid cies that occupy the Nile corridor today are also found both north barrier is contradictory. For example, genetics indicates that and south of the Sahara. This paucity of Nileotic fauna highlights the Sahara has been a barrier to the dispersal of Macroscelidea (elephant shrews) since the late Miocene when the desert first Author contributions: N.A.D. and R.M.B. designed research; N.A.D., R.M.B., S.J.A., C.S.B., formed (6), whereas pollen analysis of terrestrial sediments from and K.H.W. performed research; N.A.D., R.M.B., and S.J.A. analyzed data; and N.A.D. and the central Sahara demonstrates that during the Holocene humid R.M.B. wrote the paper. period an arid belt existed north of 22° (7). Furthermore, arid The authors declare no conflict of interest. plant taxa are found continuously through Atlantic Ocean cores This article is a PNAS Direct Submission. between 23° and 27° north for the last 250 ka suggesting a long- 1To whom correspondence should be addressed. E-mail: [email protected]. lasting Saharan arid belt at this latitude (8). In contrast, a green This article contains supporting information online at www.pnas.org/lookup/suppl/ Sahara route has been proposed whereby this currently hyperarid doi:10.1073/pnas.1012231108/-/DCSupplemental. 458–462 ∣ PNAS ∣ January 11, 2011 ∣ vol. 108 ∣ no. 2 www.pnas.org/cgi/doi/10.1073/pnas.1012231108 Downloaded by guest on September 30, 2021 the importance of a green Sahara corridor and indicates that Chad, and Nile Basins form a single biogeographic province (20) the Nile route is, potentially, less significant. A further 13 species (Fig. 2A). The low degree of endemism indicates that these of animal are not found in the Nile corridor but are found both palaeohydrological interconnections happened relatively recently north and south of the Sahara yet not in the center (SI Appendix, (15), perhaps during the early Holocene “climatic optimum” Table S1). Although they must have dispersed across the Sahara, when the Sahara was last humid (21). whether they used the Nile or Green Sahara route cannot be de- Freshwater mollusc biogeographic provinces (22) have a simi- termined without additional fossil evidence. lar spatial distribution to that of the fish (SI Appendix, Section 5 Fish species and molluscs are found both in oases across the and Fig. S5) suggesting that they dispersed across the Sahara Sahara and along the Nile, suggesting they crossed the Sahara using the same routes (SI Appendix, Section 2). That they have using both the Green Sahara and Nile routes. The majority of no endemic genera, and only 11 endemic species (22), again these species are thought to only disperse via water (15) (SI indicates a recent dispersal into the Sahara. In contrast, the Appendix, Section 2), thus current drainage only explains the Nile divide between amphibian biogeographic provinces (SI Appendix, populations and another mechanism needs to be established to Figs. S7 and S8) is located roughly equidistant from the head- explain the present trans-Saharan distribution. waters of the Nile, Chari, and Niger rivers and the Atlas Moun- Mapping the palaeohydrology of the Sahara, using satellite tains (23). The latter presumably acted as refuges for these imagery and digital topographic data, yields a tenable model animals during arid periods. The contrasting spatial distribution (Fig. 1 and SI Appendix, Section 3). The Sahara was formerly of fish to amphibians is explained by the restriction of fish to riv- covered by a dense palaeoriver network with many channels con- taining very large alluvial fans where rivers divide into multiple branches in an inland area. Where these fans are located on the boundary between two river catchments, their distributary chan- nels can temporarily link adjacent river systems, thus allowing water-dependant life to transfer from one basin to the next (SI Appendix, Section 4).