Early to Middle Pleistocene Homo dispersals from Africa to Eurasia: Geological, climatic and environmental constraints
Introduction
Human dispersal out of Africa to Eurasia is one of the most intriguing and debated issues of paleoanthropology. Discussion deals on what caused this event; various authors have proposed the climatic-ecological variations and demographic pressure as the main causes. A statistical approach to evaluate the influence of various factors has been developed by Mithen and Reed (2002). According to Bar-Yosef and Belfer-Cohen (2001), the possibility of surviving tropical diseases was enhanced by humans’ movement toward healthier northern regions.
Items including the age of dispersal, the number and success of dispersal events, and the possible returns to Africa have been put forward by Templeton, 2002, Dennell, 2003, Dennell and Roebroeks, 2005, Olivieri et al., 2006 and Dennell (2010). For the issue of the routes of dispersal, there is a relatively easier answer for Homo sapiens through genetic studies (Mellars, 2006, Olivieri et al., 2006, Templeton, 2007, Campbell and Tishkoff, 2008, Oppenheimer, 2009). However, focusing on earlier Homo species makes the tracking of their paths difficult because of scarce and often poorly dated human fossil records and archeological sites. Moreover, a successful colonization for short or extended periods depends on factors that are difficult to assess, including the strategies and cultural technologies acquired in the homeland, and those implemented along the way and in colonized areas. Further debated subjects about hominin movement include the possibility of crossing sea arms, lakes and large rivers (Straus, 2001, Derricourt, 2005), and the crucial migrant survival problem of needing daily access to drinking water (Oppenheimer, 2009).
This paper will focus on the geological, morphological, ecological and climatic conditions from the Early to Middle Pleistocene that could have influenced human dispersal, with comment on the traditional and generally accepted route of dispersal (the Nile River valley–Levantine corridor) and an additional analysis of the transit through the Bab el Mandeb Straits, Sicily Channel, Strait of Gibraltar, and trans-Sahara megalake and riverine corridors (Fig. 1). The 1995–2010 field research in Eritrea (Abbate et al., 1998, Abbate et al., 2004) and Sudan (Khasm El Girba, Atbara valley, Abbate et al., 2010; Red Sea western coastal area) provides new elements for a more reliable evaluation. These two regions constitute strategic areas where the hominins had to choose the best pathway for migration.
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Morphological and geological constraints on dispersal pathways
In addition to human and archeological records, the reconstructions of possible dispersal pathways from Africa to Eurasia rely mostly on sea-level fluctuations and faunal evidence. These elements are generally placed in a paleogeographic context based on the present-day situation. However, during the Late Pliocene and Pleistocene times, tectonic deformation and volcanism significantly affected mountain chains, rift areas and marine basins. This is particularly true for the East Africa triple
Sea-level changes
Global sea-level fluctuations are related to plate tectonic processes involving accretion and fragmentation of supercontinents, variable volume of mid-oceanic ridges and thermal cooling of oceanic lithosphere. These geodynamic processes are responsible for the first-order changes of sea-level with a hundred million year cyclicity (Haq et al., 1987). The high-frequency sea-level changes that characterize the Late Pliocene and Quaternary are imputed to glacioisostasy with the waxing and waning of
Hydroclimatic and ecological records
Based on the marine isotope stages (Raymo et al., 1992) and increases in lithic rafted material in high-latitude North Atlantic sediment cores at about 2.6 Ma (Shackleton, 1984), the climatic deterioration marks the onset of major glaciations in the northern hemisphere. After these global events, rapid climatic variations occurred during the Pleistocene. These oscillations were less intense, with a periodicity of 40 ky during the Early Pleistocene, but become well marked at the Early/Middle
Faunal African affinities in the Africa to Eurasia transit areas
The previous considerations of the geological/climatic conditions governing the possible dispersal routes can be discussed in light of the analyses of the faunas that were present in the Africa to Eurasia transit areas during the Early Pleistocene (Arribas and Palmqvist, 1999). These faunas will also provide information on paleoecological conditions and food availability both for animals and humans.
In Sicily, the rodents Pellegrinia panormensis and Asoriculus burgioi, both found in the Monte
Out of Africa phases of dispersal and migration routes
According to Bar-Yosef and Belfer-Cohen (2001), at least three main dispersal waves occurred during the Early Pleistocene between 1.8 and 0.8 Ma. To test this hypothesis, hominin and lithic industry sites with currently uncontested or commonly accepted dates were selected from the literature (Table 1, Table 2, Table 3, Table 4, Table 5, Table 6). Eighty-five sites were considered as suitable candidates, and their ages were plotted, grouped in one hundred thousand year intervals, and
Discussion
Although numerous well-dated anthropic sites have been published in the last decades, doubts still remain about the chronological and geographical data distribution of vast unexplored areas, particularly in Northern Africa and Central Asia as evidenced in the maps of Fig. 6, Fig. 7, Fig. 8. Crucial information about the routes of dispersal could be derived from surveys of the shelfal areas that repeatedly emerged during the Plio-Pleistocene, but now drowned due to the Holocene sea-level rise.
Conclusions
During the Pleistocene climate changes, environmental variations as well demographic pressure drove human populations to leave the African motherland and disperse toward more resource-rich and healthy Eurasian regions. This diaspora has allowed global diffusion and, possibly, species survival. Humid corridors that were active for a short time, such as the Nile River valley and the trans-Sahara megalake and riverine belts, have been exploited by humans to cross temporarily inhospitable North
Acknowledgements
We thank Bienvenido Martínez Navarro, Luca Bondioli, Roberto Macchiarelli and Lorenzo Rook for their comments on an early draft of the paper. We gratefully acknowledge the corrections and suggestions provided on the manuscript by two anonymous referees. Financial support for this research from the Italian Ministry of Foreign Affairs, DGPCC, Settore Archeologia, and the Italian Ministry for the University and Research (COFIN 2005).
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2022, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :This is the case because a permanent land connection to Eurasia existed there during the Pleistocene and is documented by the dating of human fossil and archaeological sites (Beyin, 2011; Breeze et al., 2016; Groucutt et al., 2021). Besides terrestrial routes, sea straits across the Mediterranean Sea or across the Red Sea offer additional routes to enter Eurasia, e.g., via the Gibraltar Strait (Alimen, 1975; Arribas and Palmqvist, 1999; Villa, 2001; Gibert et al., 2003; Clyde Winters, 2011; Rolland, 2013; Soria-Boix et al., 2017), Sicily Strait (Alimen, 1975) and/or the Bab-al-Mandab Strait (Bailey, 2009; Abbate and Sagri, 2012; Rohling et al., 2013; Beyer et al., 2021). The configuration of these sea straits varied along with climatic stages.