Upper Pleistocene Human Dispersals out of Africa: A Review of the Current State of the Debate

Abstract

Although there is a general consensus on African origin of early modern humans, there is disagreement about how and when they dispersed to Eurasia. This paper reviews genetic and Middle Stone Age/Middle Paleolithic archaeological literature from northeast Africa, Arabia, and the Levant to assess the timing and geographic backgrounds of Upper Pleistocene human colonization of Eurasia. At the center of the discussion lies the question of whether eastern Africa alone was the source of Upper Pleistocene human dispersals into Eurasia or were there other loci of human expansions outside of Africa? The reviewed literature hints at two modes of early modern human colonization of Eurasia in the Upper Pleistocene: (i) from multiple Homo sapiens source populations that had entered Arabia, South Asia, and the Levant prior to and soon after the onset of the Last Interglacial (MIS-5), (ii) from a rapid dispersal out of East Africa via the Southern Route (across the Red Sea basin), dating to ~74-60 kya.

Figure 1

Map showing the two potential dispersal routes out of Africa and hypothetical scenarios for Upper Pleistocene hominin dispersals: (a) 150–100 kya, Homo sapiens first appeared in East Africa [19] and dispersed to Arabia, SE Asia, and Levant after 150 kya [1, 5, 38]; (b) 100–60 kya, Homo sapiens continued to expand from Arabia and from the Levant toward Southeast Asia. The reasons for these dispersals may include range expansion in response to overpopulation, competition, and climate change. Gene flow between Homo sapiens and Neanderthals may have occurred in scenario (a), and some hybrid groups may have spread to South and East Asia afterwards. Scenario (b) also depicts the timing of the purported rapid coastal dispersal from East Africa to Southeast Asia via the SR [11].

Figure 2

Potential source populations for the colonization of Eurasia in the Upper Pleistocene and a tentative reconstruction of Homo sapiens and Neanderthal relationship based on recent genetic study by Green et al. [64]. On the basis of a recent Paleolithic discovery in the United Arab Emirates [5], the first out of Africa dispersal of early modern humans may have occurred in the range of 150–130 kya. This may have stimulated successive eastward migrations of hominins up to Southeast Asia, resulting in the occupation of Zhirendong (South China) around 100 kya [38]. The author believes that the out of Africa dispersal scenarios shown by B, D, and D′′ may not have resulted in replacement of the preceding inhabitants of the Levant, Arabia, and Southeast Asia (be it Neanderthals, archaic humans, or those Homo sapiens carrying Neanderthal genetic material).

Figure 3

Map showing the geographic distribution of ancestral mtDNA haplogroups of modern humans in Africa and Eurasia between 60 and 30 kya, after Forster ([, page 258], Figure 2). Note that all the ancestral haplogroups now found in Eurasia are believed to have branched from L3, which originated as a founder in eastern Africa around 85 kya [11]. The thick bold line indicates the general pathway of the southern dispersal route (SR) along the circum-Indian Ocean.

Figure 4

Sea level changes and early modern human dispersal events in the Upper Pleistocene [3, 46, 79]. Sea level chart redrawn from Siddall et al. ([, page 143], Figure 2).