When first mooted, the Out of Africa model for the spread of anatomically modern humans (AMH) centred on a single exodus from African to Eurasia, which researchers broadly agreed to have occurred about 60 thousand years ago. That was when an advance of continental glaciers and sea level fall narrowed to manageable proportions the obstacle presented by the Red Sea. The only archaeological drawback was that AMH had occupied the Levant at around 110 ka. That was formerly considered to have been a temporary occupation corralled by hyperarid conditions immediately to the east and a mountain barrier to the north, with the Mediterranean Sea to the west. Yet, during humid periods there was every chance that the eastern barrier would occasionally have been permeable. Plumping for the 60 ka exit model was a conservative view stifled by a lack of high-quality dates for scattered suggestions of an Asian AMH presence, such as occurrences of stone tools resembling those of early moderns and even rarer, incomplete and often ambiguous skeletal remains. The ‘modern-looking’ tools that occurred both above and below the 74 ka Toba ash deposit in southern India were disposed of as ‘advanced’ tools of earlier migrants; probably Homo erectus. In retrospect, the established fact of earlier occupation of Eurasia by such ‘primitive’ African migrants, as long ago as ~1.8 Ma in the case of Homo fossils in Georgia, should have encouraged the view that culturally better-endowed AMH would have had less problem in diffusing eastwards once they found an escape route from Africa.
Whatever, the flurry during the last couple of decades of more skeletal and archaeological remains of AMH in Asia, genetic evidence for their interbreeding in the west and east with earlier human groups and, principally, improvements in dating ancient sites suggests a more complex geographic flow. Christopher Bae of the University of Hawaii and colleagues based in the UK, Germany and the US have reviewed this growing wealth of new data to put forward various scenarios for Out of Africa dispersal through Asia (Bae, C.J. et al. 2017. On the origin of modern humans: Asian perspectives. Science, v. 358, p. 1269 (summary); online full paper DOI: 10.1126/science.aai9067). They highlight growing evidence for at least one pre-60 ka dispersal, and probably several, to reach the Levant, Arabia, India, China, Laos, Indonesia, the Philippines and Australia before that date. This tallies with Neanderthal and Denisovan DNA segments within the genomes of living Eurasians that indicate interbreeding before 60 ka.
Bae and colleagues also assemble data that bear on where AMH managed to move out of Africa. They resolve the dispute between routes around the northern shores of the Red Sea and crossing the southern Straits of Bab el Mandab by concluding ‘why not both’. Where the migrations went to is currently suggested by the distribution of sites that reveal either pre- or post-60 ka occupancy. The earlier dispersals may have been dominated by following coastlines along the Mediterranean in North Africa to the Levant and via Bab el Mandab across the Persian Gulf, along the northern Indian Ocean littoral to south-east and east Asia. The later, more ‘adventurous’ movements using both routes led to Europe and deeper into continental Asia and thence to its north east. The review conveniently covers in seven pages much the same geoarchaeological and anthropological ground as Earth-Pages has visited bit-by-bit as it has unfolded since 2000. Clearly, great swathes of Asia have not been explored by palaoanthropologists. As in most geographic sciences there is a tendency to follow up known sites year after year – often decade after decade – to ensure publishable results, and that will consume lots of economic and human resources. It is more risky to try and fill in the gaps, but that basic field work is urgently needed to supply new material.