Tag Archives: AMH

Multiregional human evolution in Africa

Africa is not only a large continent, but is subdivided into many different climatic zones and ecosystems and these have changed drastically over the last 2 Ma. It is further subdivided by terrain features, such as the courses of major rivers, large plateaus, tectonic rift systems and the mountains that frequently define their flanks. Getting around Africa is not easy today, was more difficult before modern transport, and many geomorphic provinces may have been mutually inaccessible in the distant past. For instance, the Sahara Desert forms a major barrier to travellers on foot because access to surface water is non-existent except at widely spaced oases. Without boats or rafts the Nile and Congo cannot be crossed for a thousand miles or more. Migration was perhaps a very rare event outside of periods of widespread humid climates or when great environmental stress forced people either to move or perish. Despite these physical and ecological divisions and barriers palaeoanthropologists have, until recently, tended to regard the evolution of Homo sapiens and earlier human and hominin species as having occurred within single populations: a linear view forced on them by scanty fossil remains and limited methodologies. Logically, when human numbers were small Africa probably had several isolated population Physical isolation would have engendered genetic isolation in which our ancestors evolved for tens of thousand years.

Anatomically modern human (AMH) remains found at Jebel Irhoud in Morocco turned out to be 315 ka old, displacing those from Ethiopia (190 ka) as the earliest known examples of AMH. Several more archaic H. sapiens fossils have turned up in southern Africa and as far afield as the Middle East, suggesting that the early evolution of AMH was in an Africa-wide context rather than in one area – the rift system of Ethiopia and Kenya – from which a new species radiated outwards. This breadth of finds has encouraged Eleanor Scerri of Oxford University and her many international colleagues to resurrect what was once a widely discarded hypothesis; a multiregional model of modern human origins, originally proposed to have arisen from pre-sapiens groups in Eurasia by Milford Wolpoff but which was sunk once genetic connections among living humans turned out to be rooted in Africa. (Scerri, E.M.L. and 22 others 2018. Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology & Evolution, v. 33, p. 582-594; (PDF) doi: 10.1016/j.tree.2018.05.005). Scerri et al’s model is sited in Africa and the paper’s authors include several leading palaeoanthropologists who once opposed multiregionalism and established the Recent African Origin hypothesis on the back of the early genetic data.

early homo

Different early AMH cranium shapes: left Jebel Irhoud, Morocco (315 ka), right Qafzeh, the Levant (85 ka) (credit: Scerri et al, 2018; Figure 1)

From region to region in Africa, the oldest AMH crania show significant differences from each other, but within a distinct combination of features that clearly distinguish us from our fossil relatives and ancestors, such as Homo heidelbergensis from Zimbabwe and the primitive-looking H. naledi found in a South African cave in 2015. Improved dating now shows that the Zimbabwean H. heidelbergensis and H.naledi remains are roughly the same age as the Jebel Irhoud AMH specimens. The first has long been held as the progenitor of AMH and descended from H. antecessor, perhaps the common ancestor for AMH, Neanderthals and Denisovans about 700 ka ago. The three human species cohabited Africa early in the evolutionary history of AMH. It is now abundantly clear from ancient and modern genomes that AMH, Neanderthals and Denisovans interbred in Eurasia. The proximity in time and space of earlier African AMH to two more ancient human species opens up a similar possibility earlier in the emergence of all living humans. There is evidence for that too: Yoruba people living in West Africa, whose genomes have been analysed, carry up to 8% of genetic ancestry that originated in an unidentified ancient population that was non-sapiens. At present, DNA analysis with the same high precision and information content from other living Africans has not been performed, and deterioration of ancient DNA in African climates has so far thwarted genomic studies of ancient African fossils.

The new view of our origins points to repeated hybridisation involving other coexisting human species, as well as evolution in isolation, from the outset. It continued through later times while Neanderthals and Denisovans survived. Even recent human genetic history is peppered with intermingling of a great variety of migrants passing through all the habitable continents. Another issue: In the earliest times, were cultures exchanged as well as genes? The first appearance of AMH coincides with that of a new stone technology (Levallois technique), moving away from the earlier dominance by handaxes towards more delicate, leaf-shaped points, that characterise the African Middle Stone Age. Similar techniques reached Europe with the Neanderthals. Was this an invention of the earliest AMH or a joint venture?

You can find an excellent review of these issues in the September 2018 issue of Scientific American (Wong, K. 2018. Last hominin standing.  Scientific American, v. 319(3), p. 56-61) along with several other articles on human evolution.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Early human dispersal through Asia

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.