Tuesday, August 25, 2015

Theories Of Dispersal Of Homo Sapiens From Africa

Huw S. Groucutt and colleagues in Evolutionary Anthropology lay out the evolving story of the dispersal of Homo sapiens from Africa. The review brings together fossil, genetic  and archaeological data which now strongly leans towards a scenario of multiple migrations of Homo sapiens out of Africa beginning more  than hundred thousand years ago. These migrations followed ecological  windows of opportunity.  Interglacial phases resulted in wetter climate in the Levant and Arabia and may have made viable migration routes following either coastal contours or more interior passages towards the rest of Europe and Asia.

An Excerpt:

A variety of dispersal models (Table 1) address the period between the widely accepted African origin of Homo sapiens by around 200-150 ka and the arrival of our species at the margins of the Old World, including Australia, Siberia, and northwest Europe, by 50-40 ka.1–4 The evolutionary, demographic, and cultural processes between these milestones remain unclear, but a variety of recent studies add important new data.Whereas earlier models focused on assessing the geographical origins of our species based on fossil data, more recent approaches seek to combine fossil, genetic, archeological, and paleoenvironmental data to illuminate the nuances of dispersal into Asia (Table 1). These models emphasize different hypotheses concerning factors such as when dispersals began, how many occurred and which routes were followed. Recent models have largely fallen into two broad categories, emphasizing Marine Isotope  Stage (MIS) 5 (early onset dispersal model) or post-MIS 5 (late dispersal model) time frames (Table 1). This, however, is not a rigid dichotomy. For example, models proposing an early onset to dispersal are consistent with subsequent post-MIS 5 dispersals having also played an important role in patterns of human diversity.

The map below shows the distribution of Middle Paleolithic sites plotted on a modeled precipitation map of the last interglacial (MIS 5). The abundance of sites in the interior of Arabia speaks against a strictly coastal migration route into India. The interior of Arabia during humid phases would have been a mix of grasslands and riparian corridors offering potential dispersal routes into India and the rest of Asia.

Source: Huw S. Groucutt et. al. 2015

What is the Indian context?  The generally accepted earliest  modern Homo sapiens skeletal record in South Asia are ~35 k old fossils in Sri Lanka. But the tool record indicates presence of modern humans in India much before that. This review suggests that the totality of the tool records favors the theory that Homo sapiens may have entered India during MIS 5 more than a hundred thousand years ago, followed by additional  migrations beginning around fifty thousand years ago. Groucutt et. al. mention that future fossil discoveries from South Asia have the potential to transform ideas about the dispersal of Homo.

As of date the skeletal record of Homo in India consists of just a few fossils . Research by A.R.  Sankhyan and colleagues show that all of these have been found in the Narmada valley at Hathnora and a few km away at Netankheri . At Hathnora, hominin fossils occur in fluvial conglomerate and sand layer. One is a partially preserved calvarium and has been identified as a "robust" late Homo erectus or an archaic Homo sapien. Its cranial capacity is estimated to be 1200 cc to 1400 cc putting in the range of modern humans. It is associated with a collection of heavy duty large flake Acheulian hand axes and cleavers and chopping tools. The other fossil find at Hathnora consists of two clavicles and a partial 9th rib, interpreted to be belonging to a separate population of "short and stocky" archaic Homo sapiens associated with smaller Middle Paleolithic implements.  The cranium has been dated to the Middle Pleistocene ~ 250 k, while the clavicles and 9th rib appear to be younger with an estimated date to be ~150K range. A change in the ecology of this region is seen in the younger deposits based on the faunal content. The large flake tool industry disappears at this point in time . This has been interpreted to mean a migration of the larger robust archaic hominin away from this area based on appearance on this tool typology further north of this region and as far southeastwards to the Bastar region of Bihar.

At Netankheri, a partial femur and a humerus have been found. The femur occupies the same stratigraphic level as the Hathnora calvarium  and has been interpreted as belonging to late Homo erectus -archaic Homo sapien. The humerus though is of the "short and stocky" morphology and interpreted to represent an early modern Homo sapiens. Delicate bone implements have been found along with this fossil.  It is thought to be much younger, dated to be around 75 k, based on its stratigraphic position just below the Baneta Formation which contains Younger Toba Ash layers (ash deposits of the Toba eruption). The researchers interpret this to mean evolution from an archaic to a modern form, population continuity and continuous occupation of this area by this morphologically distinct hominin through the Middle and Late Pleistocene.

In summary, the skeletal and tool record points to presence of two culturally and physically distinct archaic hominin populations occupying the Narmada valley in the Middle Pleistocene. The tool record shows that Homo has been present in India for more than a million years and these physically distinct Middle Pleistocene hominins may be indicating the evolution of distinct hominin lineages in India. Or, was this population differentiation and morphological evolution inherited from an older African population structure, representing separate Middle Pleistocene migration episodes? And how they fit into the broader story of modern Homo sapiens dispersal and occupation of India remains to be worked out.

What is the margin of error on the 150 k date of the "short and stocky" hominin. Could they be younger and represent the early MIS 5 dispersal from Africa ( 100-125 K)?  Of interest are the ~35 K old Homo sapiens fossils from Sri Lanka which are physically distinct from the Netankheri "short and stocky" population. This points to another more recent (MIS 3) migration from Africa. Did these recent arrivals interbreed with the resident archaic hominins?  More fossils from South Asia are needed to fill in these gaps in our understanding of hominin evolution in India.  The authors of the Narmada hominins paper suggest that the "short and stocky" population may have contributed ancestry to later short bodied  populations of South Asia including the pygmies. Certainly, recent genetic work shows interbreeding between modern humans and other differentiated hominins like Neanderthals and Denisovans in Europe and East Asia respectively. Perhaps the Indian story is also one of assimilation of the earlier hominin populations with later human entrants.


  1. Suvrat - this is fascinating, thanks. When I was writing the desert book, I much enjoyed digging into the conventional wisdom of the "out of Africa" story and discovering the controversies, differing viewpoints, variety of evidence and seemingly endless possibilities, so this paper is right up my street! It is intriguing how the newly available revelations from Arabia are changing the story - and complicating it!

  2. Michael- thanks... glad the paper will be useful to you.. Besides Arabia, India too has a long more than 1 million record of Homo, and possibly entry of Homo sapiens pre Toba eruptions i.e. before around 75 k. so many potential stories to be found in India too. hoping for more fossils to be found here in India though :)