Mystery of human migration: Why did modern human beings disperse out of Africa?

By Syed Akbar

Scientists world over believe that modern humans had dispersed from 
Africa into Eurasia about 60,000 years ago. But they are not sure what 
had forced them to leave Africa after living there for about 1,00,000 
years.
Modern man evolved about 1,60,000 years ago in the African continent 
and of this, he had spent about 1,00,000 years there. And suddenly he 
started migrating from Africa about 60,000 years ago into Asia and 
Europe.
A number of major technological, economic and social developments 
in southern Africa between 80,000 and 60,000 years ago were 
responsible for the dispersion of modern human beings to different 
parts of the world, reveals a research study by Dr Paul A Mellars of the 
University of Cambridge, UK.
Dr Mellars, who is the professor of prehistory and human evolution in 
the department of archaeology at Cambridge, was in Hyderabad to 
participate in an international workshop on "Human evolution and 
disease". In his paper, "archaeological evidence for modern human 
origins and dispersal," presented at the workshop, Dr Mellars based his 
study on a number of recently investigated sites in South Africa.
"The technological and other innovations associated with these 
developments would seem to have given the modern human 
populations the crucial adaptations that were necessary to expand from 
Africa and to colonise a range of new and alien environments, and to 
rapidly expand their range over most areas of Asia and Europe," he 
pointed out.
According to Dr Mellars, recent discoveries in India and Sri Lanka 
show some striking similarities to sties in eastern and southern Africa, 
which must be very close in time to the period when the first dispersal 
from Africa took place, between 50,000 and 60,000 years ago. It would 
seem that the further dispersal eastwards of these populations 
throughout southern and south-eastern Asia was accompanied by a 
progressive loss in the complexity of the associated technologies, 
through a succession of repeated demographic and cultural founder 
effects, leading to the comparative simplicity of the earliest Australian 
technologies.
It is known that populations that were essentially modern in both 
genetic and anatomical terms had already emerged in Africa by at least 
1,50,000 years ago but it is not clear why did it take these populations a 
further 1,00,000 years to disperse to other regions of the world? 
Another question that has been haunting scientists is what were the 
crucial evolutionary and adaptive developments that allowed these 
populations to colonise a range of entirely new and alien environments 
and to successfully compete with, and replace, the long-established, 
and presumably well adapted, archaic populations in these regions? 
It is interesting to see that two separate approaches to the analysis of 
mtDNA (mitochondrial DNA) patterns in present-day African lineage 
point strongly to an episode of rapid population growth in the ancestral 
Africa populations centered broadly within the time range from 60,000 
to 80,000 years ago, i.e., some 1,00,000 years after the inferred most 
recent common ancestor of mitochondrially modern populations in 
Africa. 
Clearly, the precise age of these inferred population expansions 
depends on the accuracy of the assumed mutation rate of mtDNA, but 
the evidence as a whole points strongly to a major and apparently rapid 
increase in African population numbers much earlier than that 
experienced in either Asia or Europe and apparently involving 
expansion by means of a demographic "diffusion wave" from a 
relatively small population nucleus (probably confined to a fairly small 
region of Africa) to other parts of the continent.  
The central question is what could have caused this apparently dramatic 
expansion in African populations between 60,000-80,000 before 
present, and it is here that recent archaeological research in southern 
and central Africa becomes central to the interpretation of the 
demographic data.
The most relevant evidence at present comes from a number of sites 
located close to the southern tip of Africa in Cape Province, most 
notably from Blombos Cave and Klasies River on the southern coast 
and those of Boomplaas Cave and Diepkloof, further to the north and 
west.
Dr Mellars points out that the excavations at these sites revealed "soft 
hammer" techniques of flaking; new forms of both specialised skin 
working tools (end-scrapers) and tools for the controlled shaping of 
bone and wooden artefacts (so-called burin forms); a range of 
extensively shaped bone tools, apparently used as both tips of throwing 
spears and sharply pointed awls for skin working; new forms of 
carefully shaped stone inserts, probably used as tips and barbs of either 
hafted throwing spears or conceivably wooden arrows; large numbers 
of perforated estuarine shells, evidently used as personal ornaments of 
some kind; and large quantities of imported red ochre, including two 
pieces from the Blombos cave with carefully incised and relatively 
complex geometrical designs on their surfaces. These designs represent 
the earliest unambiguous forms of abstract "art" so far recorded.
Equally significant in these sites is the evidence for the large-scale 
distribution or exchange of both high-quality stone for tool production 
and the recently discovered shell beads from the Blombos cave, in both 
cases either transported or traded over distances of at least 20-30 km. 
All of these features show a striking resemblance to those which 
characterise fully modern or "Upper Palaeolithic" cultures in Europe 
and western Asia, which first appeared with the initial arrival of 
anatomically and behaviourally modern populations at about 
45,000-50,000 BP (before present) i.e., some 20,000 years later than 
their appearance in the African sites.
Moreover the total population numbers in Africa decreased 
significantly at this time, owing to the onset of extremely dry 
conditions in many parts of Africa between 60,000 and 30,000 BP.
"The point is simply that increased levels of technological efficiency 
and economic productivity in one small region of Africa could have 
allowed a rapid expansion of these populations to other regions and an 
associated competitive replacement (or absorption) of the earlier, 
technologically less "advanced," populations in these regions," Dr 
Mellars argues. 
The pivotal question, of course, is what caused these radical changes in 
the technology, economy, and social patterns of African groups about 
80,000-70,000 BP, asks Dr Mellars.
He says the emergence of distinctively modern patterns of culture and 
technology was due to a sudden change in the cognitive capacities of 
the populations involved, entailing some form of neurological 
mutation.
"Or alternatively (and more prosaically), we could look for an 
interpretation in terms of some major shift in the adaptive and selective 
pressures to which the human populations were subjected, perhaps 
precipitated by some major episode of climatic and environmental 
change. In this context, the obvious candidate would be the sharp 
oscillations between wetter and drier climatic conditions that marked 
the transition from oxygen isotope stage 5 to stage 4, as reflected in the 
deep-sea core and ice-core climatic records," he says.
The final, and most controversial, issue at present is exactly when and 
how these anatomically and genetically modern populations first spread 
from Africa to other parts of Asia and Europe. Here there are two main 
possibilities. According to Dr Mellars, the first is that the initial 
expansion occurred via North Africa and the Nile valley, with 
subsequent dispersal to both the west into Europe and to the east into 
Asia. The second is that the initial dispersal was from Ethiopia, across 
the mouth of the Red Sea, and then either northward through Arabia or 
eastward along the south Asian coastline to Australasia-the so-called 
"southern" or "coastal" route.