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HOME > PAST ISSUE > July-August 2009 > Article Detail

FEATURE ARTICLE

Human History Written in Stone and Blood

Two bursts of human innovation in southern Africa during the Middle Stone Age may be linked to population growth and early migration off the continent

Zenobia Jacobs, Richard G. Roberts

… and Demographic Contractions

On the flip side of the demographic coin are records of population isolation also preserved in mtDNA. There is an emerging consensus that our species experienced several such episodes in sub-Saharan Africa, and did not always form a single, geographically continuous population. Particular attention has been paid to the L0 haplogroup, which is the most ancient of the African mtDNA lineages and has two branches (L0d and L0k) that are most prevalent among the Khoi and San (Khoisan) “Bushmen” of southwest Africa. The Khoisan still live a hunter-gatherer lifestyle and are famous for speaking a “click” language. Recent studies of complete mtDNA genomes have revealed that the Khoisan became genetically isolated more than 90,000 years ago. Only with the advent of the Later Stone Age—tens of millenia later—was there renewed genetic admixture in the Khoisan mtDNA pool, supporting a deep ancestry for this language family. A similar demographic pattern has been uncovered in tropical central Africa, using the L1c haplogroup to trace the mtDNA history of isolation of Pygmy hunter-gatherers. The ancestors of these equatorial rainforest dwellers became genetically isolated no later than about 74,000 years ago, with gene flow resuming about 40,000 years ago. At present, we also cannot rule out the possibility that the L3 haplogroup experienced episodes of population isolation, superimposed on a general trend of growth and lineage diversification between 80,000 and 60,000 years ago. Is the similarity in timing of these major demographic shifts in sub-Saharan Africa and the ages for the Still Bay and Howieson’s Poort mere coincidence? We think not.

Successive pulses of population expansion and contraction in southern Africa might explain why the Still Bay rose to prominence so abruptly across such a large area and then vanished in less than a millennium, and why the Howieson’s Poort began 7,000 years later and lasted about 5,000 years. For technological and behavioral innovations to be spread widely and rapidly, a cohesive network of social contacts is needed to promote the transmission of new ideas and inventions. Periods of population expansion of the L3 haplogroup could conceivably have created such a network and prompted geographically widespread trade and exchange of high-quality stone and symbolic artifacts across southern Africa. In this hypothesis, the gap between the Still Bay and the Howieson’s Poort represents a period of population contraction, during which social networks weakened or collapsed. The reasons for this calamity remain an enigma, but perhaps the prevailing cooler conditions altered the balance of nature, triggering a change in the distribution, diversity, predictability and productivity of resources needed by hunter-gatherers. If people were forced to abandon habitation sites and to move to find sustenance, links in the social network could have been strained or broken, isolating new innovations to smaller groups with limited geographic dispersal.

It might have taken another explosion in population size to reinvigorate this social network across southern Africa, resulting in the widespread transmission of the latest technological innovation associated with the Howieson’s Poort (backed blades for hunting weapons). This integrated, subcontinental network of hunter-gatherer communities was maintained for more than five millennia, but then disappeared about 60,000 years ago, perhaps in response to the population contractions and isolations identified by genetic studies. Similarly sophisticated stone-tool technology did not reappear for another 20,000 years—the end of the Middle Stone Age in East Africa—when there is evidence for renewed gene flow south of the Sahara.




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