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
Clearing the Haze
In 2006, we began to clear the haze obscuring the Still Bay and Howieson’s Poort industries. We did it by deploying a single dating method, OSL, on the same type of material from a number of key sites, using identical procedures and equipment. OSL offers many advantages. It can capture ages of objects that originate or events that occurred during the entire Middle Stone Age. It produces ages with a precision of better than plus or minus 5 percent. If a single operator can perform the dating, then the precision can be improved further, as can the chances for consistency.
OSL exploits the fact that buried quartz grains absorb energy from natural ionizing radiation sources (mainly uranium and thorium, their radioactive decay products, and potassium) in the surrounding ground. A small fraction of that energy is stored by electrons trapped at defects in the crystal structure of the grains. Using blue or green light, the electrons are released from their traps in the laboratory and the resulting OSL is detected with a filtered photomultiplier. This allows an estimate to be made of the equivalent radiation dose. By also measuring the rate of supply of ionizing radiation to the quartz grains from the surrounding deposit (the environmental dose rate), it is possible to calculate when the grains and nearby artifacts, whether they be stone tools or Stone Age ornaments, were buried.
OSL dating was introduced by David Huntley of Simon Fraser University and his colleagues in 1985, primarily to date geological sediments. But the method underwent a revolution near the turn of the millennium, when measurement procedures and instrumentation enabled equivalent radiation doses (and, by inference, burial ages) to be obtained from single grains of quartz as small as 0.1 millimeter in diameter. That made it feasible to routinely measure hundreds of individual grains for every sample, providing multiple, independent estimates of age that could be compared for self-consistency and any signs of sample contamination or disturbance. For our dating of the Still Bay and Howieson’s Poort artifacts, article co-author Zenobia Jacobs collected and analyzed all of the samples in a systematic fashion, eliminating inconsistencies from site to site that plagued earlier dating efforts.
Judging when an archaeological tradition might have started and ended is not simple. It cannot be done solely from artifacts collected at one site because it is unlikely that the entire duration of an industry will be represented at one location. Archaeologists must assemble a series of “snapshots” from multiple sites to get a fix on first and last appearance dates. This is easier said than done, especially in the case of the Still Bay, which has been found at only a handful of spots throughout southern Africa. The situation is much better for the Howieson’s Poort, with around 30 confirmed or claimed occurrences in the high mountains of Lesotho, the desert fringes of Namibia, the continental interior and subtropics of South Africa and the coastal areas and hinterlands along the continent’s southern margin. All the sites are natural shelters and caves that hunter-gatherers used as temporary camping places. Remnants of their everyday activities—fireplaces, food remains and tools—have been excavated at the sites.
Our subcontinental survey included 11 locations that contained one or both of the Still Bay and Howieson’s Poort industries, including some of the classic Middle Stone Age sites of southern Africa, such as Klasies River. Other sites were either not accessible for sampling or, in the case of Blombos Cave, had been dated previously by Jacobs. We intentionally selected geographically widespread sites in diverse environmental settings to capture the full range of natural variability in climatic conditions affecting the region at different times. The deposits at two of the sites proved unsuitable for OSL dating, but a sufficient number of samples (54 in total) were dated from the remaining nine sites to allow us to establish the time of first and last appearance of the Howieson’s Poort and, with less confidence, the Still Bay.
As we reported in
in 2008, both industries appear to have been extremely short-lived, appearing suddenly and then vanishing soon after. Remarkably, this pattern is seen over an area of 2 million square kilometers, cutting across a range of climatic and ecological zones. Because of the lack of any spatial variation in timing, we pooled all of these data in a statistical model, devised by Rex Galbraith of University College London, to estimate the first and last appearance dates of the Still Bay and Howieson’s Poort. We concluded that the Still Bay lasted no longer than perhaps 1,000 years (from approximately 72,000 to 71,000 years ago). Howieson’s Poort debuted 7,000 years later in the archaeological record, around 65,000 years ago. This industry ended abruptly five millennia later, followed by another gap of about 3,000 years before the first of the subsequent periods of less sophisticated Middle Stone Age technology. The margin of error on these estimates is only a few millennia at the 95 percent confidence interval—sufficiently precise to resolve the ephemeral and punctuated nature of these bursts of technological and behavioral innovation.
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