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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

The Chronological Haze

A general problem encountered in trying to date archaeological events or objects is that multiple, independent age estimates are typically more scattered than would be expected with normal statistical variation, even when margins of error for each estimate are considered. Much of this extra spread is an artifact of experimentation—an unwanted but unavoidable outcome of different materials being dated by different methods, using variable equipment, calibration standards, measurement procedures and data-analysis techniques. This complication often arises even when a common set of samples (or a single sample) from one site is dated using the same technique in different laboratories. Unless all experimental conditions, instrument specifications, reference standards and analytical software are identical, extra spread in results should be expected. We call this the chronological “haze” because it obscures the true age of the sample or event of interest.

South Africa timelineClick to Enlarge Image Many attempts have been made to determine when the Still Bay and Howieson’s Poort industries started and ended, using a range of numerical and relative dating methods. Numerical methods produce quantitative age estimates that can be placed on a standard timescale, commonly expressed as years before present. Relative dating methods produce ages that can be ordered relative to one another but need to be calibrated against a numerical age to get anchored on a timescale. The first and best-known numerical technique is radiocarbon dating, which under ideal circumstances can pinpoint when a plant or animal died in the last 60,000 years. In practice, however, sample contamination makes it challenging to reliably date samples more than half that age, so alternative numerical methods are used for older samples and nonplant or nonanimal materials. Uranium-series dating of calcite formations in caves, electron spin resonance dating of tooth enamel, thermoluminescence dating of burnt stones and optically stimulated luminescence (OSL) dating of sun-bleached sediments have all been applied with varying success to southern African archaeological sites over the past three decades. So has dating of ostrich eggshell by amino acid racemization, a relative dating method. Resulting age estimates for the Still Bay ranged between about 130,000 and 50,000 years ago, whereas the Howieson’s Poort estimates were slightly younger, 100,000 to 40,000 years ago. Such coarse-grained chronologies leave plenty of room for imaginative speculation about the likely duration of these two industries and the reasons for their origins and demise.





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