DATING AND STRATIGRAPHY OF MIDDLE STONE AGE DEPOSITS AT BLOMBOS CAVE, SOUTH AFRICA, USING AMINO ACID RACEMIZATION IN OSTRICH EGGSHELL: IMPLICATIONS FOR THE EVOLUTION OF MODERN HUMAN BEHAVIOR

Blombos Cave is an important Middle Stone Age (MSA) locality on the south coast of South Africa at 34°25’S, 21°13’E, at 35 m above sea level. Excavations by C.S. Henshilwood, J.C. Sealy and others since 1993 have yielded an extraordinary series of finely made bone tools, and engraved objects including ocher fragments with the oldest known complex geometric incised designs. Sterile dune sand separates the Middle Stone Age layers from the overlying Holocene Later Stone Age occupations. The dune sands have recently been dated using optically stimulated luminescence to approximately 67-71 ka by Jacobs et al. (2003a, 2003b). To directly determine the age of the MSA levels themselves, we used liquid chromatography to analyze the degree of epimerization of L-isoleucine to D-alloisoleucine in 50 samples of ostrich eggshell, using high performance liquid chromatography. The samples derived from five different meter-square excavation units and encompassed six successive stratigraphic units within the MSA. In one square (E4), epimerization reflected in D-alloisoleucine/L-isoleucine ratios was greatest in the top unit, suggesting either a stratigraphic reversal or slight heating of this square’s deposit in antiquity. Other squares (e.g., F3 and E2) displayed D-allo-/L-isoleucine values that increased generally with depth, from approximately 0.35 to 0.50. ¹⁴C-dated calibration samples from other sites in the Cape Province suggest that this should correspond to an age of at least 70 ka for the youngest samples, and 90-100 ka or more for the oldest ones. No samples suggested younger ages or intrusive penetration of material from the overlying Later Stone Age horizons. These results confirm the considerable antiquity of symbolic and technological behaviors characteristic of modern humans in the African Middle Stone Age. We gratefully acknowledge the financial support from the National Science Foundation, and the George Washington University Committee on Research.

Jacobs, Z., et al., 2003a. Optcal dating of dune sand from Blombos Cave, South Africa: I-multiple grain data and II-single grain data. J. of Human Evol. 44: 599-612 and 613—625.