Catalysts for Stone Age innovations

Abstract

Fossil and genetic evidence suggest the emergence of anatomically modern humans (Homo sapiens) in sub-Saharan Africa some time between 200 and 100 thousand years (ka) ago. But the first traces of symbolic behavior – a trait unique to our species – are not found until many tens of millennia later, and include items such as engraved ochres and eggshells, tools made from bone, and personal ornaments made of shell beads. These behavioral indicators appear in concert with two innovative phases of Middle Stone Age technology, known as the Still Bay (SB) and Howieson’s Poort (HP) industries, across a range of climatic and ecological zones in southern Africa. The SB and HP have recently been dated to about 72–71 ka and 65–60 ka, respectively, at sufficiently high resolution to investigate the possible causes and effects. A remarkable feature of these two industries is the spatial synchroneity of their start and end dates at archaeological sites spread across a region of two million square kilometers. What was the catalyst for the SB and HP, and what were the consequences? Both industries flourished at a time when tropical Africa had just entered a period of wetter and more stable conditions, and populations of hunter-gatherers were expanding rapidly throughout sub-Saharan Africa before contracting into geographically and genetically isolated communities. The SB and HP also immediately preceded the likely exit time of modern humans from Africa into southern Asia and across to Australia, which marked the beginning of the worldwide dispersal of our species. In this paper, we argue that environmental factors alone are insufficient to explain these two bursts of technological and behavioral innovation. Instead, we propose that the formation of social networks across southern Africa during periods of population expansion, and the disintegration of these networks during periods of population contraction, can explain the abrupt appearance and disappearance of the SB and HP, as well as the hiatus between them. But it will take improved chronologies for the key demographic events to determine if the emergence of innovative technology and symbolic behavior provided the stimulus for the expansion of hunter-gatherer populations (and their subsequent global dispersal), or if these Middle Stone Age innovations came into existence only after populations had expanded and geographically extensive social networks had developed.

Acknowledgements

This study was supported by the Australian Research Council, through grant DP0666084.

Figures and Tables

Figure 1 Oxygen isotope data (expressed in per mil) for the period 90-30 ka obtained from the Byrd26 and EPICA Dronning Maud Land (EDML)24 ice cores from West and East Antarctica, respectively. The vertical grey bands delineate the start and end dates, and durations, of the SB and HP industries and the timing of the immediately post-HP pulse.15,16 The solid black squares and horizontal grey bars denote the mean ages and 95% confidence intervals for the late and final MSA periods.22