Catalysts for Stone Age innovations

Abstract

Fossil and genetic evidence suggests the emergence of anatomi- cally 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 indica- tors 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 were the catalysts 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 communi- ties. 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 popula- tion expansion, and the disintegration of these networks during periods of population contraction, can explain the abrupt appear- ance 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 *Correspondence to: Zenobia Jacobs; School of Earth and Environmental Sciences; University of Wollongong; Wollongong NSW 2522 Australia; Email: zenobia@uow. edu.au Submitted: 12/29/08; Accepted: 01/02/09 Previously published online as a Communicative & Integrative Biology E-publication: http://www.landesbioscience.com/journals/cib/article/7743 www.landesbioscience.com 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 geographi- cally extensive social networks had developed.