Population, Culture History, and the Dynamics of Culture Change

The "Weak Garden of Eden" model for the origin and dispersal of modern humans (Harpending et al., 1993) posits that modern humans spread into separate regions from a restricted source, around 100 ka (thousand years ago), then passed through population bottlenecks. Around 50 ka, dramatic growth occurred within dispersed populations that were genetically isolated from each other. Population growth began earliest in Africa and later in Eurasia and is hypothesized to have been caused by the invention and spread of a more efficient Later Stone Age/Upper Paleolithic technology, which developed in equatorial Africa. Climatic and geological evidence suggest an alternative hypothesis for Late Pleistocene population bottlenecks and releases. The last glacial period was preceded by one thousand years of the coldest temperatures of the Later Pleistocene (approximately 71-70 ka), apparently caused by the eruption of Toba, Sumatra. Toba was the largest known explosive eruption of the Quaternary. Toba's volcanic winter could have decimated most modern human populations, especially outside of isolated tropical refugia. Release from the bottleneck could have occurred either at the end of this hypercold phase, or 10,000 years later, at the transition from cold oxygen isotope stage 4 to warmer stage 3. The largest populations surviving through the bottleneck should have been found in the largest tropical refugia, and thus in equatorial Africa. High genetic diversity in modern Africans may thus reflect a less severe bottleneck rather than earlier population growth. Volcanic winter may have reduced populations to levels low enough for founder effects, genetic drift and local adaptations to produce rapid population differentiation. If Toba caused the bottlenecks, then modern human races may have differentiated abruptly, only 70 thousand years ago.

Certain aspects of what archaeologists have traditionally called stylistic variation can be understood as the result of the introduction of selectively neutral variation into social-learning populations and the sampling error in the cultural transmission of that variation (drift). Simple mathematical models allow the deduction of expectations for the dynamics of these evolutionary mechanisms as monitored in the archaeological record through assemblage diversity and interassemblage distance. The models are applied to make inferences about the causes of change in decorative diversity and interassemblage distance for Woodland ceramics from Illinois.