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4. Modernity as an Emergent Property of Social and Demographic Factors

The apparent failure of the biological-essentialist programme has promoted the development of alternative explanations for the acceleration of innovation and the proliferation of symbolism in the later palaeolithic. These have focused on the role of demographic and ecological variables in the transmission of culture through social formations. Shennan's exploration of the relation between population size and the rate of cultural innovation has been especially influential. Using a mathematical model derived from population genetics he argues that, even presuming no change in individuals' proclivity for devising technical innovations, increased population size (where a population is essentially taken to mean a hunter-gatherer band) leads to an acceleration in the rate at which innovative practices are adopted by the population (Shennan 2001). In the model, increased population size mitigates the impact of functionally unsuccessful innovations, enhances the likelihood that beneficial innovations arise, and therefore increases mean Darwinian fitness. This underwrites in turn further population growth, so a virtuous feedback circle of innovation and population growth can develop. Shennan attributes the emergence of a coherent 'modern' behavioural package at the beginning of the African LSA around 50 kya to rapid population growth in Africa from 60 kya, following a period of environmentally forced population bottlenecks and local extinctions from 71 kya (Shennan 2001, 13). A similar argument has been advanced by Richerson et al. (2009), while a comparable model has been applied to explain the cultural loss of skills in Holocene Tasmania (Henrich 2004).

More sophisticated models of the relations between demographic variables and rates of innovation in the palaeolithic have since been developed. An important element in this has been the recognition that the regional metapopulation—that is, the wider population of local populations or subpopulations—is for some purposes a more appropriate unit of analysis. Powell et al. (2009) present a mathematical model in which the accumulation of innovations is enhanced both by an increase in the packing density of subpopulations within a region and by the rates of interaction between subpopulations in terms of migratory activity. Here population growth is understood primarily as an increase in the number of subpopulations within the metapopulation, rather than of the number of individuals in a subpopulation. The authors conclude that their model can adequately explain the observed archaeological patterns of short-lived outbreaks of modern behaviour in Africa in the MSA before 50 kya, followed by the crystallisation of the modern human behavioural complex in the LSA and its expansion into Europe in the Upper Palaeolithic, without invoking any evolutionary transformation in the heritable cognitive capacities of the Homo sapiens brain (Powell et al. 2009, 1301).

A different approach has been pursued by Premo and Kuhn (2010), who model the impact of rates of subpopulation extinction within a stable metapopulation. Their results suggest that the accumulation of innovations and levels of behavioural diversity are both promoted by longer-lived subpopulations. The demographic and ecological factors that impinge upon subpopulation longevity have in turn been considered by Hopkinson (2011), who identifies increased subpopulation size, number and proximity; expanded territorial range; logistically organised mobility; and enhanced levels of between-subpopulation migration, as key variables promoting subpopulation longevity and thus the persistence and accumulation of innovations. Support for these ideas has come from the modelling of the effects of mobility strategies on rates of encounter and cultural transmission in hunter-gatherers (Grove 2010; Perreault and Brantingham 2011). From a quite different perspective, several authors have recognised the significance of developments in hominin life-cycle parameters, such as retarded maturation rates, an extended childhood, the emergence of a discrete adolescence phase and the contribution of post-reproductive adults to child rearing, for the primary generation and subsequent adoption of innovations within palaeolithic subpopulations (Nowell and White 2010).

In these models modernity is conceived as an emergent property of particular conjunctures of social, demographic and ecological dynamics. And while both Shennan (2001) and Powell et al. (2009) continue to treat modernity as exclusive to Homo sapiens, both Premo and Kuhn (2010) and Hopkinson (2011) explicitly recognise that the archaeology of the Eurasian Middle Palaeolithic affords no reason to think that any innate cognitive ceiling precluded the possibility of modernity emerging from the dynamics of Neanderthal society, demography and ecology.

From the perspective adopted here, these theoretical and methodological developments in the study of modern human origins are welcome in that they offer an escape route from the problems of biological essentialism, and because they are more effective at explaining observed archaeological facts. Nevertheless, they are not enough, for two reasons:

1. Demographic models of modernity focus on cultural transmission, or the social transmission of knowledge. Consequently, although they deal with the issue of innovation in the palaeolithic, they have little or nothing to say about symbolism. The demographic school might have shown that, given the appropriate conditions, innovative behaviours could persist and accumulate in the cultural repertoires of any hominin species, but they have not shown how or why any of those innovations would carry a symbolic charge. We are left still relying on the biological-essentialist appeal to the evolutionary appearance of a heritable neuroanatomy, a genotype, that conferred symbolism on its possessors as a phenotypic trait.
2. As Gamble has observed, demographic models of the appearance of modernity are, at root, numbers games (Gamble 2012). More people means modernity. Consequently, modernity is still referred to a single prime mover — not the evolution of the brain, but the inexorable growth of hominin populations, possibly in a positive feedback loop with technological evolution. While short- and medium-term environmental and demographic processes, operating on local and regional scales, can lead to a loss of skills from cultural repertoires (Henrich 2004; Hopkinson 2011), demographic and social models of modernity nevertheless continue to rely on palaeolithic history delivering 'more' in the end; more subpopulations, more migration, more territory, more logistical mobility, more years before local subpopulations suffer extinction. Unless this issue is resolved, demographic approaches to modernity will remain as vulnerable to reductionism, priorism and to the teleological fallacy as is biological essentialism.

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