"Duration of agriculture and distance from the steppe predict the evolution of large-scale human societies in Afro-Eurasia"

From Nature:

Abstract

Understanding why large, complex human societies have emerged and persisted more readily in certain regions of the world than others is an issue of long-standing debate. Here, we systematically test different hypotheses involving the social and ecological factors that may ultimately promote or inhibit the formation of large, complex human societies. We employ spatially explicit statistical analyses using data on the geographical and temporal distribution of the largest human groups over a 3000-year period of history. The results support the predictions of two complementary hypotheses, indicating that large-scale societies developed more commonly in regions where (i) agriculture has been practiced for longer (thus providing more time for the norms and institutions that facilitate large-scale organisation to emerge), and (ii) warfare was more intense (as proxied by distance from the Eurasian steppe), thus creating a stronger selection pressure for societies to scale up. We found no support for the influential idea that large-scale societies were more common in those regions naturally endowed with a higher potential for productive agriculture. Our study highlights how modern cultural evolutionary theory can be used to organise and synthesise alternative hypotheses and shed light on the ways ecological and social processes have interacted to shape the complex social world we live in today.

Introduction
The size and complexity of modern human societies is on a scale unmatched in other species. Yet for much of our evolution humans lived in small-scale, internally undifferentiated groups, and it is only in the last several thousand years that larger-scale societies with more complex forms of organisation began to develop resulting in what we can label “macro-states” or “empires” involving millions of individuals. Anecdotal and empirical research indicates that historically the largest human societies tended to be situated in a relatively narrow band of the Afro-Eurasian landmass, stretching from Western and Central Europe and the Mediterranean in the West, through to China in the East (Fig. 1) (Diamond, 1997; Turchin et al., 2013). Understanding how and why humans are able to form functioning societies on such a scale, and why large, complex societies have tended to form more readily in certain places are questions of long-standing interest across a range of disciplines (Carneiro, 2003; Flannery and Marcus, 2012; Sanderson, 2015). The strong geographic patterns noted above suggest that ecology may play an important role yet a number of other factors have been proposed to be important in driving the evolution of social complexity such as the development and productivity of agriculture (Diamond, 1997; Nielsen, 2004), information processing (Morris, 2013), warfare (Turchin et al., 2013), the geography of continental land masses (Diamond, 1997), technology (Morris, 2013), and religion (Norenzayan et al., 2014). However, there have been relatively few empirical tests of these competing ideas within a common theoretical framework. Here, we employ modern cultural evolutionary theory to systematically develop and empirically test a range of alternative hypotheses involving the socio-ecological factors that may ultimately promote or inhibit the formation of large, complex human societies.

Deriving cultural evolutionary hypotheses
Cultural evolutionary theory (CET) is a conceptual framework concerned with understanding how and why socio-cultural traits emerge and spread (Boyd and Richerson, 1985; Henrich, 2015; Mesoudi, 2011). In CET cultural traits are seen to exhibit variation that is inherited from one individual or group to another, and when there is competition then selection and adaptation can occur; mirroring the key processes of biological evolution (Futuyma, 2013). For groups to remain politically unified as they expand their territory, either through the physical movement of people, or the joining together or annexation of other groups, cultural norms and institutions must be developed that structure social interactions and enable social cohesion (Fukuyama, 2011; North, 1990; Turchin, 2016; Turchin et al., 2018). For example, establishment of formal leaders with the authority to punish free-riders can solve collective action problems (Smith et al., 2015), while more hierarchical organisation and specialised, bureaucratic forms of political organisation can improve coordination over larger distances (Carneiro, 1981; Spencer, 2010; Turchin and Gavrilets, 2009). While a large number of processes may be involved in the evolution of large-scale societies, our focus here is on factors that have systematically affected the geographic and temporal distribution of such groups.

From a CET perspective, the variation seen in the occurrence of large-scale societies, could be due to differences in different parts of the world relating to (i) the benefits and costs of large-scale organisation (selection), (ii) the generation of different socio-cultural traits involved in large-scale organisation (variation), (iii) the transmission of these traits across time and space (inheritance). Here, we develop specific hypotheses about the factors in the real-world relating to these processes of selection, variation, and inheritance.

In humans, competition between groups is potentially a strong-selective force. While warfare has probably occurred throughout history, the intensity of between-group conflict has varied across space and time. A major historically attested factor intensifying warfare was the development of horse-based military technologies such as chariots and cavalry (Turchin et al., 2013). These first developed in the pastoralist societies of the Eurasian steppe and enabled such groups to raid settled agricultural societies in regions neighbouring the steppe, sometimes inflicting severe casualties (Turchin, 2010). It is hypothesised that pressure from the steppe selected for the unification and scaling-up of agricultural societies into larger groups to more effectively counteract these incursions. This in turn would select for greater size in pastoralist communities, and also other neighbouring agricultural groups who were now relatively smaller and at a competitive disadvantage with their neighbours. This effect would be amplified by the diffusion of such military technology from the steppe. Under this hypothesis we would predict that there is relationship between the occurrence of large-scale societies and distance from the Eurasian steppe (relating to the presence of intensive forms of horse-based warfare), with large-scale societies occurring more frequently nearer the steppe (the “steppe warfare” hypothesis)....

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