It is generally held that human cooperation first evolved in our ancestral past, when we tended to live in small groups composed mostly of family members, and when cooperation could therefore be selected for by kin selection. This narrative is also sometimes invoked to explain the origin of cooperative mechanisms that are primarily about cooperation between nonkin. For example, in the iterated Prisoner’s Dilemma, cooperation between nonkin can be maintained by the tit-for-tat strategy, and a population of tit-for-tat players can resist invasion by defectors. However, tit-for-tat strategists cannot invade a population of defectors (presumably the primordial strategy), and so the question remains how it got started in the first place. Axelrod & Hamilton (1981) proposed that one way could have been if dispersal was low enough in the past that cooperative types tended to cluster together.
Over the course of the human lineage, there is a general pattern expanding social networks and declining relatedness between interacting individuals. DNA analysis back to 45 ka shows a general decline in background relatedness over time, with a marked change at the Neolithic Demographic Transition (Ringbauer et al., 2021), when the advent of farming in each region coincided with a sudden increase in population size (Bocquet-Appel, 2011). In industrialised societies, falling mortality and fertility has also reduced the size of kin networks (David-Barrett, 2019), motivating new bases for social identity (David-Barrett, 2020). In the popular imagination, the lifeways of hunter-gatherer people represent our closest analogue to what the deep ancestral past must have been like; however, modern hunter-gatherers also maintain expansive social networks with hundreds of unrelated individuals (Hill et al., 2011; Bird et al., 2019), and groups congregate seasonally for communal hunting and socialising (Kelly, 2013; Balme, 2018).
Not a lot is known about social structure before 45 ka (Graeber and Wengrow, 2018), and so inferences must be made on the basis of fossils and other material evidence. One potential indicator of social structure is the distance that materials were transported from their source. Before around 1.6 Ma, raw-material transport distances are comparable to chimpanzee home-range sizes (∼ 13 km), indicating relatively isolated social groups composed mostly of kin (Marwick, 2003). Distances and occurrences of material transport subsequently increased over the course of the Early and Middle Stone Age. For example, approximately 295-320 ka, obsidian and ochre were transported 25-50 km (as the crow flies) (Brooks et al., 2018). After ~130 ka, raw-material transport distances frequently exceeded 300 km (Marwick, 2003). These distances may be indicative of networks of exchange, which implies increased language abilities (Marwick, 2003) and notions of relatedness beyond genetic kin (Moutsiou, 2012). However, we must also be cautious when trying to infer what these exchanges meant, because humans are quirky and sometimes transport materials over long distances for unexpected reasons (Graeber and Wengrow, 2018).
The types of materials that are found can also be indicative of something social. Tool sophistication and symbolic development may be indicative of cognitive and social development, and their stylistic diversity indicative of cultures and the flow of information between them. In the early period (~1.5-0.4 Ma), evidence of material innovation is scarce; but nonetheless, encephalisation increased over this period, which some authors attribute to the demands of increasing social complexity (Gamble et al., 2011). The transport of ochre mentioned above, from 295-320 ka, is notable because ochre is used by modern people as a pigment, either for artwork or body ornamentation, and as a potential indicator of one’s group identity (Brooks et al., 2018).
The appearance of beads (> 142 ka Sehasseh et al., 2021) may be important because they can be used to communicate social identity (e.g., group membership and marital status) to strangers. The use of beads greatly increased around the same time that population sizes increased (40-45 ka, Kuhn et al., 2001), which further supports the idea that beads were used in this way. Beads were also transported long distances; for example, shell beads found in the Kimberly, Australia, from 30 ka, were transported > 300 km from their source (Balme and Morse, 2006). It is interesting to note that some modern hunter-gatherers use the exchange of beads as the substrate for indoctrinating children into socially defined notions of kinship (Wiessner, 1998). We also find long-distance transport of other materials potentially used to communicate identity, e.g., ochre from 32 ka transported 125 km in central Australia (Smith et al., 1998).
An expanding social network could have provided the opportunity to experiment with different styles of large-scale collective action (Graeber and Wengrow, 2021). For example, the use of nets is an indicator of communal hunting, particularly of the integration of labour from women, children, and the elderly (Soffer et al., 2001). Evidence of large-scale fishing operations occur from 27 ka in Australia (Balme, 1995).
Acquiring meat may have been one of our ancestors’ first collaborative activities, and collaborative foraging in general has been linked to the early stages of human cooperation (Tomasello et al. 2012). Unfortunately, it is unclear exactly how animal products were first acquired by our ancestors: were they working together to take down large prey in some way (Domínguez-Rodrigo et al., 2021), or were they opportunistically finding and/or snatching scraps from animals taken down by other carnivores (Pobiner, 2020)? Perhaps scavenging itself was a collaborative activity if that meant confronting and scaring away large, dangerous carnivores who were still at their meals (Bickerton & Szathmáry, 2011)?
In the figure below, I’ve sketched out a rough timeline of some of the elements above. This should help orient different narratives about how cooperation evolved.
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