When and Why Did Humans Start Living Together in Larger Societies?/info


Introduction

The transition from small, mobile foraging bands to large-scale, sedentary societies constitutes one of the most consequential transformations in human evolutionary history. For the vast majority of the genus Homo's existence — spanning roughly two million years — social organisation centred on kin-based groups of perhaps 20–50 individuals. The emergence of communities numbering in the hundreds, and eventually the thousands, required novel cognitive capacities, institutional frameworks, and mechanisms for managing cooperation among unrelated individuals. Understanding when and why this transition occurred remains a central problem in evolutionary anthropology, behavioural ecology, and the archaeology of complex societies.

Multiple disciplinary perspectives converge on this question. Palaeoanthropologists track changes in brain size, site occupation patterns, and material culture for evidence of expanding social networks. Evolutionary theorists model the game-theoretic conditions under which cooperation among strangers becomes stable. Ethnographers of contemporary foraging societies provide comparative data on the levelling mechanisms — gossip, ridicule, ostracism, and ultimately lethal sanctions — that suppress dominance hierarchies in small-scale groups. The challenge is to integrate these lines of evidence into a coherent account of how and why Pleistocene band-level organisation gave way to the hierarchical, institutionally governed societies of the Holocene.

What we know

Robin Dunbar's social brain hypothesis, first articulated in 1992, established a quantitative relationship between neocortex size and social group size across primate species. Extrapolating from this allometric relationship, Dunbar predicted a natural group size for humans of approximately 150 individuals — a figure that has since been identified in a range of organisational contexts, from Neolithic village sizes to modern military company structures. The hypothesis implies that the cognitive architecture for managing larger social networks evolved in tandem with encephalisation during the Middle and Late Pleistocene.

Christopher Boehm's work on reverse dominance hierarchies, published in Current Anthropology in 1993 and expanded in his 1999 monograph Hierarchy in the Forest, demonstrated that egalitarianism in hunter-gatherer societies is not a passive default but an actively maintained political arrangement. Coalitions of subordinates systematically suppress would-be dominants through public opinion, ridicule, disobedience, and in extreme cases, execution. Boehm argued that this capacity for collective action against bullies was a prerequisite for the cooperative enterprises — communal hunting, food sharing, collective child-rearing — that characterise human social life.

Michael Tomasello's interdependence hypothesis, elaborated in a 2012 paper in Current Anthropology, proposed a two-step model for the evolution of human cooperation. The first step involved the emergence of joint intentionality in small-scale collaborative foraging contexts during the Middle Pleistocene. The second step, coinciding with behavioural modernity and the expansion of group sizes, involved the development of collective intentionality — shared norms, conventions, and institutions that could coordinate behaviour across large populations of non-kin.

Archaeological evidence for the transition to larger settlements is most clearly visible in the Pre-Pottery Neolithic of the Levant (c. 10,000–8,500 BCE), where sites such as Jericho and Çatalhöyük reached populations of several hundred to several thousand inhabitants. However, evidence from Late Pleistocene sites in Africa, Europe, and Southeast Asia suggests that periodic aggregation of dispersed bands into larger groups — for trade, ritual, and mate exchange — predates sedentism by tens of thousands of years.

Powers, van Schaik, and Lehmann (2021) argued in Evolutionary Anthropology that the transition to large-scale cooperation can be explained through changes in institutional rules — the "rules of the game" governing social interactions — without requiring the evolution of novel prosocial motivations. Their model suggests that self-interested individuals, operating under appropriate institutional constraints (norms of fairness, punishment of free-riders, redistributive obligations), can sustain cooperation at scales far exceeding those predicted by kin selection or reciprocal altruism alone.

Szathmáry and colleagues, writing in the Philosophical Transactions of the Royal Society B (2023), framed the transition to large-scale cooperation as a major evolutionary transition in individuality, analogous to the earlier transitions from single-celled to multicellular organisms. On this view, the suppression of within-group competition through egalitarian norms and institutions was functionally equivalent to the suppression of selfish cell lineages in the evolution of multicellularity.

The genetic and demographic evidence suggests that the effective population size of Homo sapiens expanded dramatically around 50,000–70,000 years ago, coinciding with the Out of Africa dispersal and the Upper Palaeolithic revolution. This demographic expansion likely both required and was enabled by new forms of inter-group cooperation, including long-distance trade networks, alliance formation, and the cultural transmission of cooperative norms.

Classification of theories

A. Plausible explanations (supported by evidence, not refuted):

  • Institutional-rules hypothesis: Changes in social norms and enforcement mechanisms (punishment, ostracism, reputation) enabled cooperation among non-kin at increasing scales, without requiring novel prosocial psychology (Powers et al. 2016, 2021).
  • Cultural group selection: Competition between culturally differentiated groups favoured populations with more effective cooperative norms, driving the ratcheting up of group size and institutional complexity (Richerson & Boyd 2005; Henrich 2016).
  • Two-step interdependence model: Joint intentionality in dyadic foraging contexts preceded collective intentionality in large-group settings; both cognitive capacities evolved under selection for cooperative mutualism (Tomasello et al. 2012).
  • Reverse dominance / egalitarian enforcement: Active suppression of dominance by coalitions of subordinates was a prerequisite for the trust and food-sharing behaviour that enabled larger aggregations (Boehm 1993, 1999).

B. Possible explanations (raised by researchers, less well supported):

  • Ecological forcing: Climate instability during the Late Pleistocene forced aggregation at resource-rich refugia, creating selection pressures for cooperation at larger scales.
  • Warfare and defence: Inter-group conflict incentivised the formation of larger, more defensible communities, with cooperation emerging as a by-product of military necessity.
  • Technological thresholds: Advances in food storage, agriculture, and construction lowered the per-capita cost of large-group living.

C. Highly unlikely but argued by some:

  • Purely genetic explanations: The hypothesis that a single mutation or genetic shift produced "prosocial" humans has no empirical support and fails to account for the extraordinary cultural variation in cooperation norms.
  • Alien or external catalysis: Fringe theories attributing the transition to extraterrestrial intervention have no evidential basis.

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Research Papers

Landmark Studies

1. Dunbar, R.I.M. (1998). "The social brain hypothesis." Evolutionary Anthropology, 6(5), 178–190. Full text: https://onlinelibrary.wiley.com/doi/10.1002/(SICI)1520-6505(1998)6:5%3C178::AID-EVAN5%3E3.0.CO;2-8 [publisher paywall] [Agent-generated summary] Dunbar presents the social brain hypothesis, arguing that the unusually large neocortex of primates evolved primarily to manage the cognitive demands of complex social relationships. He extrapolates an allometric equation relating neocortex ratio to group size across primate species, predicting a natural group size for humans of approximately 150. Full-text notes: The paper establishes the quantitative framework that has become foundational for discussions of human social evolution. Dunbar marshals comparative data from dozens of primate species to demonstrate the neocortex ratio–group size correlation. He acknowledges that group size in humans is not rigidly fixed but clusters around 150 in a range of organisational contexts. The paper's primary limitation is its reliance on a single neuroanatomical variable, and subsequent work has debated whether neocortex size is the most appropriate predictor.

2. Boehm, C. (1993). "Egalitarian Behavior and Reverse Dominance Hierarchy." Current Anthropology, 34(3), 227–254. Full text: https://www.journals.uchicago.edu/doi/10.1086/204166 [publisher paywall] [Agent-generated summary] Boehm argues that egalitarianism in human foraging societies is an actively maintained political system, not a default state. Drawing on ethnographic data from dozens of hunter-gatherer and tribal societies, he identifies mechanisms — public opinion, criticism, ridicule, disobedience, and extreme sanctions — by which coalitions of subordinates suppress would-be dominants, creating what he terms a "reverse dominance hierarchy." Full-text notes: Boehm's framework resolves the paradox of human egalitarianism: given that our closest great ape relatives display pronounced dominance hierarchies, how did human foragers achieve relative equality? The answer lies in collective political action. The paper includes extensive commentary from peers, several of whom challenge the universality of the pattern. Boehm's response strengthens the claim by documenting the cross-cultural consistency of levelling mechanisms. This work laid the groundwork for his later monograph and for broader theories linking egalitarianism to the evolution of morality.

3. Tomasello, M., Melis, A.P., Tennie, C., Wyman, E., & Herrmann, E. (2012). "Two Key Steps in the Evolution of Human Cooperation: The Interdependence Hypothesis." Current Anthropology, 53(6), 673–692. Full text: https://www.journals.uchicago.edu/doi/10.1086/668207 [publisher paywall] [Agent-generated summary] Tomasello and colleagues propose a two-step model in which collaborative foraging in small groups selected for joint intentionality (shared goals, complementary roles), while the later expansion to larger group sizes selected for collective intentionality (shared norms, institutions, cultural conventions). Full-text notes: The paper integrates developmental psychology, comparative cognition (especially chimpanzee studies), and evolutionary modelling. The authors argue that obligate collaborative foraging — not kin selection or reciprocal altruism — was the initial engine of human cooperation. The second step, driven by inter-group competition and demographic expansion, required the cultural invention of institutions. The paper is notable for its synthesis across disciplines but has been critiqued for underspecifying the archaeological evidence for the transition between steps.

4. Powers, S.T., van Schaik, C.P., & Lehmann, L. (2016). "How institutions shaped the last major evolutionary transition to large-scale human societies." Philosophical Transactions of the Royal Society B, 371(1687), 20150098. Full text: https://royalsocietypublishing.org/doi/10.1098/rstb.2015.0098 [open access] [Publisher abstract] What drove the transition from small-scale human societies centred on kinship and personal exchange, to large-scale societies comprising cooperation among thousands to millions of unrelated individuals? We propose that the unique human capacity to negotiate institutional rules facilitated this transition. Full-text notes: Powers and colleagues build a formal evolutionary model demonstrating that institutional design — particularly rules governing resource distribution and punishment — can explain the scaling of cooperation from small to large groups without invoking novel prosocial motivations. The model predicts that institutional complexity should increase with group size, a prediction supported by cross-cultural data. The paper represents an important challenge to approaches that emphasise psychological change (e.g., evolved altruism) over institutional innovation.

5. Richerson, P.J. & Boyd, R. (2005). Not by Genes Alone: How Culture Transformed Human Evolution. Chicago: University of Chicago Press. Full text: https://press.uchicago.edu/ucp/books/book/chicago/N/bo3615170.html [publisher — book] [Agent-generated summary] Richerson and Boyd present a comprehensive theory of gene-culture coevolution, arguing that cultural inheritance — the transmission of information through social learning — created a second inheritance system that dramatically accelerated human adaptation. They develop the theory of cultural group selection, proposing that competition between groups with different cultural norms drove the evolution of large-scale cooperation. Full-text notes: This monograph is foundational for the cultural group selection framework. The authors argue that conformist social learning and norm-enforcement mechanisms create stable between-group cultural variation, enabling group-level selection to operate. The work synthesises formal models with ethnographic and historical evidence. Critics from the inclusive fitness tradition (e.g., West et al.) have questioned whether group selection adds explanatory power beyond individual-level models, but the framework remains highly influential.

6. Boyd, R. & Richerson, P.J. (2009). "Culture and the evolution of human cooperation." Philosophical Transactions of the Royal Society B, 364(1533), 3281–3288. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC2781880/ [open access] [Agent-generated summary] Boyd and Richerson argue that cultural evolution, particularly the development of norms enforced through punishment, provides the mechanism by which human cooperation was scaled from small kin-based groups to large anonymous societies. They present models showing that punishment can stabilise cooperation in large groups even when cooperation is individually costly. Full-text notes: This paper distils the theoretical core of the cultural group selection programme. The authors show formally that punishment (including third-party punishment) can sustain cooperation in large populations where reciprocity and reputation-based mechanisms fail. They emphasise that punishment itself is a cultural norm that must be sustained by meta-norms — creating a hierarchy of institutional enforcement. The paper is especially valuable for its clarity in distinguishing ultimate and proximate explanations.

Recent Studies

7. Powers, S.T., van Schaik, C.P., & Lehmann, L. (2021). "Cooperation in large-scale human societies — What, if anything, makes it unique, and how did it evolve?" Evolutionary Anthropology, 30(4), 280–293. Full text: https://onlinelibrary.wiley.com/doi/full/10.1002/evan.21909 [publisher paywall] [Agent-generated summary] A comprehensive review arguing that the transition to large-scale cooperation is best explained by institutional evolution rather than by novel prosocial psychology. The authors synthesise evidence from experimental economics, cross-cultural studies, and evolutionary modelling. Full-text notes: This paper updates and extends the authors' 2016 model. They argue that the same self-interested motivations observable in small-scale societies are sufficient to sustain cooperation in large societies, provided institutional rules are appropriately structured. The paper challenges the assumption that large-scale cooperation requires psychological mechanisms absent in other primates. It provides a useful taxonomy of cooperation-sustaining institutions across societal scales.

8. Muthukrishna, M., Bell, A.V., Henrich, J., et al. (2020). "Human large-scale cooperation as a product of competition between cultural groups." Nature Communications, 11, 702. Full text: https://www.nature.com/articles/s41467-020-14416-8 [open access] [Publisher abstract] A fundamental puzzle of human evolution is how we evolved to cooperate with genetically unrelated strangers in transient interactions. Group-level selection on culturally differentiated populations is one proposed explanation. Full-text notes: Using experimental economic games administered across a global sample, the authors test predictions of cultural group selection theory. They find that cooperative behaviour in anonymous interactions is positively associated with markers of cultural group identity and between-group competition, consistent with cultural group selection models. The study provides some of the strongest experimental evidence for the framework, though the authors acknowledge that alternative explanations (e.g., market integration) cannot be fully excluded.

9. Szathmáry, E., et al. (2023). "Human cooperation and evolutionary transitions in individuality." Philosophical Transactions of the Royal Society B, 378(1872), 20210414. Full text: https://royalsocietypublishing.org/rstb/article/378/1872/20210414/109095/ [open access] [Publisher abstract] The authors posit that new challenges in the human niche set in motion a positive feedback loop in selection pressure for cooperation that ratcheted coevolutionary changes in sociality, communication, brains, cognition, kin relations and technology, eventually resulting in egalitarian societies with suppressed competition and rapid cumulative culture. Full-text notes: This paper frames the emergence of large-scale human cooperation as a major evolutionary transition, drawing explicit parallels with earlier transitions (e.g., from unicellular to multicellular life). The analogy highlights the suppression of within-group competition as the key mechanism enabling higher-level organisation. The authors argue that egalitarian norms in foraging societies functioned analogously to mechanisms preventing cancer in multicellular organisms. The paper is conceptually ambitious but the analogy has limits — human institutions are culturally, not genetically, transmitted.

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Current Discussions

  1. "The Social Brain Hypothesis — Thirty Years On" — Dunbar (2024) revisits and updates his original framework in Annals of Human Biology, addressing three decades of criticism and new evidence.

Full text: https://www.tandfonline.com/doi/full/10.1080/03014460.2024.2359920 [publisher paywall]

  1. "Teaching is associated with the transmission of opaque culture and leadership across 23 egalitarian hunter-gatherer societies" — Garfield & Lew-Levy (2025) in Nature Communications provides new cross-cultural evidence on how knowledge transmission functions in egalitarian societies.

Full text: https://www.nature.com/articles/s41467-025-58764-9 [open access]

  1. Smarthistory: Çatalhöyük — An accessible overview of the archaeological evidence from one of the earliest large-scale settlements, illustrating the material correlates of the transition to larger societies.

URL: https://smarthistory.org/catalhoyuk/

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Future Research Directions

The integration of ancient DNA studies with archaeological settlement data offers the potential to directly track kinship structures within and between early villages. If the institutional-rules hypothesis is correct, early large settlements should exhibit low average relatedness among co-residents, distinguishable from the kin-clustered pattern expected under inclusive fitness models.

Advances in computational modelling — particularly agent-based simulations that incorporate cultural transmission, institutional design, and demographic dynamics — may allow researchers to test competing models of the transition against empirical settlement-size distributions from the archaeological record.

Cross-cultural experimental economics, particularly studies conducted with small-scale societies at different stages of the transition to market integration, can illuminate whether cooperative norms scale with institutional complexity in the manner predicted by the Powers et al. framework. The expansion of these studies beyond WEIRD (Western, Educated, Industrialised, Rich, Democratic) populations remains a priority.

The role of ritual and religion in sustaining cooperation in large groups — the "Big Gods" hypothesis of Norenzayan and colleagues — requires further testing against archaeological proxies for ritual activity in early complex societies. The relationship between monument construction, communal feasting, and group-size expansion is a particularly promising avenue.

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Summary of Existing Research and Public Opinion

The scholarly consensus holds that the transition from small bands to large societies was a gradual, multi-causal process driven by the coevolution of cognitive capacities, cultural norms, and institutional structures. No single factor — brain size, agriculture, warfare, or religion — is sufficient on its own, but the interplay among them provides a robust explanatory framework.

Public understanding of this topic tends to be shaped by popular accounts that emphasise either the "noble savage" narrative (humans are naturally egalitarian and were corrupted by civilisation) or the Hobbesian alternative (life in small groups was "nasty, brutish, and short," and civilisation represents progress). The academic literature supports neither extreme: egalitarianism in foraging societies was a hard-won political achievement, and the transition to hierarchy was neither inevitable nor uniformly beneficial.

The cultural group selection framework, though controversial among some evolutionary biologists, has gained substantial traction in anthropology and has been popularised by Joseph Henrich's The WEIRDest People in the World (2020). This has brought the institutional-rules perspective into broader public discourse.

There remains genuine disagreement over the relative importance of bottom-up versus top-down processes. Some scholars emphasise the spontaneous emergence of cooperative norms through repeated interaction; others stress the role of elite imposition and coercion. Archaeological evidence from early complex societies supports both pathways operating in different contexts.

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Where Do I Come In?

This episode of Dig Labs invites readers to engage with one of the most fundamental questions about human nature: are we inherently cooperative or competitive, and what enabled the extraordinary scaling of social life that distinguishes us from all other primates? The question is not merely historical — it speaks directly to contemporary debates about the design of institutions, the limits of trust in anonymous societies, and the conditions under which cooperation breaks down.

Readers with backgrounds in evolutionary biology, anthropology, psychology, or economics will find entry points into the primary literature through the studies cited above. The Powers et al. (2016, 2021) framework is particularly accessible and has direct implications for institutional design in modern contexts — from governance structures to platform cooperatives.

For those interested in the archaeological evidence, the Pre-Pottery Neolithic of the Levant and the early urban centres of Mesopotamia offer rich case studies. The transition from Çatalhöyük's apparently non-hierarchical organisation to the stratified city-states of Sumer represents a compressed version of the broader evolutionary trajectory.

Observatory.wiki's research community can contribute by synthesising emerging evidence from ancient DNA studies, by evaluating the predictive success of competing models against newly excavated settlement data, and by developing accessible summaries that bridge the gap between technical evolutionary theory and public understanding of human social origins.