Original ArticleAmazonian horticulturalists live in larger, more related groups than hunter–gatherers
Introduction
Although still hotly debated in some circles, a key paradigm of evolutionary biology remains Hamilton's (1964) idea that consanguineal relatedness among individuals drives the evolution of cooperative behavior (Abbot et al., 2011, Forster et al., 2006; cf. Nowak et al., 2010, Wilson, 2005). Cooperation (behaviors that benefit another individual and are selected for because of beneficial effects on recipients, West, Griffin, & Gardner, 2007a) is generally thought to form around kinship, from bacteria (Gardner, West, & Griffin, 2007) to primates (Silk, 2002), although cooperation often extends to non-kin (Clutton-Brock, 2009). Humans are an unusual species in that we live in large and cooperative social groups with embedded families formed around stable pair-bonds that create sets of full siblings with life-long bonds (Alexander, 1979, Chapais, 2008, Rodseth et al., 1991). Hunter–Gatherers, the best contemporary representatives of earlier periods of human evolution, often have fluid fission/fusion social dynamics that break up the co-residence of sibling sets and lead to rather low levels of group relatedness (r ≤ 0.05, Hill et al., 2011). Models of contingent cooperation show that reciprocating strategies can increase when rare only at the high range of empirical estimates of human group relatedness (r > 0.05, Boyd, Schonmann, & Vicente, 2014).
Bugos (1985) distinguishes intensive versus extensive human kinship systems. An extensive kinship system is characterized by marriages among genealogically and geographically distant individuals to create a diffuse kinship network (Fix, 1999, MacDonald and Hewlett, 1999). Extensive kinship networks may be more adaptive for nomadic peoples in unpredictable environments by providing more residential options and insurance in times of crises (Fix, 1999, Yellen and Harpending, 1972). In contrast, intensive kinship systems often include marriage alliances between co-resident lineages which generate cousin marriages and converging networks of kin (Lévi-Strauss, 1949). In addition to endogamy and limited dispersal, social norms that favor kin marriages (Flinn and Low, 1986, Walker and Bailey, 2014) and population dynamics like lineal fissions (Neel and Salzano, 1966, Walker and Hill, 2014) increase the average relatedness of human groups under intensive kinship regimes. Here, groups are defined as people living close together in local residential communities which may be temporary (e.g., hunter–gatherer camps) or more permanent (e.g., horticultural villages). Group relatedness is important because it governs how strongly kin (or group) selection favors cooperation (Hamilton, 1964, Queller, 1992, West et al., 2007a, West et al., 2007b). Some pertinent examples of cooperation and collective action problems in humans include building structures, clearing fields, sharing feasts, raiding enemies, keeping the peace, and forming institutions; all of which appear to have increased in importance and scale across human history from hunting-and-gathering ancestors to more recent and intensive modes of agricultural-based subsistence (Johnson & Earle, 2000).
Bowles (2006) has argued that hunter–gatherer groups may have had sufficiently high relatedness to explain human cooperation in a between-group competition model. However, others have discounted between-group genetic differentiation as a primary driver of human cooperation (Langergraber et al., 2011). These two previous studies used empirical estimates of group relatedness based on genetic differentiation among human groups yet have yielded opposite conclusions. The current study uses comparative genealogical datasets to estimate human group relatedness across hunter–gatherer and horticultural subsistence regimes. An advantage of genealogies and census data is that analyses include all adults living together in a relatedness matrix without having to rely on genetic sampling issues or assumptions of population structure such as demes within ethnolinguistic boundaries. Particular attention is given here to horticultural populations where the potential for dense networks of kin, due at least in part to limited dispersal of sibling sets and more inbreeding, leads to higher group relatedness.
Section snippets
Methods
Most genealogies and censuses used in this study are available online at KinSources (http://kinsources.net). Additional societies have been added from recent studies of co-residence in hunter–gatherers (Hill et al., 2011) and lowland horticulturalists (Walker et al., 2013) (Table 1). Both of these publications only analyzed primary kin (parents, offspring, and siblings) living together in the same group. A difference in relatedness is anticipated since an average hunter–gatherer lives with 1.8
Results
The average group size for horticulturalists in the sample is 35 adults (range 3–192) which is more than twice that for hunter–gatherers of 16 adults (range 3–118). Despite larger groups, the average coefficient of relatedness for horticultural villages is 0.11 (bootstrapped 95% confidence interval 0.090–0.126), higher than that for hunter–gatherer camps at 0.08 (0.076–0.090). The difference cannot be explained by differential data quality because both samples have the same average genealogical
Discussion
Results show that horticulturalists live in larger and more related groups, at least in part because of increased inbreeding. Inbreeding can arise from higher reproductive skew (successful males, especially headmen, married polygynously) producing more sets of at least half siblings whose children may intermarry, and from social norms of marriage patterns like endogamy, prescribed kin marriages, and sister exchange systems, all common strategies to form alliances between lineages (Chapais, 2008
Acknowledgments
This paper benefited from the genealogies and documentation made available at KinSources (http://kinsources.net) and from data, help, and advice from Drew Bailey, Woodrow Denham, Kim Hill, Ed Hagen, Karen Kramer, and Jeremy Koster. Several anonymous reviewers provided helpful corrections. Financial support was provided by a National Geographic Society Research and Exploration grant (#9165-12).
References (47)
Sexual selection under parental choice: the role of parents in the evolution of human mating
Evolution and Human Behavior
(2007)- et al.
Unity and disunity in the search for a unified reproductive skew theory
Animal Behaviour
(2013) A model for the evolution of despotic versus egalitarian societies
Animal Behaviour
(1983)- et al.
Multiple dimensions of male social statuses in an Amazonian society
Evolution and Human Behavior
(2008) - et al.
Body counts in lowland South American violence
Evolution & Human Behavior
(2013) - et al.
Marrying kin in small-scale societies
American Journal of Human Biology
(2014) - et al.
Evolutionary explanations for cooperation
Current Biology
(2007) - et al.
Calculation and use of inbreeding coefficients for genetic evaluation of United States dairy cattle
Journal of Dairy Science
(1995) - et al.
Inclusive fitness theory and eusociality
Nature
(2011) Darwinism and human affairs
(1979)
Group competition, reproductive leveling, and the evolution of human altruism
Science 314.5805
Hunter-gatherer population structure and the evolution of contingent cooperation
Evolution and Human Behavior
An evolutionary ecological analysis of the social organization of the Ayoreo of the Northern Gran Chaco
Genealogy, solidarity, and relatedness: limits to local group size and patterns of fissioning in an expanding population
Yearbook of Physical Anthropology
Fission in an Amazonian tribe
New York Academy of Sciences
Mate competition, favoring close kin, and village fissioning among the Yanomamö Indians
Primeval kinship: how pair bonding gave birth to human society
Cooperation between non-kin in animal societies
Nature
Migration and colonization in human microevolution
Resource distribution, social competition, and mating patterns in human societies
Kin selection is the key to altruism
Trends in Ecology and Evolution
Is bacterial persistence a social trait?
PloS One
The genetical evolution of social behaviour I & II
Journal of Theoretical Biology
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