Émergence et évolution de la parenté
|Biological Substrates of Human Kinship
The View from Life History Theory and Evolutionary Ecology
Résumé
Le but de cet article est de montrer, dans la lignée de R. Fox, que les notions de résidence, d’affiliation et d’accouplement – qui distinguent les humains des primates – doivent jouer comme autant de contraintes pesant sur le cours ultérieur du développement proprement humain, après la mise en place des règles symboliques de filiation et des catégories de parenté. D’un point de vue darwinien, c’est l’avantage sélectif qui gouverne l’histoire de la vie et même si les règles de parenté sont, de ce point de vue, des fictions, encore faut-il pouvoir dire quels sont les types de fiction qui prévalent. L’auteur veut démontrer que toute coopération entre mâle et femelle (interprétée comme contrainte pesant sur les règles ultérieures de parenté) doit reposer sur une coopération préalable entre femelles. L’hypothèse de l’« homme chasseur », troquant la viande qu’il a chassée contre un commerce sexuel auprès d’une femelle unique et une certitude de paternité touchant sa progéniture n’est plus crédible du point de vue archéologique et c’est plutôt l’hypothèse d’un gain en prestige par une attitude de libéralité à l’égard de la collectivité toute entière qui est aujourd’hui retenue. En faisant appel aux modèles de la théorie des jeux, on se rend compte que les dons en provenance des mâles et en direction des femelles sont susceptibles de se produire d’autant plus facilement que le dimorphisme sexuel a tendance à diminuer (au moins dans le cas où les femelles ont développé des stratégies visant à punir les mâles non coopérateurs) pour des raisons de coût énergétique. On parvient à la même conclusion d’un point de vue archéologique : à partir de 2 millions d’années, la taille du cerveau d’Homo ergaster a doublé par rapport à celui des chimpanzés et le dimorphisme sexuel a diminué, principalement par une augmentation de la taille des femelles, ce qui implique une plus grande dépense d’énergie de leur part et sans doute une plus grande coopération des mâles envers elles. Si l’encéphalisation joue un rôle de pression de sélection pour les femelles, il y a deux façons pour elles de répondre aux demandes énergétiques croissantes que cette encéphalisation requiert : (i) en modifiant la façon dont elles font usage de leur énergie. Par exemple, un meilleur régime alimentaire peut permettre de réduire le coût de maintenance des boyaux, dont le coût énergétique est élevé ; (ii) en demandant de l’aide aux autres – que ce soit d’autres femelles ou des partenaires masculins potentiels. Un certain nombre de traits typiques de la vie humaine doivent être pris en considération : la durée pendant laquelle les petits humains sont complètement dépendants implique un meilleur régime alimentaire que celui des primates ; le retard dans la maturité sexuelle lié à une plus grande espérance de vie ; la ménopause et les vingt ans en moyenne qu’une femelle humaine peut escompter vivre après son temps de reproduction. L’« hypothèse de la grand-mère », c’est-à-dire d’une coopération entre les femelles, rend compte de tous ces traits spécifiques : une grand-mère est le candidat le plus sûr pour assurer la garde des enfants longtemps dépendants ; elle permet également un plus grand succès reproductif à sa fille. L’hypothèse ne va d’ailleurs pas à l’encontre de celle de l’homme chasseur mais la réintègre autrement puisque les mâles recherchent les femelles ayant le plus grand succès reproductif et que celles-ci recherchent des protéines dont leur progéniture a particulièrement besoin : ainsi les deux hypothèses se renforcent-elles mutuellement.
Texte intégral
1 This paper deals with the biological substrates and constraints on kinship affiliations through human evolution drawing on the latest modelling from life history theory and evolutionary ecology. As far as evolving hominins go, we are dealing with periods preceding prehistory, lacking any evidence for symbolic culture or kinship. For early hominins with ape-like life histories, it is reasonable to adopt terms from primatology in describing residence patterns (e. g. female philopatry where females stay and males move from natal groups; male philopatry, males stay, females move) or allegiance (e. g. male kin-bonded with brothers as allies, or female kin-bonded with mothers, sisters, daughters as main allies). These are not equivalents to human symbolically constituted residence and descent rules. However, as Fox argues, the pragmatics of residence, affiliation and mating on the ground must precede and constrain what subsequently develops in terms of descent rules and categoric kinship systems. So, too, in human evolution, the residence and genetic kin alliances we start from determine certain possible pathways, while excluding others.
- 1 E. Charnov, Life History Invariants, Oxford, Oxford University Press, 1993.
- 2 J. Krebs and N. Davies, An Introduction to Behavioural Ecology, Oxford, Blackwell Press, 3rd ed., 1 (...)
- 3 A. Hughes, Evolution and Human Kinship, New York, Oxford University Press, 1988.
2The areas of Darwinian theory most applicable to kinship studies are life history theory1 and behavioural ecology.2 The first addresses how and why organisms apportion effort to maintain survival and reproduction through their lifetimes. It is based on an essential trade-off between time and energy used in somatic maintenance and that used to reproduce. For mammal species, including primates, human and non-human, it asks, how long is spent in gestation, in lactation? How long does it take to reach adulthood and first reproduction? How long are the intervals between reproductive events and how long will you live? The second area, evolutionary ecology, assumes that every organism behaves in ways to promote its own survival and reproduction, but then explores the variability of strategies for maximising reproductive fitness given different ecological conditions and constraints. The kinds of evidence that can be used to test hypotheses on the evolution of human life history include comparative data across extant primate species, archaeological and fossil data. Evolutionary ecological hypotheses can potentially be tested by looking at observable fitness effects in contemporary populations. We may be the only symbolic species, but we are still also animals who are likely to act in ways that promote fitness. Ultimately, where kinship is concerned, evolutionary anthropologists should investigate the underlying fitness interests which lead to the evolution of different types of kinship system.3 Symbolic systems of kinship terminology do indeed consist of communally entertained “fictions” which may contradict genetic realities; but there will still be Darwinian causes for one set of fictions prevailing over another.
- 4 S. Washburn and C. Lancaster, “The Evolution of Hunting”, in R. Lee and I. Devore (eds.), Man the H (...)
- 5 G. Isaac, “The Food Sharing Behavior of Protohuman Hominids”, Scientific American, 238, 1982, p. 90 (...)
- 6 See review in J. F. O’Connell, K. Hawkes, K. D. Lupo and N. G. Blurton Jones, “Male Strategies and (...)
3In narratives of human social evolution, it is no longer credible to make the assumptions that shored up the old “man the hunter” hypothesis.4 Ideologically motivated, these imposed nuclear family structures onto the Pleistocene past, reading a sex division of labour and family life into the juxtapositions of fossils, tools and animal bones in archaeological assemblages.5 The old story ran that as Plio-Pleistocene males became increasingly successful at hunting, they brought back meat to females and juveniles at central places for foraging, tending to establish pair-bonds and trade such regular provisioning for “paternity certainty” each with their own woman. The idea that we have been quasi-hunter-gatherers for the past2 million years has not stood up to scrutiny of the archaeological record on Pleistocene scavenging,6 nor is it consistent with evidence from hominin fossils on sexual size dimorphism and life history schedules.
- 7 R. Trivers, Social Evolution, Menlo Park (CA), Benjamin Cummings, 1985.
- 8 K. Hawkes, J. F. O’Connell and N. G. Blurton Jones, “Hunting Income Patterns Among the Hadza: Big G (...)
- 9 N. G. Blurton Jones, F. W. Marlowe, K. Hawkes and J. F. O’Connell, “Paternal Investment and Hunter- (...)
4But the main challenge to the old “man the hunter” ideas comes as a matter of principle from “selfish-gene” theory. Because the sexes get genes into the next generation by different ways, they have different calculations about fitness. Females are forced into high levels of parental investment. For a male, investment in an offspring may come at a high opportunity cost if he has chances of mating elsewhere.7 While it is likely that some form of male care or paternal solicitude is ancient in primates as a guard against risk of infanticide, this is different from male parental investment (MPI) involving provision of energy to mother or offspring. The latter is rarely if at all seen among non-human primates, and we have to account carefully for its evolution in terms of fitness benefits and costs to males. The work of evolutionary ecologists on hunter-gatherers today has undermined beliefs in MPI as the main motivation for men’s hunting8 proposing instead that men are “showing off” and interested in extra matings. In the Hadza case, a man’s kill is distributed throughout a camp, not to one specific nuclear family. If men are not trading provisioning for pair-bonds and paternity certainty, we need different models for the emergence of a social division of labour.9
- 10 S. Blaffer Hrdy, “Cooperative Breeders with an Ace in the Hole”, in E. Voland, A. Chasiotis and W. (...)
5Recently, Sarah Hrdy has broken the old “man the hunter” mould by characterising evolving human ancestors as “cooperative breeders”, tending to have multiple extended family forms that were flexible in response to demographic uncertainty.10 But if evolutionary anthropology has anything to say about the evolution of human forms of kinship, it is necessary to investigate the main pathways that natural selection would operate on to produce early forms of social cooperation, including divisions of labour. Critical here are the energetic costs of reproduction for each sex. These have altered during the course of human evolution particularly because of increases in brain size – the brain being an especially costly organ, the costs falling on mothers of encephalised offspring – and also increases in body size, which require extra energy for maintenance.
- 11 C. A. Key and L. C. Aiello, “A Prisoner’s Dilemma Model of the Evolution of Paternal Care”, Folia P (...)
6Cathy Key used game theory models to explore the effect of the relative reproductive costs of the sexes on their likelihood of cooperation, both within each sex and between the sexes.11 The main determinant of sex difference in energetic costs is body size. If males need to be much bigger than females to succeed in reproductive competition, their costs will be similar to female costs. Key found that where female costs of reproduction were high, female-female cooperation was strongly selected. Males were not likely to cooperate with females where their costs were relatively high (i. e. their body sizes significantly bigger), but, when female costs rose relative to males, males would become “unconditionally cooperative” with females. This means they would give females benefits (such as gifts of high-energy food) even where they might receive nothing in return. So, a flow of benefits from males to females becomes more likely as sexual size dimorphism decreases. But it is important that this outcome depended on the condition that females developed strategies to “punish” non-cooperative males by long-term refusal of cooperation. There is also no necessary basis of “paternity certainty” in this model. Both males and females are likely to « trade » with more than one partner.
7How can Key’s abstract model be related to the actual fossil record? Among earlier hominins, australopiths prior to2 million years who retained significant climbing abilities, brains and bodies were relatively small, with high size dimorphism between the sexes (see Table 1).
Table 1 – Indicators of life history change in hominins.
Hominin grade | Brain/body size | Date | Life history |
Australopiths, Encephalised early “Homo” | Small brains/bodies – brains increase, bodies stay small | Pliocene, before 2 mya | Ape-like growth schedule and mortality |
h. ergaster/erectus | Bigger brains and bodies | Early Pleistocene, from 1.75 mya | Intermediate between apes and modern humans; reduced mortality |
h. heidelbergensis, ancestor to us and Neanderthals | Large brain, large robust body | Middle-Late Pleistocene, from 0.5 mya | Virtually modern human schedule/lifespan |
- 12 H. M. McHenry, « Sexual Dimorphism in Fossil Hominids and its Socioecological Implications”, in J. (...)
- 13 H. Kaplan, K. Hill, J. Lancaster and A. M. Hurtado, “A Theory of Human Life History Evolution: Diet (...)
8 From about2.5 million years, some of these species began to encephalise while bodies remained quite small and apparently still highly dimorphic. This suggests increasing costs for females, indicating more pressure for femalefemale cooperation, while males still had high body-size costs and were less likely to be cooperative. These encephalised early Homo species led to the emergence of Homo ergaster after2 million years, the first hominin with body proportions like us, bodies that were bigger and designed for walking not climbing. Their brains, though not more encephalised (that is a relative measure of brain to body weight) than earlier Homo, were absolutely twice the size of chimpanzee/australopithecine brains. Sex size dimorphism had reduced, largely because female Homo ergaster increased body size disproportionately more than males.12 With female costs rising relative to males, significantly more cooperation by males with females can be expected from this time. But this is based on the prior evolution of interfemale cooperation. We know from hunter-gatherer economies that male hunters do significantly contribute to and subsidise female reproductive costs.13 Already in early Pleistocene scavenging economies, males may have been giving females significant benefits – but we should be careful not to make assumptions about MPI. We need to consider the implications of female cooperative strategies as the necessary basis for the emergence of male-female cooperation.
9Across species, brain and body size are generally closely related to life history variables such as age at first reproduction and lifespan. Therefore fossil remains can provide important evidence about the life history of those species (see Table 1). For evolving Homo females of the Plio-Pleistocene who came under selection pressure of, encephalization, there are two basic areas for evolutionary change to enable females to meet their costs. She can:
- alter the ways she allocates her own energy and resources, leading to change in stages of life history, patterns and rates of growth, etc.;
- get other people to help! Possible candidates – female kin, or actual/ potential mates. Which is she likely to turn to first? Key’s model says it must be her own relatives first.
- 14 L. C. Aiello and P. Wheeler, “The Expensive Tissue Hypothesis: The Brain and the Digestive System i (...)
- 15 P. C. Lee and J. E. Bowman, “Influence of Ecology and Energetics on Primate Mothers and Infants”, i (...)
10 Under the first category (changing the ways she uses her own energy) comes the brain/guts trade-off (which applies to both sexes). By the expensive tissue hypothesis14 it is possible for an organism to run a larger brain without increasing basal metabolic rate if expensive tissue from another part of the body is reduced. The gut is the part that can be most readily reduced, but only if the animal finds a higher-quality diet. Relative to early Homo, Homo ergaster/erectus females increased body size proportionately more than males did. Why the strong selection pressure on females? Small gut size means higher quality diet, which requires larger foraging areas. Homo ergaster females had to travel further, they needed larger bodies of the right shape to give them more efficient bipedality, and thermoregulation in increasingly arid environments. Larger body size also aided females who had to carry offspring that is helpless for longer. Another benefit is that the bigger the mother in relation to the offspring, the more efficient lactation.15 This means that the reduction of sexual size dimorphism critical to Key’s model for emergence of male-female cooperation is driven originally by females meeting their own costs. It is not driven by changes in behaviour between the sexes (i. e. reduction of male sexual competition). But Key’s model says that increasing male-female cooperation may be an outcome.
11Life history changes also come into the first category, but may be affected or triggered by changes in foraging behaviour and new strategies of allocare, which shades into the second area – recruiting energy from others. How could femalefemale cooperation affect life history?
12Special life history characteristics of humans today are:
- secondary altriciality;
- childhood;
- delayed sexual maturity, taking the form of adolescence;
- menopause.
- 16 R. A. Foley and P. C. Lee, “Ecology and Energetics of Encephalization in Hominid Evolution”, Philos (...)
13 Secondary altriciality refers to the extreme helplessness of a human infant, owing to the rapidity of brain development in first year and lack of development of motor skills or digestive function. Compared with a chimp, a human baby is retarded in development. That makes sense for the mother, because she has significantly more energy to find than a chimp mother, so she slows the whole process down.16 But, the paradox is, we have a much shorter interbirth interval (IBI) than chimps. How can this be?
14 Childhood is special to humans because unlike primate juveniles, human children, once weaned, are still dependent on adult help. Childhood is characterised by immature dentition; very small guts relative to body and brain size; and still growing brains. Children need special diets.
15Delay in sexual maturity is only possible if an animal expects to live longer. A later age of first reproduction will only be favoured if adult mortality rates improve. Brain size across species correlates well with first age of reproduction and overall lifespan. So the increased brain size of Homo ergaster indicates delay of sexual maturity. So does body size. By Charnov’s life history model, if mortality rates are reduced and lifespan increases, it pays to spend time growing a larger body, because you should reproduce more successfully when you do start.
- 17 K. Hawkes, J. F. O’Connell, N. G. Blurton Jones, H. Alvarez and E. L. Charnov, “Grandmothering, Men (...)
16 Menopause. A classic problem of evolutionary theory. Why select for living after last reproduction? The “grandmother” hypothesis – senior females add to their fitness by helping with daughter’s offspring – offers a neat solution, but is it viable? As originally modelled, “grandmothering” was made to compete against further mothering, i. e. carrying on having more offspring. And in terms of overall reproductive success, it cannot win, if that is the alternative. But Hawkes and her colleagues, O’Connell and Blurton Jones, argue that we should model it starting from the “chimp” baseline. A female chimp and a female human have roughly the same reproductive lifespan. The chimp starts earlier, ends a little earlier, but both have finished reproduction between 40-45. At that point, a chimp has very short life expectancy. A hunter-gatherer woman has life expectancy of20 or more years. What we have to explain is selection for these extra years, with no selection for extra child-bearing.17
- 18 J. F. O’Connell, K. Hawkes and N. G. Blurton Jones, « Grandmothering and the Evolution of Homo Erec (...)
17The “grandmother hypothesis” elegantly accounts for the evolutionary onset of all these special human characters.18 The mother’s mother is the most reliable candidate for offering allocare for weanling children. A mother who has a long-lived, vigorous mother will be able to shorten her IBIs and increase reproductive output relative to other females. This sets up selection directly for longer lifespan and greater allocation of somatic effort to maintaining the body post reproduction. The consequent reduction of mortality rates allows delay in sexual maturity. O’Connell and colleagues place these developments in a specific evolutionary context of change in climate and foraging strategies that triggered these new social strategies. With the increasing aridity of the Early Pleistocene, it became harder for weanlings especially to find accessible resources. Adults and older siblings would have to help. This climate favoured such resources as roots and tubers (underground storage organs or USOs) which became widely available (associated with geographic range of Homo ergaster/ erectus) but could only be processed by adults. These dietary strategies imply new social strategies based in female kin-bonded structures. Available resources allowed larger groups of related females to congregate together, leading to increased protection of juveniles, and reduction of mortality (among primates, weanlings have a specially high mortality rate, because they have to go it alone).
- 19 C. A. Key, “The evolution of human life history”, World Archaeology,31,2000, p. 329-350.
18In modelling the effects of body size on reproductive costs for chimps and hominins, Key shows that female Homo ergaster could not have evolved her larger body (and with it the longer lifespan) without drastically shortening IBI relative to chimps.19 This is because breast-feeding is the most expensive aspect of reproduction. Shortening lactation greatly reduces costs, but it is only possible if there is someone to help with allocare. We cannot reasonably suppose that males were the first to step in. “Grandmother” hypothesis challenges the old assumptions of “man the hunter”, “provisioning for paternity”, mainly on the grounds that modern hunter-gatherer men do not appear to target the meat they hunt to specific offspring. Also, although modern day hunters do produce a significant overall contribution to diet, any individual male would be too unreliable in providing the daily sustenance that children require. Observation of Hadza hunter-gatherers shows that older women can do that, providing staple resources including USOs.
19What this means is that to become the animal we are today, with our special life history characteristics, our ancestors necessarily evolved in female kin-bonded social structures. We know this process commenced from the Early Pleistocene because of the evidence of body size change. Despite some problems with the model (e.g. How many females would live long enough?), it appears that “grandmothering” was vital to our evolution.
20 So is “man the hunter/scavenger” dead and buried? Not necessarily. “Grandmother” leads us to a modifed view of “man the hunter”. Key’s model said that, when females have relatively high costs, males become more inclined to help. But which females will they help most? The most attractive to males will be those females who are most frequently fertile, i. e. those with reduced IBIs. So, females with older female kin who take the weaned children off their hands will get more male help. Meat is especially valuable for children because of micronutrients. Females who get meat gifts from males will be able to reduce their IBIs even further and/or their children will survive better. So “grandmother” and “man the cooperative scavenger” become mutually reinforcing. Males will come to actively choose females who have senior female kin support, because their children will survive better. But we can expect mating effort rather than paternal strategies initially.
- 20 C. Power and L. C. Aiello, “Female Proto-Symbolic Strategies”, in L. D. Hager (ed.), Women in Human (...)
21In the final phase of encephalization, from 500 000 years among Homo heidelbergensis, the ancestor of us and the neanderthals, female costs again rose steeply. Female coalitions adopted strategies to force males to cooperate in biggame “show-off” hunting. Males became more productive, fuelling the larger brains of the offspring. Ultimately, these female coalitionary strategies led to the emergence of ritual and symbolism.20 Because of the importance of strategies of female-female cooperation, there is little reason to suppose the prior structures of female kin-bonding would have been altered at this stage. Therefore we are led to the conclusion that modern humans and their immediate ancestors were biased to matrilocality, forming a basis for matrilineal priority in the earliest symbolic kinship systems.
Notes
1 E. Charnov, Life History Invariants, Oxford, Oxford University Press, 1993.
2 J. Krebs and N. Davies, An Introduction to Behavioural Ecology, Oxford, Blackwell Press, 3rd ed., 1993.
3 A. Hughes, Evolution and Human Kinship, New York, Oxford University Press, 1988.
4 S. Washburn and C. Lancaster, “The Evolution of Hunting”, in R. Lee and I. Devore (eds.), Man the Hunter, Chicago, Aldine, 1968, p. 293-303.
5 G. Isaac, “The Food Sharing Behavior of Protohuman Hominids”, Scientific American, 238, 1982, p. 90-108.
6 See review in J. F. O’Connell, K. Hawkes, K. D. Lupo and N. G. Blurton Jones, “Male Strategies and Plio-Pleistocene Archaeology”, Journal of Human Evolution, 43,2002, p. 831-872.
7 R. Trivers, Social Evolution, Menlo Park (CA), Benjamin Cummings, 1985.
8 K. Hawkes, J. F. O’Connell and N. G. Blurton Jones, “Hunting Income Patterns Among the Hadza: Big Game, Common Goods, Foraging Goals, and the Evolution of the Human Diet”, Philosophical Transactions of the Royal Society, London B,334, 1991, p. 243-251.
9 N. G. Blurton Jones, F. W. Marlowe, K. Hawkes and J. F. O’Connell, “Paternal Investment and Hunter-Gatherer Divorce Rates”, in L. Cronk, N. Chagnon and W. Irons (eds.), Adaptation and Human Behavior, New York, Aldine de Gruyter,2000, p. 69-90.
10 S. Blaffer Hrdy, “Cooperative Breeders with an Ace in the Hole”, in E. Voland, A. Chasiotis and W. Schiefenhövel (eds.), Grandmotherhood. The Evolutionary Significance of the Second Half of Life, New Brunswick (NJ) and London, Rutgers University Press,2005, p. 295-317.
11 C. A. Key and L. C. Aiello, “A Prisoner’s Dilemma Model of the Evolution of Paternal Care”, Folia Primatologica, 71,2000, p. 77-92.
12 H. M. McHenry, « Sexual Dimorphism in Fossil Hominids and its Socioecological Implications”, in J. Steele and S. Shennan (eds.), The Archaeology of Human Ancestry, London, Routledge, 1996, p. 91-109.
13 H. Kaplan, K. Hill, J. Lancaster and A. M. Hurtado, “A Theory of Human Life History Evolution: Diet, Intelligence and Longevity”, Evolutionary Anthropology, 9,2000, p. 156-185; F. Marlowe, “Male Contribution to Diet and Female Reproductive Success among Foragers”, Current Anthropology, 42,2001, p. 755-760.
14 L. C. Aiello and P. Wheeler, “The Expensive Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution”, Current Anthropology,36, 1995, p. 199-221.
15 P. C. Lee and J. E. Bowman, “Influence of Ecology and Energetics on Primate Mothers and Infants”, in C. R. Pryce, R. D. Martin and D. Skuse (eds.), Motherhood in Human and Nonhuman Primates, Basel, Karger, 1995, p. 47-58.
16 R. A. Foley and P. C. Lee, “Ecology and Energetics of Encephalization in Hominid Evolution”, Philosophical Transactions of the Royal Society, London B,334, 1991, p. 223-232.
17 K. Hawkes, J. F. O’Connell, N. G. Blurton Jones, H. Alvarez and E. L. Charnov, “Grandmothering, Menopause, and the Evolution of Human Life Histories », Proceedings of the National Academy of Sciences, 95, 1998, p. 1336-1339.
18 J. F. O’Connell, K. Hawkes and N. G. Blurton Jones, « Grandmothering and the Evolution of Homo Erectus”, Journal of Human Evolution,36, 1999, p. 461-485.
19 C. A. Key, “The evolution of human life history”, World Archaeology,31,2000, p. 329-350.
20 C. Power and L. C. Aiello, “Female Proto-Symbolic Strategies”, in L. D. Hager (ed.), Women in Human Evolution, New York and London, Routledge, 1997, p. 153-171; and see Knight’s paper.
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