Cognitive consequences of our grandmothering life history: cultural learning begins in infancy

Abstract

Postmenopausal longevity distinguishes humans from our closest living evolutionary cousins, the great apes, and may have evolved in our lineage when the economic productivity of grandmothers allowed mothers to wean earlier and overlap dependents. Since increased longevity retards development and expands brain size across the mammals, this hypothesis links our slower developing, bigger brains to ancestral grandmothering. If foraging interdependence favoured postmenopausal longevity because grandmothers' subsidies reduced weaning ages, then ancestral infants lost full maternal engagement while their slower developing brains were notably immature. With survival dependent on social relationships, sensitivity to reputations is wired very early in neural ontogeny, beginning our lifelong preoccupation with shared intentionality.

This article is part of the theme issue ‘Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals’.

1. Introduction

I elaborate the hypothesis and point to associated evidence that distinctive aspects of learning and social cognition came along with the evolution of the life history that distinguishes us from the other great apes. This builds on Sarah Hrdy's insights that ancestral dependence on allomothers and survival benefits for social precociousness in infants may explain why a unique propensity for other-regarding cognition and behaviour evolved in our lineage of hominids [1–5].

Among our closest living evolutionary cousins, mothers rear each offspring to weaning when it feeds itself. Human youngsters always need others' help (e.g. [1,3,6–10]). The grandmother hypothesis specifies the socioecological context that likely propelled the evolution of this distinctive human life history. When savannah resources that youngsters could not handle for themselves gave reliable return rates to gregariously foraging adults, older females' economic productivity subsidized dependent grandchildren as their own fertility was ending. Still-fertile females could invest less in each offspring and bear next babies sooner. More robust older females could subsidize more descendants, favouring mutations that increased allocation to somatic maintenance and propelling the evolution of postmenopausal longevity [11–18]—with some of the consequences to be noted below [5,19–24].

2. Background

Since the nineteenth century, the hypothesis favoured for the evolution of distinctive features in our lineage has focused on hunting by ancestral males to provision their mates and offspring. That hunting hypothesis as elaborated in the last several decades proposes that spreading savannahs in ancient Africa made hunting a promising way to make a living. Since dependent offspring could interfere with prey pursuit, ancestral mothers did better to pair with a hunting mate who ‘brought home the bacon’, resulting in a sexual division of labour with nuclear families the elementary economic and social units (e.g. [25–27]). Researchers' own ideals of separate nuclear families combined with the fact that men, unlike other primate males, provide goods and services we all consume, contribute to the appeal of that hunting hypothesis. Stone tools and bones of multiple large animals in early archaeological sites can seem hard evidence that ancestral hunters brought their kills home to provision their mates and offspring (e.g. [28]).

But the ecological context, site structure, and assemblage composition of the early sites compared with faunal assemblages that result from modern people hunting and gathering in similar habitats show the ancient sites likely represent near-kill/ambush/scavenging locations not ancient home bases [20,29–33]. Systematic behavioural observations among those modern people quantify the daily failure rates for big game hunting and aggressive scavenging and the wide claims on periodic bonanzas. Even with fully modern brains and powerful bows and arrows, average success rates per hunter-day are below 5%, with marked seasonal and annual variation and most meat claimed by others [34–37]. While men differ from other primate males in the contributions their activities make to overall consumption [38], seeking big carcasses is not suited to meeting anyone's daily nutritional needs—let alone those of dependent children ([32,34–37,39–43], and see [44] for nutritional quality of game).

The salience of hunting reputations (e.g. [45,46]) and the rich ethnographic evidence that men's alliances dominate community affairs (e.g. [47–49]) make male status competition a better candidate than paternal provisioning to explain risky hunting [19,35–41]. Across a sample of hunter–gatherer societies it is lack of other paternity opportunities not father effects that correlate with higher pair bond stability [50]; and the wide variation in paternal contributions to offspring care—within and among individuals and across cultures—are not consistent with an evolutionary legacy of obligate paternal effort (e.g. [7,51]).

With women's postmenopausal life stage and spermatogenesis continuing in older men, human sex ratios in the fertile ages are male-biased [52], a bias that favours mate guarding (e.g. [53–57]), which links human pair bonding habits to the evolution of our grandmothering life history [52]. If that life history has consequences for wiring infant cognitive sensitivities early as outlined below, then those sensitivities cascading through lifetimes keep social relationships dominant priorities as survivors continue to respond and adjust mutual understandings (e.g. [58,59]). The longer sharing intentions persist with particular others, both adults and children, the more grievous the loss of that relationship.

3. Grandmothering

Like the hunting hypothesis, the alternative framework privileged here ties the evolution of human life history to a shift in ancestral foraging strategies when ancient climate change constricted African forests and expanded more open habitats. But instead of risky hunting, the focus is on reliable daily fare. In forests, ancestral hominid populations would have relied on fruits and leaves as great apes continue to do now. Nursing infants, carried by their foraging mothers, could begin to pick and eat the same foods that she was eating long before weaning, and become independent feeders when she bore her next offspring. With ancient climate change, forests retreated, savannahs spread, and plants that sequestered nutrients in hard-shelled nuts and underground storage organs flourished: opportunities that ancestral populations could exploit. The scenario I and others hypothesize [5,11–13,19–22,33,60–62] can be summarized as follows.

Ancestral adults colonizing more open habitats could take advantage of savannah resources, pursuing and processing more than enough food for their own consumption. But, as with modern humans today, youngsters were too small to earn high enough return rates on those foods to feed themselves (e.g. [61,63–65]). As offspring were dependent longer, mothers in those habitats might have invested more in each one and lengthened their birth intervals. But unlike mothers' milk, these foods could also be supplied by others big and strong enough to handle them.

Of special importance, the savannah foods of interest do not promote scramble or interference competition where more consumers reduce return rates for all. Amounts of savannah foods available for consumption increase with extraction and processing effort. Foragers get mutualistic advantages from gregarious acquisition. With deeply buried geophytes, return rates initially increase for additional effort, favouring accumulation for bulk processing. Rather than extracting, processing and consuming items one at a time, there are clear economies of scale that multiply with cooking [20,22,62] where start-up costs for cooking fires increase marginal benefits for mutual processing in quantity.

Production of food in lumps, in contrast to hand-to-mouth, eat-as-you-go foraging, also results in opportunities for others to appropriate shares [66]. This was noted by Richard Wrangham [67] in association with his review of evidence for the importance of cooking in human evolution. He inferred that risks of robbery for batches of resources would be grounds for enlisting a mate as guard. But we have drawn attention instead to the likely appropriators most immediately at hand: dependent juveniles [19,22,62].

Our ethnographic observations of age and sex differences in foraging among Hadza hunter–gatherers underpin this grandmother hypothesis about the evolution of human life history [5,11–13,19–22,31,33,61,63,64,68]. We observed high foraging productivity of older women [69], and active foraging by children [63], but youngsters are too small to be very effective at digging the deeply buried tubers that are year-round staples [63,64,70]. Children's weight gains depended on their mothers' foraging effort. However, when she had a new baby that relationship disappeared; then weaned children's gains depended on grandmothers' foraging [11,13]. Older women join residence groups where their productivity can raise their inclusive fitness most [68]. Grandmothers have large effects on grandchild survival in this population [61].

If modern mothers and grandmothers foraging in such habitats leave more descendants with this division of labour, then similar trade-offs could have driven the evolution of human age structures. Female fertility ends at about the same age in humans and other great apes, a basis for assuming this to be a synapomorphy, shared from our most recent common ancestors. But, unlike humans, great apes become decrepit with age in their mid-30s [71] and rarely out-live their fertility [13,72–75]. Humans have much lower adult mortality rates. People who survive to adulthood remain healthy and productive decades longer, and women usually live well past menopause [13,76,77]. Our postmenopausal life stage [78], long of particular interest to evolutionary life-history theorists (e.g. [79,80]), is not shared by other primates [81].

Our Hadza ethnography was underway as evolutionary biologist Eric Charnov was investigating mammalian life-history evolution. Wide differences in the pace of life cycles had long been of interest (e.g. [82]), but it took decades to accumulate records of age-specific fertility and mortality in mammal populations [83–85]. When comparative analyses began, Charnov noted that average adult lifespan and age at maturity increased with body size at the same allometric rate and constructed a model of female life-history evolution aimed at recovering these allometries ([86], see more discussion in [60]). Average adult mortality and age at first birth vary widely from mice to elephants, but for each taxon, the product of those two averages is nearly constant. In Charnov's modelling, that invariance results from the trade-off between benefits of growing longer before reproducing and risks of dying first. Adult mortality determines the cost of delay. Selection can only favour waiting longer when that cost goes down.

Charnov [86] plotted primate data [84] to show the correlation between age at first birth and average female adult lifespan. At the time, the other great apes were classified together as pongids, with humans the only hominids, and his figure includes a point for each. Both fitted nicely on the regression, with humans—of course—the highest point. Unremarked at the time, this should have been puzzling. Unlike the other primates, human females do not continue bearing offspring throughout adulthood. Both Fisher [87] and Hamilton [80] had noted that post-fertile women reproduce their genes through grandmothering. If contributions to the ancestry of future generations from ancestral grandmothers explain why humans fit that mammalian invariant, it implies another distinctive, measurable consequence within the model. Rates of baby production should be higher (birth intervals shorter) than predicted for non-grandmothering mammals with our age at first birth. Demographic estimates of baby production rates in great apes and ethnographic hunter–gatherers are directly consistent with that expectation [12,61,88].

But could such subsidies actually propel the shift from ancestral great ape-like life histories to human-like ones? Mathematical modelling can address that question. Peter Kim built two-sex models using the longevity trade-offs for females in Charnov's mammal model plus a longevity trade-off for males. Greater longevity meant more paternity chances but lower competitive success against shorter-lived males. Agent-based simulations began with parameters set at a great ape-like longevity, so very few females outlived their fertility. When those few post-fertile females could subsidize dependents, mothers could bear another offspring sooner; and grandmothering subsidies drove simulations to new age structures like those of living hunter–gatherers, where about a third of the adult females are past their childbearing years [14,15].

Those models fixed the end of childbearing at age 45 based on the similarity between humans and great apes. The question ‘why 45?’ was explicitly left for subsequent work with the aim to see whether, given that end to female fertility, grandmothering could nevertheless propel the evolution of postmenopausal longevity. Since the end of female fertility varies across the mammals—earlier in monkeys than in apes, and extend to even older ages in some mammals (e.g. two decades longer in elephants, [89])—we surmised it was selection, not some constraint of biochemistry, that accounts for the timing of menopause in humans. Fitness benefits from continued baby production on one hand trading off with more grandmothering subsidies on the other seemed likely. Subsequent modelling [16–18] took up that trade-off. Using similar assumptions about life-history trade-offs and allowing both longevity and the end of female fertility to evolve, both partial differential equations that allow easier exploration of parameter values and simulations found that when grandmothering subsidies are added to an initial chimpanzee-like life history those subsidies drive populations to human-like longevities while holding the end of female fertility below 50.

4. Evolving mammalian brains

Average adult lifespans determine the duration of maturation and rate of baby production in Charnov's model to explain mammalian life-history variation. The fundamental role of adult mortality and the correct scaling relationships among life-history traits are central insights [86]. Mammalian brains also scale with longevity. Barbara Finlay and colleagues have explained how this scaling results from phylogenetic constraints on neural ontogeny. The longer after conception neural structures cease differentiating, the larger they will be. Finlay [90] summarizes the explanation this way:

A central process underlying these predictable patterns of brain scaling is duration of neurodevelopment, which begins from the kinetics of stem cell and neuron proliferation, and extends to the production of processes, synapses, myelin, and other supporting cells … . Brain regions that cease neurogenesis early, like the medulla, change relatively little from one mammalian species to the next, while regions that proliferate for an extended period become larger nonlinearly, reflecting the underlying exponential factor of cell division in stem cell pools … .

Finlay and colleagues have demonstrated that the order of developmental events in neural maturation is astonishingly closely conserved across eutherian mammals of widely varying brain size [91]. Because the neocortex develops later, it necessarily increases faster than whole brain size [92]. As increasing longevity extends the duration of development, brain size expands, with the neocortex becoming a larger and larger fraction of larger brains.

Human brains are just the size, with just the expected neocortex proportion, that results from the process of neurodevelopment in eutherian mammals with our lifespans because both final brain size and neocortex proportion depend on the duration of development, which in turn varies with longevity. Finlay [90] tallies costs of the mistaken belief that our neocortex is exceptional:

… this belief has caused both neuroscientists and psychologists to prematurely assign functions distributed widely in the brain to the cortex, to fail to explore subcortical sources of brain evolution, and to neglect genuinely novel features of human infancy and childhood … [Emphasis added].

Consequences for distinctive features of human cognition are especially large because our life history combines earlier weaning with slower neural development [23,24]. That combination escalates survival consequences of harms and comforts from social interactions, prioritizing social responses notably early as baby brains wire to their developing somatosensory sensory world.

5. Cognitive consequences

Sarah Hrdy recognized that shifting away from the independent mothering of our closest living evolutionary cousins to the interdependencies of our own lineage would have major cognitive consequences. She (e.g. [2,3,4]) connected the evolution of human babies' capacities to delight and engage others with Michael Tomasello's findings about differences in social learning between humans and the other great apes (e.g. [93–95], etc.). Tomasello and collaborators have continued to devise experiments that identify distinctive features of human mind reading and perspective taking: our appetite for shared intentionality [96].

Those differences between us and our evolutionary cousins are important to the argument here. But so is evidence accumulated in pursuit of other questions. For decades, developmental psychologists have focused on human infants' and children's solutions to the mysteries of the world, categorizing foundational domains of knowledge as baby physics, baby biology and baby psychology [97]. One productive view of those findings was captured by Alison Gopnik, Andrew Meltzoff & Patricia Kuhl's [98] book title, The scientist in the crib. The ‘theory theory’ proposes that infants construct expectations—theories—they test and revise in light of their experience. As Gopnik & Meltzoff [99] said, ‘it is not that children are little scientists, but that scientists are big children’. Pushing that further here: so are all adults. In the distinctively interdependent grandmothering socioecology of ancestral populations, the welfare of infants and children depended on support from others. Babies' responses to others’ intentions constructed and tested expectations of comfort and harm. Social sensitivities were wired early and continued to have lifelong inclusive fitness consequences.

Children test and revise their theories with imaginary friends, other ‘counter-factuals’, and pretend play (e.g. [98–101]). Here the supposition is that experiments like those had survival consequences in the ancestral socioecology that favoured the evolution of our grandmothering life history. Gopnik [101] suggests that, ‘… the evolutionary purpose of childhood is to provide a protected period in which variation and innovation can thrive. Play is the most striking manifestation of that strategy. Play is precisely an activity with no apparent goal or purpose or outcome’. But Gopnik herself develops the alternative emphasized here. A goal that play serves for individuals—infants, children and adults—is to test and revise hypotheses about the physical world, other living things, social relationships and about shared intentions, purposes that have lifelong fitness-related consequences.

Other animal infants also arrive at the magic show of gravity and object permanence as their somatosensory systems mature. Great apes display that understanding when they recognize and solve physical problems, and use some basic psychology when identifying problems and solutions in the intentions and perspective of others [102]. Differences between them and us include the salience and effectiveness of social cues for learning about both problems and solutions (e.g. [96,103]).

6. Cultural learning

Particular attention to learning from social cues was stimulated by David Premack & Guy Woodruff's [102] paper ‘Does the chimpanzee have a theory of mind?’ The question was provocative at a time when theory of mind was generally assumed to be a distinctly human cognitive capacity. Premack & Woodruff reported that a human-raised chimpanzee, shown a film of one of her keepers confronting a problem, would correctly choose the tool the keeper needed to solve it. To do so she had to understand the human's perspective and intentions.

When Michael Tomasello, Ann Kruger & Hillary Ratner [94] sought to identify what is distinctive about human responsiveness to social cues, they considered that example, attributing more human-like perspective-taking skills of chimpanzees raised by humans to their experience ‘in an environment in which joint attention to objects is a regular and important part of their social lives with their human caregivers. This has led them to develop more fully their latent capacities for engaging in joint attention and for taking the perspective of intentional agents' [94, p. 507].

Capacities presumed latent in our common ancestors that evolved to promote cultural learning have been characterized in many ways. Tomasello and colleagues enumerated some of them this way:

… learners do not just direct their attention to the location of another individual's activity; rather, they actually attempt to see a situation … from inside the other's perspective … . This qualitative difference is possible because human beings are able … to take the perspective of the other … , to engage in mindreading of the other … , to understand the other as a ‘person’ … , or to participate with the other intersubjectively …. [94, p. 496]

This, they said [94, pp. 495–496] was ‘echoing a theme’ of Lev Vygotsky, but Vygotsky (e.g. [104]) focused ‘almost exclusively on the important role of culture, neglecting for the most part what the individual organism brings to the process of enculturation. [In contrast] … we attempt to focus on the individual capacity for acquiring culture, that is to say, on the social-learning processes whereby human children acquire the skills and conventions of those around them’.

Subsequently, Kruger & Tomasello [105] offered an amendment to the proposal just quoted that clarifies the issue of evolutionary causality I mean to raise here. They say:

… based on the responses … especially from cultural anthropologists and cultural psychologists … our theory of cultural learning … focused too narrowly on the cognitive capacities of the individual child. For a more balanced and complete theory we need to complement our focus on what the child brings to the culture with a focus on what the culture brings to the child … the role of the pre-existing structure of the culture: its games, institutions, rituals, cultural models, intentional scripts, and communal activities … . [105, p. 370]

The point here is that when there is such a pre-existing structure, the initial evolution of the human capacities and tendencies from an ancestral condition without them remains unexplained. This is the problem that Hrdy highlighted when she identified challenges to ancestral infants that might explain why distinctive social cognition evolved in our ancestors but not our great ape cousins. The hypothesis here is that those challenges to ancestral infants arose with particular features of ancestral socioecology.

7. Ancestral socioecology

Unlike other apes, humans are descendants of ancestral babies who grew up in economically interdependent groups of mixed ages as modern hunter–gatherer babies do today (e.g. [106]). Among Hadza foragers in northern Tanzania, for example, we tabulated the locations and activities of camp residents [11,107], documenting daily interactions among children of different ages with each other and with adults both in and out of camp (see also [65]). Mixed-age interactions recur from infancy, often in conjunction with joint consumption of foods that are acquired and processed by others. Unlike other apes, humans, throughout life, do not acquire all they eat, or eat all they acquire [108].

This is an expansion of Hrdy's hypothesis that preferences for participation, engagement and sharing attention and intentions with others that emerge in infancy evolved as survival responses to the cognitive ecology of ancestral infancy [1,3,4,5,109]. Infant attempts to align perspectives with others are the beginning of preferences for shared intentionality that mature along with their somatosensory systems and experiences through childhood and into adulthood. Those preferences endure because economically interdependent foraging makes fitness-related payoffs at each age depend directly on social relationships. Youngsters are active agents doing from the beginning what they will continue to do as adults. Our efforts to influence each other construct and reconstruct both continuity and diversity in human cultural life. This perspective aligns with arguments from ethnographers of childhood who dispute claims (e.g. [110]) that teaching provides the universal foundation for cultural persistence. Instead, those studying childhood across cultures, including David Lancy (e.g. [111–113]) and Barbara Rogoff [114], report that formal teaching is often rare or absent. Youngsters learn by joining in and ‘pitching in’.

Investigators living in Western middle-class communities readily assume that the small, isolated nuclear families familiar to us are typical human social units. Misleading consequences were reviewed by Joseph Henrich, Steven Heine & Ara Norenzayan [115], who found that:

Behavioral scientists routinely publish broad claims about human psychology and behavior … based on samples drawn entirely from Western, Educated, Industrialized, Rich, and Democratic (WEIRD) societies. … Here, our review of the comparative database from across the behavioral sciences suggests … that WEIRD subjects are particularly unusual compared with the rest of the species – frequent outliers … . The findings suggest that members of WEIRD societies, including young children, are among the least representative populations one could find for generalizing about humans . [115, p. 1]

Commentary on that paper expanded concerns about unappreciated variation in infant/child/adult interactions and patterns of attachment. Of particular interest for assumptions about development and learning, children are frequently the caretakers of children (e.g. [106]). By contrast, WEIRD families tend to be small, separated from other kin, with very low fertilities and closely spaced births. So, we rarely grow up living with babies or children of widely varying ages.

The view I take here emphasizes the survival challenges that ancestral infants faced owing to reduced maternal investment in each offspring. Challenges continued after weaning because juveniles, still unable to feed themselves, depended on relationships. This perspective emphasizes not only the benefits from subsidies, but the perils of dependency. From that perspective, distinctive features of learning in our lineage are the legacy of ancestral infants who successfully coped with those perils, which did not end with infancy. Managing social reputations continued to dominate learning throughout life.

Hrdy [1,3,116,117] noted that allomaternal care is more frequent in the primate order than in mammals generally as mothers can gain fitness benefits when others hold their infants and relieve them to forage freely. A common impediment to netting these evident benefits is maternal protectiveness, while the proximity of trusted matrilateral kin makes allocare more likely [3,116]. The grandmother hypothesis identifies both a distinctive kind of allomaternal help, reliable daily provisioning, and the socioecology in which supplying it was the consequence of efficient foraging for daily consumption. The particular ancestral allomothering that likely propelled the evolution of the life-history distinctive to our human radiation was the economic productivity of older females. With that came cognitive consequences that Hrdy [3] linked to the evolution of shared intentionality [94,95].

Tomasello recently agreed that Hrdy's cooperative breeding hypothesis [3,4], and my elaborations on it [5,109] have merit [118,119]. But there, and elsewhere (e.g. [120]), he emphasizes differences in social strategies at different developmental stages: infants' and toddlers’ attention, focused on adults, whereas children are oriented toward peers and so toward collaborating effectively in groups. In a 2018 paper, the evolutionary importance of infant strategies, recognized by Hrdy and built on here, disappears when Tomasello describes his overarching proposal

… that infants solve the infant tasks using general great ape social-cognitive abilities evolved for competing with others, whereas older children solve the classic tasks using uniquely human social-cognitive abilities evolved for coordinating mental states with cooperative partners, abilities also known as ‘skills and motivations of shared intentionality’. [121, p. 8491]

Children growing up in WEIRD societies do not experience continually interdependent mixed-age groups that likely characterized most of human experience—experience beginning long before the emergence of modern humans. In ancestral socioecologies, likely similar to those of hunter–gatherers today, the attention of others is a dominant feature of everyone's cognitive ecology. Ancestral social sensitivities that would have emerged when priority to engaging attention and aligning interests was a survival strategy for infants resulted in ‘learning without teaching’ [122,123], ‘learning from no one’ [113] and ‘learning by pitching in’ [114]. Attending to the intentions of others and responding to influence them is not just the Bayesian problem-solving process that wires immature baby and toddler brains [124]. Strategies to attract, engage and persuade persist through the counter-factual play of infants [98,98], the pretend play of children [100,101] and the continued alliance building and reputation sensitivity of adults.

8. Language

Distinctive features of human communication that serve our shared intentionality were explored by Tomasello (e.g. [125]) using philosopher of language Paul Grice's [126] theory of meaning. Cultural anthropologist Dan Sperber and colleagues (e.g. [127,128]) also rely on Grice and identify persuasion as the salient aim of human communication; In their account we use reason, not because logic gets at truth, but to improve the persuasiveness of arguments; hence, the inevitability of confirmation bias [128]. Neither Grice nor Sperber gives serious attention to the likely social cognition of ancestral populations before the evolution of human language. But Tomasello uses Grice's analysis to identify features that distinguish human social cognition and communication. Both as children and as adults, humans seek to persuade as we try to find shared understandings. If those motivations preceded the evolution of language, they would make its subsequent emergence less mysterious [3,125,129].

But did they precede language? Cecelia Heyes, a cognitive scientist who has persistently and persuasively criticized claims about special human capacities for mind reading proposes otherwise. Heyes [130] argues that ‘language comes first’. Through language, children learn to read minds. She assembles evidence that ‘the mindreading mechanism is built by conversation, storytelling and collective reminiscence’ [130, p. 204]. My claims here entail expectations that, with language, conversation, storytelling and collective reminiscence are continuing learning opportunities. Language is a pervasive part of human experience now, used continuously to evaluate and adjust shared intentions. But that does not address the question why it and associated distinctive aspects of social cognition evolved in our lineage to begin with.

Heyes herself provides an opening for my argument when she reviews evidence that ‘human and nonhuman social learning are continuous, and social learning is adaptively specialized – it becomes distinctively “social” – only when input mechanisms (perceptual, attentional and motivational processes) are phylogenetically or ontogenetically tuned to other agents' [131, p. 193]. As argued here, infant motivational processes were tuned to other agents by the ancestral socioecology in which interdependent foraging for savannah resources that youngsters could not handle effectively made grandmothering subsidies propel the evolution of human life history.

Richard Moore (e.g. [132–134]) agrees with Heyes that positing evolved cognitive or language modules to explain distinctively human capacities is not ‘satisfactory, because to posit a modular solution is not to explain how the problem is solved, but simply to stipulate that at some point in our evolutionary history it was' [133, p. 313].

Moore's framing of the problem follows others in turning to Grice's analysis of meaning:

On standard accounts of Gricean communication, the cognitive mechanisms that support intentional communication presuppose sociocognitive abilities that our nearest non-human cousins - chimpanzees and bonobos – lack … . As a result, before our early hominin ancestors could become Gricean communicators, they needed to undergo a sociocognitive revolution … . Since Gricean communication became possible only after the revolution, it could not have contributed to its occurrence. [133, p. 322]

Moore also identifies a solution. If great ape gestural communication demonstrates meaning,

…what our ancestors acquired may not have been a whole new system of communication powered by vastly superior social cognition, so much as incremental improvements in a structurally similar system of communication. These improvements may have enabled further forms of communication (including language) that themselves enabled the development of the metapsychology that is often thought to be a prerequisite of Gricean communication. [132, p. 229]

This accommodates Heyes's observations that talk about thinking affects ideas about how minds work and so language plays a role in mind reading. That is also consistent with the conjecture here that human experience during infancy and onward prioritizes concerns about reputations and shared intentionality.

But Moore follows Tomasello [125,135,136] in supposing the perceptual, attentional and motivational processes that increased tuning to other agents came from ancestral dependence on hunting:

…at some point in hominin history, our ancestors became dependent on using their existing but limited abilities for Gricean communication to engage in coordinated hunting … . In turn, this allowed our ancestors to eat more meat – which supported brain growth and gave rise to Hominini with more powerful abilities for inferring the communicative intentions of others. [134, p. 810].

This is a supposition that evidence I have reviewed earlier stands against. While Moore does mention Hrdy's hypothesis, it is with the qualification that his scenario ‘is intended to complement and not exclude other gradualistic stories about the evolution of hominin social motivation and attention (e.g. moderate versions of the Cooperative Breeding Hypothesis ([3][b])’ [134, p. 813]. But by arguing that the evolution of our lineage was from ‘meat to language’ he fails to take full advantage of Hrdy's insights, or evidence that hunting rarely supplies reliable daily fare, and assumes a quite different account of the evolution of our big human brains from the one favoured here.

9. Concluding summary

My conjecture is that the evolutionary foundation of distinctive features of our social cognition and learning emerged when hominid ancestors began exploiting savannah plant foods that gave gregariously foraging adults reliable, daily, benefits for economic interdependence. Those were foods that youngsters could not handle effectively enough to feed themselves, but daily return rates of older females whose fertility was ending were high enough to subsidize them, so mothers spent less on each offspring, shortening birth intervals. Since females ageing more slowly subsidized more descendants, longevity increased, delaying maturation and neural development in subsequent generations. Survival of early-weaned infants depended on others, a vulnerability that prioritized infant sensitivity to others' intentions. With slower brain development, that social sensitivity is wired especially early in neural ontogeny prioritizing responsiveness to reputations—one's own and those of others—and continued to dominate social cognition lifelong.

This scenario suggests concurrence with the distinction Gopnik (e.g. [101]) emphasizes between school learning and the different ways that WEIRD infants, toddlers and children learn outside of school. Emphasis on teaching and even on cultural transmission and cumulative culture as human specialities draws attention away from that difference and away from the relevance of that ‘outside of school’ learning for most of the human past. Language itself seems a relevant parallel. It persists from one generation to the next not because the older generation teaches language to the younger. Infants, toddlers and children learn the language spoken around them as they try to figure out what older children and adults are saying and doing, eager to be a part of it [137].

Now in middle-class Western socioecologies hearing 30 million words by the age of four may influence children's subsequent success [138–140]. That is important for both the present and the global future, but not a guide to the novel learning challenges and responses that accompanied the evolution of life history in the human radiation. Our own Bayesian priors arise from and are continually adjusted by our own mostly WEIRD experience. Recognizing that will improve arguments about what happened in the evolution of our life history and social cognition, and also help distinguish what is novel in learning challenges of the present and future.

Data accessibility

My paper summarizes data reported and analysed in other papers that are publicly available.

Competing interests

I declare I have no competing interests.

Funding

I received no funding for this study.