First few million years: hominins cognition-imagination, worldly-unworldly, time allocation calculations, solution-seeking, territoriality, layers of interaction, functionally equivalent bonding forms

May 08, 2023

Written by Robin Dunbar

Because we share a common ancestor with the chimpanzees, it makes the chimpanzees (as opposed to some kind of generic early Miocene great ape) the proper comparison for the human lineage, and in many ways the best model for early members of the human lineage (the australopithecines and their immediate predecessors).

Conventionally, taxonomists now refer to the great ape family (including humans) as hominids, while all members of the lineage leading to modern humans that arose after the split with the LCA are referred to as hominins.

… we are not just great apes. There are some radical differences.

The least interesting of these, although the ones that almost everyone has focused on, are the anatomical differences, and in particular our upright bipedal stance. In fact, most of these traits are just bits of early remodelling to allow a mode of travel that became a route out of certain extinction as the Miocene climate deteriorated and the tropical forests retreated. Much of the rest of the debate has hinged around instrumental behaviours like tool-making and tool use. But in reality these are cognitively relatively small beer – even crows make and use tools, with a brain that is a fraction the size of a chimpanzee’s.

The substantive difference lies in our cognition, and what we can do inside our minds. …

… religion and … story-telling … are entirely and genuinely unique to humans. We know they must be unique to humans because both require language for their performance and transmission, and only humans have language of sufficient quality to allow that.

… In both cases, we have to be able to imagine that another world exists that is different to, and separate from, the world we experience on an everyday basis. We have to be able to detach ourselves from the physical world, and mentally step back from it. Only when we can do this are we able to wonder whether the world has to be the way it is and why, or imagine other parallel worlds that might exist, whether these are the fictional worlds of story-telling or para-fictional spirit worlds. These peculiar forms of cognitive activity are not trivial evolutionary by-products, but capacities that play – and have played – a fundamental role in human evolution.

… the social performance of music. To be sure, many other species can be said to produce music, including songbirds and whales, to name but the best known. But only humans seem to engage in music as a social activity. …

… Seen on the grand scale of the last 6 million years, hominin brain size has been on a steady upswing in which brains trebled in size from their ape-like beginnings among the australopithecines to the brains of modern humans. …

… correlated problems… how to fuel the extra costs of these big brains, given that brains are, in energetic terms, exceptionally expensive, and how to bond the ever larger social communities that these large brains were designed to allow. Finding the time in an already busy day is the rack on which these species were all stretched – so stretched, in fact, that without finding novel solutions to the problems of time allocation, they would never have been able to break through the glass ceiling on community size that each phase represents. …

… [The] conventional focus on ‘stones and bones’ has inevitably left the social aspects of human evolution, and even more so their cognitive underpinnings, largely unexplored. The archaeologist’s understandable worry has always been that it is just too easy to infer what you will about social behaviour from the sometimes scrappy and invariably indirect evidence available in the deep time record. Yet it is precisely these social and cognitive aspects of our biology that mark out the road that led, tortuously and often uncertainly, from the Last Common Ancestor some 6–8 million years ago to the modern humans that we now represent. If we want to account for that trajectory, we have to grapple with this murky, unseen social world, however difficult that may be.

In the societies of primates (including contemporary humans), the social community takes the form of a highly structured network of individuals linked to each other through ties of kinship, friendship and obligation. The way in which these social networks are structured in terms of kin and non-kin and the way they are distributed in space both have implications for how easy it is for an individual to call for assistance, as well as … the coherence and persistence of the network …

The approach that I will adopt … exploits our much improved understanding of how primates allocate time to the various core activities (feeding, travel, rest, social bonding) that are crucial to their ability to survive in a particular habitat. …

… The key issue here is that the length of the working day is limited …

A species cannot increase its brain or body size without affecting the time it has to spend feeding, and it cannot change its feeding time allocation without affecting the time available for other equally crucial activities like travel or socializing. …

… the social brain hypothesis has … come to be an explanation for the correlated differences in cognition and sociality between primate species. The crucial feature of this hypothesis is that it provides a quantitative equation relating brain size to social group size. The fact that this relationship is extremely robust and almost independent of any direct effects due to ecology means that we have a way of predicting typical social group size in fossil species. This provides us with two key insights into time budgets.

[1] because brain size predicts group size, we can determine how much extra time is needed for bonding larger groups …

[2] … increase in brain size has to be fuelled by additional foraging time.

The question we ask … is simply this: how did they accommodate these additional demands on their time? And if their time budgets were already stretched to their limit, what novel solutions did they find to make the extra time they needed available? …

… the bread-and-butter of traditional accounts of human evolution … include bipedalism, the changes in pelvic structure and the acquisition of a plantar foot, the loss of canines, the increased gracilization (i.e. lighter body build) of modern humans, the progressive increases in brain size, the acquisition of a modern human lifehistory with delayed maturation (signalled by delayed molar eruption) and precocial babies, tools in all their complexity, hunting and artwork. Others (fission–fusion sociality, division of labour, grandmothering, the menopause, cooking, religion and pairbonding) have played a central role in anthropological discussions of human social evolution but often lack plausible archaeological signatures … in the fossil record.

Some, however, are genuinely novel and have never really been considered in the context of human evolution: these include music and dance, story-telling, religion, those forms of social cognition known as theory of mind or mentalizing, and laughter. These have, I shall argue, played a particularly important role …

2. Primate Sociality

Primates are, above all, intensely social, and their peculiar kind of bonded relationships allows them to form groups that remain stable and coherent over time.

The main reason that primates live in groups is as a defence against predators. An animal’s risk of being caught by a predator increases as it becomes more terrestrial and occupies more open habitats with fewer trees to provide refuges. Under these conditions, group size increases, and to support this their social relationships shift to being ever more intensely bonded, presumably in order to ensure that individuals stick together and come to each other’s aid when necessary.

Living in close proximity to other individuals may have its benefits, but it also has its costs.

These arise from three separate sources: direct and indirect costs, and freeriding. The direct costs arise from conflicts within the group: altercations between individuals over food or the safest resting sites. All of these inevitably increase in frequency as groups get larger.

The indirect costs are the fact that more travel time has to be allocated within the waking day as group size increases in order to allow the group to visit enough food patches to satisfy everyone’s nutritional requirements, and that means less time is available for other activities.

Furthermore, travel consumes energy, so there are knock-on consequences for feeding time as more foraging will be needed to pay for the extra energy used in travel. Travel also exposes the group to the additional risk of encountering predators.

Last but not least, a primate group is an implicit social contract (it is a collective solution to the problem of predation), and social contracts are always susceptible to being broken by freeriders – those who take the benefit of the contract but don’t pay the costs, thereby benefiting doubly at everyone else’s expense. In this case, the benefit is collective protection from predation. Individuals who exploit fellow group members impose a burden that these eventually become unwilling to bear. As with the other costs, the risks of being exploited become more acute as group size increases.

In primates, the direct costs are borne mainly by the females, because the stresses of living in a group – the casual jostlings in cramped conditions, the occasional conflicts over access to food or safe refuges – impact on female menstrual cycle endocrinology and lead to infertility. … [there follows detailed discussion of relevant studies]

As this pressure mounts with group size, females will want to live in smaller groups.

Thus female reproductive strategies become a crucial factor that puts a natural brake on runaway increases in group size. When predation pressure favours an increase in group size, primates will have to neutralize these stresses, otherwise groups will inevitably fragment and living in large groups will be impossible. …

Defusing the stresses of group-living … Monkeys and apes neutralize the stresses created by living in groups by forming coalitions that buffer their members against harassment. The intensely bonded relationships that underpin these coalitions seem to involve two separate processes that interact to create these bonds.

[1] One is an emotionally intense mechanism triggered by social grooming. Social grooming seems to work because it triggers the release of endorphins in the brain … as the stresses increase with group size, it becomes more and more essential to ensure that one’s coalition works as reliably as possible, and so animals increasingly concentrate their grooming effort on their most important social partners to the exclusion of casual grooming. …

[2] …. One aspect of primate sociality that remains conspicuously unresolved is the question of what these social bonds actually involve. In humans, they appear to be primarily emotional, which may be why they are so notoriously hard for us to define. … intuitively at least we can distinguish between relationships that are relatively more and relatively less ‘bonded’ when we see or experience them: we know that some of our relationships are stronger than others, even though we may not be able to say exactly why … face-to-face interaction is important in maintaining the quality of a relationship…

These two components [observable grooming and undefinable emotional factors] … correspond … to the elements of our dual-process model of bonded relationships in primates:

[1] ‘being close’ seems to reflect the grooming element in primate relationships (the need to be in close physical proximity and the emotional attachment that this engenders) and

[2] ’feeling close’, though undoubtedly a much more nebulous concept, seems to reflect the cognitive component (the sense of being willing to do anything for the partner – including, presumably, coming to their aid, or of having that sense of trust and obligation that is so essential in making social alliances work).

… Although the cognition that underpins this sense of ‘feeling close’ is far from clear, one thing that comparative and developmental psychologists agree on is that it involves some form of ‘social cognition’.

Our best guess as to what this entails is what has become known as ‘theory of mind’, mindreading or mentalizing. Theory of mind gets its name from the fact that it defines a state in which individuals ‘have a theory of mind’ – that is, they have an informal theory or belief about what having a mind is like. Practically speaking, this means that they recognize that other individuals have minds like their own. Formally, mentalizing involves the capacity to use words like believe, suppose, imagine, want, understand, think and intend. Philosophers of mind refer to these kinds of words by the general term intentionality, meaning the capacity to take an intentional stance or view. …

… In one sense, it doesn’t actually matter that we don’t really understand what mentalizing is in cognitive terms, the fact is that it provides a simple, reliable linear scale of social cognitive complexity. …

[1] first order intentional: monkeys understand the contents of their own mind, and are aware that they have beliefs about the world.

[2] There is some experimental evidence to suggest that great apes [chimpanzees] can just about cope with second order intentionality (formal theory of mind)

[3] … normal human adults are fifth order intentional …

Primate social evolution …

Reconstructing primate social evolution has, until very recently, largely been a speculative venture.

Everyone pretty much agrees that the ancestral primates were small and nocturnal, and lived in dispersed semi-solitary societies in which females (and their young) foraged in small individual home ranges with minimal overlap, while males lived in large territories (defended against other males) that overlapped the ranges of several females (to whom they then had exclusive access for mating). …

… [Recent] analyses suggest that by far the most likely change from the ancestral state of dispersed, individual territories and solitary foraging was directly into a multimale/multifemale form of sociality, and not into monogamy.

In other words, animals that had previously ranged alone began to gather together in groups, presumably in response to the increasing threat of predation (mainly associated with the switch from a nocturnal to a diurnal lifestyle).

From this initial multimale/multifemale state, there were two possible exits, one to harem forms of sociality and the other into monogamy. There was also a second route into monogamy from harem-based systems.

Species never returned to a semi-solitary state once they had formed groups, but did switch back and forth between the harem and multimale states …

The really important finding, however, is that there is no route out of monogamy: once a species has opted for this state, it seems that it can never escape from it. In effect, it appears that monogamy is a kind of demographic and cognitive sink, probably because the cognitive demands of monogamous partnerships are so great that once the brain has been rewired to accommodate them, this cannot easily be undone. Monogamy requires the male and female to be very tolerant of each other, but at the same time very intolerant of all other members of their own sex.

For this reason, monogamous primates always end up as territorial, with each monogamous pair occupying its own exclusive territory. This intolerance of same-sex individuals is quite unusual in mammals outside of monogamous species and makes it very difficult for several individuals of the same sex to live together, especially once they become reproductively active at puberty. Thus … full-blown obligate monogamy (where every individual is monogamous all the time) appears to be a very special evolutionary state that requires major changes in behaviour and cognition. …

… Over the years three different explanations have been offered for monogamy in mammals. These are: (1) the need for biparental care (two parents are needed to raise big-brained offspring), (2) mate-guarding by males (when females are so widely dispersed that a male cannot defend more than one at a time, he sticks with one female in order to ensure that he at least fertilizes her when she becomes receptive, and at the same time prevents other males from getting a look-in), and (3) infanticide risk (a female locks on to a male in order to use the male as a ‘hired gun’ or bodyguard to defend her against other males that might harass her and/or kill her offspring). …

… The fundamental point is that as social group size increases, females will face increasing stresses and males will be forced into competition with each other. If they cannot find solutions that defuse these constraints, they will not be able to occupy new kinds of habitats or evolve into new species with larger brains. The inevitable result will be extinction when climate change results in dramatic reduction in its favoured habitats. Since we know that they did survive, we can be certain that they must have solved these problems somehow.

This brings us to our two fundamental principles that underpin the story of human evolution: brain size and time budgets. Brain size determines social group size as a response to environmental conditions, and group size and environmental conditions between them impose demands on time budgets that must be satisfied if a new evolutionary step change is to be possible. …

3. Essential Framework

… Pairbonded individuals have to be able to factor their partner’s interests into their decisions on what to do; they need to be able to negotiate between their respective needs to find some appropriate compromise – in effect, a primitive form of mentalizing, even if it is not full-blown theory of mind.

Interpolating the neocortex ratio for modern humans into the ape equation gives a predicted group size of approximately 150. Do humans really live in such small communities? It seems that they do. One answer comes from looking at census data for hunter-gatherer societies. These societies reflect the kind of small-scale social organization that we have lived in for most of our evolutionary history. They are thus an especially appropriate place to test this prediction, given that human brain size hasn’t changed much in the last 200,000 years. … Much as most primates do, hunter-gatherers live in a multi-level form of social organization composed of several inclusive layers.

In hunter-gatherers, these consist of families, camp groups (or bands), communities (or clans), endogamous communities (or mega-bands), and ethnolinguistic units (or tribes). …

… only one of these grouping levels has a typical size of 150, namely the layer I identify as the community. In most hunter-gatherer societies, this layer is associated with a group of people who own rights of access to territory or to special resources like permanent waterholes or sacred sites; they also meet together for events like rites of passage, usually on an annual basis. …

When a community isn’t quite a community

… Despite the fact that the social brain relationship makes a very specific prediction for the natural grouping size of humans [around 150] and that there is considerable evidence to confirm this, mere evidence has not prevented debate as to what the ‘true’ community size really is for modern humans.

Archaeologists typically point to the fact that the only kinds of communities they can see in the archaeological record are overnight camps, and there has been a long tradition in social anthropology of identifying these camp groups (or bands) as the fundamental social unit for contemporary hunter-gatherers. These camp groups typically vary from 30 to 50 in size, depending on the local environment and the economy of the people …

… if the archaeologists are right and the band of 50 is the basic human community, then there has been no real social evolution since we parted company from the chimpanzees (who have an average community size of 55).

And that means there is nothing to explain …

… In fact, the resolution of this apparent dilemma is very straightforward. The ethnographic evidence tells us that hunter-gatherer camp groups (bands) are not, in fact, the fundamental units of human social organization, because they are actually quite unstable: their membership changes over time on a scale of months as individuals or families decide to join or leave.

The important point is that when a family does join another camp group, it invariably joins one whose members belong to the same 150-member community (bonded community or clan) …

… Our research over the last decade points to a striking division at the outer edge of the 150 layer: those within are people we know as individuals, based on relationships that have historical depth and involve trust, obligation and reciprocity; they are the people that we don’t really think twice about helping when they ask. In contrast, our relationships with those in the layers beyond 150 are more casual, unreciprocated and often lack history; we are demonstrably less generous to people in this outer layer.

… The 150 layer is, as we shall see [later, separate post] is important for another, explicitly social reason: it demarcates the limits within which we recognize kinship (no human culture has formal kinship terms for anyone that inhabits the layers beyond 150). Since this is the layer that corresponds to primate social groupings in the social brain relationship, it is the size of this layer that we need to worry about. …

Structural complexity in primate social systems

… large primate groups are in fact highly substructured, rather in the way that human communities are. It is probably this structuring (created by the fact that individuals focus their attention increasingly on their core allies as grooming partners) that allows primates to live in larger groups because … it is these coalitions that buffer them against the costs of living in large groups. At the same time, this structuring creates social complexity, thereby imposing the cognitive demand that is so characteristic of the anthropoid primates.

It is obvious … that hunter-gatherer communities are one layer in a hierarchically nested series of levels of social organization. In this respect, hunter-gatherer societies are typical of all human societies (we all live in nested social layers of this kind) and, as it turns out, most monkeys and apes. …

… One obvious hypothesis is that hominin social evolution consisted of progressively extending out beyond the core chimpanzee community of 50 to add, successively, the 150, 500 and 1,500 layers.

We still have no idea why the scaling factor should be almost exactly three, however – even though we find exactly the same scaling ratio in primates, elephants and orcas …

It seems likely that the difference between species with more and less complex social systems lies in the number of layers they have rather than the sizes of the layers themselves.

Humans, for example, have six layers, while chimpanzees … have only three … The capacity to maintain these multi-level social systems may well depend on a species having sufficiently developed social cognition to be able to manage several grouping levels at the same time, and hence a large enough brain to support the mentalizing capacities required to do this …

… This structuring really reflects the extent to which an individual interacts with (grooms, in the case of monkeys and apes) the other members of its group.

In our case, something like 40 per cent of our total social effort – our social capital, if you will – is devoted to the five people in our innermost circle of relationships … and about 60 per cent is devoted to the 15 people in the two innermost layers, with the remaining 40 per cent being divided between the remaining 135 people in the outer two layers. These frequencies of interaction correspond rather closely to levels of emotional closeness [MGH: which, as stated above, is undefinable] …

… In the sociological literature, these are often referred to as strong and weak ties … Notice that the sociological literature only distinguishes between two types of relationship, whereas [our analysis] clearly suggests that there are four. …

[Insert from diagram caption: The circles of acquaintanceship. Our social networks consist of a series of hierarchically inclusive layers, with each layer being three times bigger than the layer immediately inside it. Beyond the layer of 150 that makes up our personal social network lie at least two further layers, one at 500 (acquaintances) and a second at 1,500 (the number of faces we can put names to).] …

Why time is so important

The time budget models … begin from the observation that an animal can survive in a given habitat only if it meets its energy and nutrient requirements and ensures the cohesion of its social group.

Nutrient needs are met by allocating time to foraging (which includes both feeding and travel), and social cohesion is satisfied by allocating time to whatever activities enable this – in the case of primates, social grooming.

This leaves just one other major activity category to worry about, namely resting time. This is not rest in the sense of having nothing better to do, but time that the animal is forced to spend inactive either to avoid heat overload during the middle of the day or … to allow digestion to take place. …

… the fact that we only need to know three climate variables to be able to predict time budget components for each of these groups of primates. These are rainfall, temperature and some index of seasonality ….

… a simple rubric for determining how much time animals ought to allocate to grooming if they want to live in groups of a certain size. If group size starts to exceed the time the animals have available for grooming, given the other demands on their time, then it means that they will be unable to groom everyone they need to groom frequently enough to keep the bond between them at the right strength. As a result, the group will fragment, and eventually break up into two or more smaller groups that go their own way.

Notice, by the way, that the time budget model does not specify the size of group a species lives in in a particular habitat; it specifies the maximum group size that it can cope with in that habitat. It is not obliged to live in groups of that size (given the costs of doing so), but it definitely cannot live in groups that are larger. …

[1] … First, the key constraint on great ape biogeographic distribution is travel (or moving) time …

[2] Second, chimpanzees are only with us today because they can exploit a form of fission–fusion sociality to offset the high costs of travel that they face. Fission–fusion social systems are ones in which the community can split up into smaller foraging parties during the day. This allows the animals to reduce the time demand for travel drastically. Were chimpanzee communities forced to forage as a single large group, they would run out of time almost everywhere that they currently live.

[3] Third … dietary plasticity to cope with rising temperatures …

[4] … Fourth, a mapping of chimpanzee distributions onto those for their major predators (the lion and leopard) demonstrates that their current biogeographic distribution is limited by predation: they can cope with one or other of the African big cats, but not both at the same time. The presence of both cats prevents chimpanzees from occupying large areas… that, in terms of time budgets, they should have no trouble living in. This should remind us that predation risk is important even for larger-bodied primate species, and is thus likely to be relevant to early hominins.

I raise these points here to emphasize how crucial time is for animals. Pushed by local climate to spend more time on one activity, animals can easily find themselves having to devote more time than they actually have in the waking day to the essential activities of survival. If they cannot effect savings somewhere, they simply won’t be able to survive in that habitat. These are not issues that can be lightly dismissed: they are central to the whole question of whether a species survives or goes extinct.

[You have now reached the end of this Social Science Files exhibit.]

My Proposal [What I am doing here]

Michael G. Heller

April 19, 2023

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The Source of today’s exhibit has been:

Robin Dunbar, Human Evolution, Penguin 2014

Social Science Files displays multidisciplinary writings on a great variety of topics relating to evolutions of social order from the earliest humans to the present day and future machine age.

‘The Heller Files’, quality tools and creative impressions for Social Science.

Clifford Possum Tjapaltjarri, Bushfire Dreaming, Date: 2000