Dunbar is an anthropologist, specialist in primate behaviour and evolutionary psychologist, a polymath.
Relying on on his original research, although described as an Introduction, this is not an easy read. You can sense Dunbar thinking out the questions and possible answers when considering human and hominin evolution against the theories he developed and tested.
- Time budget models developed for chimps are compared to the possible life patterns of successive hominin species. How much time was required for food gathering, feeding and resting, leaving sufficient time for social grooming?
Australopithecines were bipedal, but this developed with Homo ergaster who were taller, striding, even jogging, and more nomadic. They evolved at a time of falling temperatures perhaps by two degrees in tropical Africa, which would have meant the need for less resting time.
Wrangham suggested that cooking was adopted by Homo ergaster/erectus as early as 1.8m years before present. Dunbar disagrees, as there is little evidence for hearths before 400,000 years ago. The use of fire produces light reducing effective night time, which would have significantly expanded time budgets.
- The social brain hypothesis indicates that a larger brain correlates directly with social group size, an approximation for social complexity, where there are more individuals to keep track of than in small groups.
There are clear reasons for group sizes to increase including defence against predation and raiding. It is argued therefore that group sizes increased leading to greater social complexity. Dunbar disagrees. In his model an increase in brain size provided the ability to cope with greater social complexity allowing an increase in group size. An initial question therefore becomes, at different stages in human evolution, “why did brain size increase?”
The hypothesis allows group size to be predicted from brain size, specifically the neocortex ratio (that relative to the rest of the brain.) Hence Dunbar’s famous number for modern human groups of 150.
Archaeologists have queried this, pointing out evidence in small hunter gatherer & horticultural societies for bands varying in size between between 30 and 50 individuals. Dunbar counters that ethnologists show membership of bands changes within months, as individuals and families join or leave. When a family joins it is always within the 150 member clan. He argues the rapid creation of dialects, even between generations, indicates language evolved to bind small exclusive communities.
Group organisation is structured by approximately threefold increases in the individuals in each layer, so those involved in successive layers are approximately 5-15-50-150-500-1500. Differences in group size and complexity between species derive, not from the size of the layers, but the number of layers. In hunter-gatherer societies groups at the 500 and 1500 levels provide information, resources and defence against raiding.
Neanderthals had large brains, but living in higher latitudes during the Pleistocene, had greater need for enhanced vision, so their eye sockets were 20% bigger than anatomically modern humans. A greater part of their brains would have been allocated to the visual system. Once this is adjusted for, Neanderthals are predicted to have had smaller group size, 2/3rds those of modern humans. This suggests Neanderthals may have died out because in smaller groups they would have been more susceptible to disease. Furthermore, if modern humans had already developed differently layered groups, then their larger groups could have swamped smaller and more isolated Neanderthal bands.
Dunbar sees the Neolithic as a settlement rather than agricultural revolution. Records indicate that humans dying from violence are constant at around 15% of surviving remains. In the Natufian a sedentary or semi-sedentary population was established prior to adoption of agriculture. Living in larger settlements resulted in deteriorating health and increased social stress. You would need good reasons to choose such a life. Dunbar suggests it was the need for defence against raiding.
He also suggests the Neolithic was a religious revolution moving from shamanistic to doctrinal religion. The latter took place in special buildings used for ritual, involving a priestly class interceding on the community’s behalf. There is evidence specialists appear once community size exceeds 500 individuals, with the emergence of craftsmen, mercenaries, priests and administrators.
- Social grooming As group size increase, so do the stresses between group members. Primates manage this by physical grooming which takes time, producing endorphins for the groomed animal. In larger groups, grooming is restricted to cliques.
A number of different means of social grooming allowed increased group size, improving time budgets compared to the chimpanzees’ base line time model.
Homo ergaster/erectus’s posture meant they could laugh. Tests shows laughter produces endorphins for three individuals compared to the single individual being physically groomed, so improving time budgets.
Musicality and dance may have evolved in advance of language. Both resulted in the production of endorphins. With language so did shamanistic religion and feasting. Storytelling around the fire provided social grooming and non-physical bonding. Early Neolithic cereals are more useful for brewing beer than baking bread, drinking beer again producing endorphins.
- Theory of the Mind Intentionality is working out what you and others intend. It is mentally demanding. 2nd level intentionality is achieved (just) by orangutans & chimps. Most humans achieve five levels of intentionality, which correlates with mental capacity. Levels of intentionality correspond with grammatical complexity in language. Laughter was changed for ever after language developed, as jokes almost always involve intentionality.
- Mating Ancestral primates were small and nocturnal. Females and their young foraged in small areas which overlapped with the larger ranges occupied by males. Development from there was to multi-male /multi-female groups. Species never reverted to a semi-solitary state once they formed groups, which switched back and forth between harem and multi-male states.
Monogamy could develop from either harem or multi-male groups but, once established, did not revert. Monogamy requires male-female tolerance, but intolerance of the same sex. Monogamous primates exist in exclusive territories. Generally monogamous creatures have larger brains than species which mate polygamously or promiscuously, as pair bonding is cognitively more demanding than casual relationships. However in primates and possibly some other mammals, particularly elephants & horses, social brain theory implies larger brains resulted in increased group size, whilst monogamous primates, for instance gibbons, have smaller brains .
Dunbar suggests monogamy was a response to infanticide with bi-parental care then developing. He also thinks protection against infanticide was critical for primates with females moving into harems under the protection of a dominant male. Sexual dimorphism and an increase in the ratio between the second and fourth finger is associated with either polygyny or promiscuity indicative of high levels of testosterone on conception. This continued into hominins, indicted by their relative sexual dimorphism.
Anatomically modern humans are only moderately dimorphic with men 8% taller and 20% heavier than women. Modern human behaviour similarly divides between monogamy and polygyny or promiscuity. Males split roughly 45:55 between monogamous and promiscuous phenotypes, with women showing a similar, but reversed, split.
Not an easy book. However I have already started to re-read it, its ideas influencing my thinking.