Chapter 2: Dunbar, R.I.M Brain and cognition in evolutionary perspective (pp. 21-46)

Social brain hypothesis: Argues that the evolution of the human brain is largely due to social cognitive factors – in other words, human brains evolved as social brains.Brains are metabolically expensive tissue; largely owing to the difficulty in creating and replenishing neurotransmitter supplies. Evolving a large brain must mean that brain size offered an important fitness advantage, otherwise its hard to see why it would happen.

Social Brain Hypothesis• Jerison’s encephalization quotient (EQ): how much brain is left over after

we scale its size against body size? • EQ for humans is around 7, for chimpanzees it is around 3-4. A

disproportionate increase in brain size in humans is attributable to increases in the frontal lobe. Controversy over whether human frontal lobes are larger than would be expected for a primate of our size. Dunbar says yes.

Social Brain Hypothesis• Evo-devo of brain evolution: simple process – extend

the ontogenetic time period for neurogenesis. In other words, delay the “stop making (precursor) brain cells” genetic signal. May help explain long developmental period for humans.

• Selection for large brains• Ecological vs. Social brain hypotheses• Ecological: increased demands of hunting meat or

selecting ripe fruit• Social: increased demands of living in complex social

systems.

Social Brain Hypothesis

• Dunbar argues that social brain hypo more strongly supported since most of primate brain expansion is in the neocortex which is where social computational processes would be taking place as opposed to sub-cortical structures where more ecological processing such as cognitive mapping would be taking place.

• Reader and LaLand (2002) hypo that expanded neo cortex is for more sophisticated social learning. Neocortex size and innovation related in primates.

Social Brain Hypothesis• Critical social cognition process: Theory of

Mind• Transition to second-order intentionality –

4 to 5 years of age; I know that you know that….

TOM in other species• Dolphins appear not to have TOM • Monkeys and nonprimates probably not• Great apes, some capacity, may understand

intentional vs. accidental actions, may understand goal and desires, but not evidence of understanding beliefs, theories; more complex mental states.

Social Brain Hypothesis• Some evidence that mentalizing tasks are computationally

demanding. • Possibility that spindle cells (unique to great apes) may be

important for social computation. High density in ACC; Connects ACC (working memory; self regulation) with amygdala (emotional processing). Spindle cell development tracks self-control development in children.

Social Brain Hypothesis• Total brain volume and TOM; as brain grows larger more diverse

processing units can be integrated and directed to one task. Summary:• Great ape social groups appear to be more complex than Old world

monkey groups (even though OWM groups can often be just as larger if not larger in number)

• Important transition in brain evolution appears to have taken place after split of OWM (23mybp) but before the branching off of the lesser apes (16mybp).

• Possibility that this had to do with a genetic mutation that allowed for faster neuronal transmission times.

• Possibility of fission-fusion social system being significant. Apes must disperse more widely because they cannot processes unripe fruits.