Friday, April 21, 2017

Robin Dunbar's Human Evolution

I just finished Robin Dunbar’s magnificent book Human Evolution: Our Brains and Behavior.  Dunbar presents two central hypotheses (if I understand the argument). One is that there is a robust correlation between the brain size of primate species and the size of the groups that they live in and interact with. 
The causation that is indicated by the correlation is it problematic for animals to live together.  We annoy each other.  Living in close proximity means that we can come into conflict over a wide range of things: food, mates, space, etc.  This annoyance has to be managed. 
One way to manage it is by grooming.  When one baboon grooms another (coming through the fur, looking for juicy insects that carry pathogens, it results in the release of endorphins.  Endorphins play a large part in the book.  Nothing makes it easier to tolerate the presence of another furry conspecific than a warm fuzzy feeling that she produces while she fondles my back. 
It is easier, of course, to tolerate others when the others are closely related.  Kin selection is one of the foundational theories in sociobiology.  If one of my inherited traits is to serve my offspring or my siblings, I am promoting the biological success of individuals who, mostly, inherit the same traits. 
The problem with both solutions to the problem of group living is that they don’t allow for very large groups.  Kin selection works according to Hamilton’s rule.  If the cost of cooperating with someone else is less than my relatedness to the other times the benefit I bestow, then cooperation can be selected for.  The formula can be stated simply: kin sacrifice is selected for whenever C < RB. 
For example, I am foraging with my brother and I see a predator stalking us.  Should I call out a warning?  The answer is no.  I am related to my brother by a factor of point five.  We share fifty percent of the same genes.  If my brother survives, that is a factor of one.  If the tiger nails me because I called out a warning, that is a cost of one: zero chance of future offspring.  1>.5 x 1.  Hamilton’s rule is not satisfied.  Natural selection will not favor this behavior because the genes that code for it will diminish in any population. 
What if I am foraging with seven brothers when I see the cat?  Now the calculation reverses.  My cost is still one if I die and the relatedness is still point five.  But the benefit (saving seven brothers) is seven.  1 < .5 x 7.  If my seven brothers survive and reproduce, I get more of my genes into the next generation than if I have my own offspring.  My nieces and nephews will inherit my familial piety. 
Kin selection is a robust foundation for cooperation and it explains how closely related individuals can work together.  It is limited, however, in its range.  While brothers are related by a factor of point five, cousins are related by a factor of point twelve and a half.  A willingness to take risks on behalf of cousins will need a lot more cousins to make the calculation work.  Kin selection cannot explain the emergence of communities much larger than the clan, let alone communities that include unrelated clans. 
Grooming can explain how unrelated individuals learn to tolerate one another.  It feels good to be groomed by another, regardless of our relationship.  This works wonders for a lot of primate species.  Here, Dunbar deploys a second device: a time-budget model.  There are only so many hour in a day.  Some of these must be devoted to sleeping and resting.  More must be devoted to feeding and moving from one source of food to another.  Some must be devoted to social bonding activities like grooming.  Grooming involves two individuals and so only so much of it can occupy the social bonding segment of the time budget. 
The genius of Human Evolution lies in the use of these two devices‑brain size vs. group size and the time-budget model to map out the emergence of human beings as a branch of the family tree.  Our ancestors came together in groups and the groups came together in larger groups.  This enlargement of the social contract was both a cause and a consequence of the enlargement of our mammalian brains.  Laughter (we bond over jokes), language, alcohol, and religion were the devices by which we solved the problems stated above. 

Why, for instance, did we survive where the Neanderthals did not?  Perhaps because, by the time we encountered one another, we could muster much larger coalitions of cooperative groups than they could.  

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