Brain, and inside it, neocórtex, is a extremely expensive to build and maintain organ that needs lots of energy. It must be a convincing justification if we can rewrite the evolution from the apes it terms of brain size and sophistication increments. A gradual amelioration of the quality of the diet that causes homo sapiens can allow it? The global size of the human body has increased and the proportion must be preserved? Dunbar wanted to demonstrate in his article that these thesis are against the common sense. The brain has evolved not because we can afford it, but because a clear evolution pressure; a reason because people with more brain development has more chances to survive, in other words ‘Large brains will evolve only when the selection factor in their favour is sufficient to overcome the steep cost gradient’.
Dumbar tried two hypotheses to correlate with brain evolution: the ecological one (you need to have better brain if you want to find fruit instead of eat leaves, easier to find but poorest as nourishment) and the social one (a complex brain is a requirement to cope with complexness of social and cultural relationships and rules). To evaluate which among these propositions has more weight to explain brain evolution, he verify the correlation of these two factors (measured as the percentage of fruit in the diet or the average size of the group) with brain size in several kinds of apes, simians and prosimians. There were no relationship with the fruit, but a clear one with group size, so the conclusion was obvious: the brain has evolved to allow more social interaction. (In fact, it seems widely demonstrate that there are several factors acting in parallel, giving themselves mutual feedback, without being strictly cause or effect as the increment of the brain efficacy, some diet changes, phonation organs changes, to became bipeds, habitat changes from jungle to savannah…)
Dunbar calculate the theoretical group size that would correspond to the homo sapiens following the correlation brain evolution – average group size. The number, more or less 150, is more an anecdote than a result, but curiously Dunbar article is much more known because this magic number rather than because of the key point it states: that it exists a relationship among social complexity and brain development. There are several details the magic number evangelists insist on ignore. First, the enormous simplification that implies to use the group size to evaluate social complexity (Dumbar apologize to use this measure and justify it by telling that it is the only available for a lot of species studied)… what about the group size of sardines? In fact sometimes those sizes are more influenced by environmental pressures or behaviour particularities than because of the neocórtex size. Regarding brain evolution, the same: to create graphics sometimes you ought to use something easy to measure rather than significant, as the ratio neocórtex-medulla dimension. Using measures being more verisimilar than precise we can not hope to get exact results of extrapolations but to figure out indications or tendencies: I can conclude that the cerebral sophistication is influenced by group complexity, but it is quite ridiculous to calculate as the main result of the research the exact social size that would correspond to an average brain size.
By the way. Chimps, papio, macaco and Gorilla are the champions in the neocórtex ratio according to the data (exclude humans) of Dunbar article, even if Gorilla is not very social…
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