The fact that is sticking in my mind after reading Baboon Metaphysics is that human brains continuously use as much energy as the pumping leg muscles of a runner during a marathon. The authors go on to make the point that natural selection does not evolve energy-guzzling organs without an equivalent return on that investment. (Bodies are good models for efficient, entropy-slowing machines.)
The main thesis of the book is that baboon brains (and human brains) are naturally selected to do one important task very well: monitor and respond to social interactions. This skill is contrasted with some bird brains that have evolved primarily to hide and locate seeds. A male baboon’s fitness is dependent on his ability to know his social rank and compete with other males in order to climb in rank, and form bonds with females that will result in the protection of his offspring.
The authors go on to make the hypothesis that human language evolved on the foundation of ape social intelligence. The precursors for words are in simian calls. The precursors for grammar are located in the proven ability of apes to organize sounds (alarm calls or predator growls) into subject, verb, modifier sequences. What apes appear to lack is a “theory of mind” wherein they can attribute mental states to others, or sense the intent of others. They do not feel the need to “explain or elaborate upon their thoughts” rendering them “largely incapable of inventing new words or recognizing when thoughts should be articulated” (265). Five to seven million years ago, humans split from monkeys and inhabited their own branch of the evolutionary tree. At some point, we developed a “theory of mind” that took our ancestral social intelligence skills to the next level. This in turn allowed us to take advantage of language adaptations like the development of vocal cords to put distinct words to all the new thoughts we were having.
Anyway, brain size measured by something called the index of cranial capacity (ICC). This is a ratio of brain volume and body size. The bigger the body, the larger the brain. But other factors also seem to influence brain size. Animals that have long life spans plus a lover period of juvenile development have bigger brains than animals that don’t. Group size also plays a factor. Animals that live in large groups have to keep track of more individuals and relationships, necessitating a bigger brain. Baboon ICC is 7.3, whereas chimpanzee ICC is 8.2. Chimps live in larger groups than Baboons.
The book is an easy read and reflects cutting-edge research.