Notes on Chapter 4: Brains Arrive

This chapter has a number of points that are central to the book:

• The olfactory system is associative instead of being point to point.
• When brains get larger, they do so disproportionately.
• Big brains are expensive.

The Olfactory Brain Regions are Associative
The important thing about associated vs. point to point is that the authors argue that the foundation for the neocortex  already existed long before primates with their larger brains showed up.  One of the primary abilities of the cortex is its ability to associate anything with anything else - something the olfactory system must be able to do in order to associate a particular smell with some attribute.

This makes for a more "natural" transition from the smaller brains to larger human brains.  Nothing special is required to explain how the cortex becomes associative: it evolved from the olfactory regions.

When Brains Become Larger, they do so Disproportionately
Disproportionate growth in brain areas means that, rather than the motor and sensory parts of the brain increasing at the same rate as the cortex, more expansion can go to the higher brain functions instead of the lower ones.  According to the authors, this trend is already apparent with birds and mammals, whose growth patterns are different from those in fish and reptiles.

Another point about brain evolution is that the proportional sizes of different brain regions are more or less constant for different classes of animals.  That is, the brain regions of a larger animal such as a moose, are proportionally the same size as those of a smaller animal such as a mouse.  Because the moose is bigger, it has a larger brain, but the relative sizes are more or less the same.

Large Brains are Expensive
The authors point out additional costs of big brains later on in the book.  For now, the point about the metabolic cost is highlighted.  In looking at the way that humans will keep dairy animals and noting that this gives them access to an extra 100 calories or so, it is clear that having a larger brain that confers no corresponding advantage is serious drawback.

And just to make the cost more onerous, 50% of calories for nervous tissue must come from carbohydrates.  Normally, calories come from starches and sugars, so this is not an issue, but under starvation conditions, these calories come at the expense of cannibalizing lean body tissues.