Brain size and evolution - complexity, "behavioral complexity", and brain size

Evolution of complexity?: "Behavioral complexity", brain size, and the relation between the two

Organisms have indeed gotten more "complex" over evolutionary time, at least on a broad scale

Organisms differ in "behavioral complexity"

Organisms differ in brain size

There is some relation between "behavioral complexity" and brain size, but humans do not have the largest brains.

There is a better relation between "behavioral complexity" and brain size in relation to body size.

from Harry J. Jerison, Paleoneurology and the Evolution of Mind, Scientific American, January, 1976

Why should such a relation exist? Is the slope of the line 2/3 (surface to volume) or 3/4 (metabolic rate)?

Interesting, so my "behavioral complexity" intuitions do have something behind them. But .... this only says that there are interesting differences between brain sizes in different animals. That's at best indirect evidence for changes in brain size over evolutionary time. Is there any direct evidence?
I'm still more interested in whether there is life on Mars?.