Question

what is the conclusion of the evolution of the brain of vertebrates?

Answer 1

Some conclusions:

Brain size is not, in and of itself, a particularly interesting measure, but ...

Brain size is lawfully related to body size. "Why?" is an interesting question.

Across species, brain size relative to body size seems to be related to a poorly defined but intuitively meaningful concept of "behavioral complexity". The sigificance of the intuitive concept and of the correlation are both interesting questions.

Brain size (and relative brain size) has increased during evolution in a variety of different lineages.

In many cases, these increases are increases in the range of the size distribution rather than involving replacement of smaller brains by larger ones.

Larger (both absolute and relative) brains are not , in general, "better" than "smaller" brains in any meaningful biological sense.

Evolution likes to "play", and this "playfulness" probably accounts to some extent for the "directionality" apparent in the evolutionary record. Neither bigger (nor more complex) necessarily means "better".

Brain size has dramatically increased in the human lineage, apparently with a general trend of larger-brained organisms replacing smaller-brained ones. Is this more "playfulness" or is it "selected for"?

Brain size increase in the human lineage seems to represent a continuation and acceleration of a vertebrate trend toward increased relative size of a particular subsystem of the brain: the neocortex and related regions.

The neocortex may be particularly significant in "playing", i.e.in both generating and dealing with ambiguity/unpredictability.

Increases in playfulness may create a selection pressure for neural systems better able to create/deal with playfulness, creating an evolutionary positive feedback loop.

Positive feedback loops provide another way of accounting for directionality in evolution. In this case, the selection pressure is generated by evolutionary change itself, and "adaptedness" is entirely context dependent.

"Behavioral complexity" may relate to the extent to which the brain functions as an "amplifier of evolution".