Anderson's three reasons for a unitary theory (From Anderson 1983):
Encephalization Hypothesis:
Encephalization is the increase in brain size over the course of
mammalian evolution. The hypothesis is that human mental ability
is simply a result of this trend (Jerison 1973, Passingham 1982).
Our brains are bigger and thus more complex, and that can account
for our greater abilities.
Evidence for the Encephalization Hypothesis: Human brain size is exactly what one would expect if you extrapolate from earlier primate brain expansions. This also hold for many substructures of the brain. There is no gross neuroanatomical evidence for modularity (that would distinguish us from earlier primates).
p10:
A strong assertion would be that humans primarily evolved a
generalized capacity for cultural innovation. In support of this idea
is the fact that archeological data shows that over the course of
history the rate of cultural change increaced. Also, it would explain
our ability for language, higher thinking and representation of the
environment.
Dunbar (1990) suggested that we evolved to be in social groups, and that cultural invention may have been a by-product.
The main difficulty with the cultural evolution proposal is that it is vague about what biological mechanism could be responsible for cultural invention. The link from biology to culture requires cognitive theory.
p11:
Cultural evidence must play a role in any evolutionary theory.
Certain mental functions, like reading and spelling, are not
cross-cultural, and thus are a result of cultural influences.
p13:
So rather than evolving special purpose modules, special purpose
modules may be learned culturally. This is supported by cortical
plasticity. A young brain grows connections indiscriminately
(Changeux 1985). Good brain connections are then selected (an
idea similar to Hebb 1949).
p16:
There were three major evolutionaary changes that made human cognition
what it is: