A little more new nervous development had to evolve in order to support a slightly different mutant morphology. Anyway, as a living thing , plant or animal, changed to be more fit to compete in a particular ecological niche, the brain mechanism had to adapt to the added level of environmental complexity. For example, gill-like slits are noted during the early weeks of human fetal development. And the environment is similiar to that inhabited by one's simpler ancestor - it's aqueous. All animal fetuses during the first days of development look remarkably alike.
Mental ontogeny recapitulates mental phylogeny.
As the stages of evolution unfold as the fetus develops, so do the corresponding stages of mental development. At the age of five days, the thinking devlopment of the fetus may be no different from what that of whatever lower form of life has the comparable mental capacity and limitation. I know that I relate to my house plants in a different manner now, after the adoption of my theory of a mental evolutionary mutuality. I feel that we are of similiar origins and of comparable levels of mental operation when I go into deep sleep. We both, side by side, occupy a similiar space, while both our basic biological needs are taken care of automatically, without deliberate decision making or cognition necessary.