The brain was already perfect, why should it keep growing?
Is the answer to the question of the end of brain growth really that simple? In the passage already quoted, Jürgen Habermas also overlooks the problem of the end of the growth of the human brain:
Once the growth of the human brain stopped, cultural learning processes began to take the place of genetic adaptation. What other animal species lack is the transfer of symbolically stored knowledge from generation to generation, such that it can be revised and expanded light of new experiences.
If genes cannot induce human adaptation, does culture have to take over? But where does culture come from? Who discovered cultural learning processes?