Has our brain grown too big to think effectively?

A variant of microcephalin, MCPH1 gene, was introgressed about 37,000 years ago into Homo sapiens genetic pool from an archaic (Homo erectus) lineage and rose to exceptionally high frequency of around 70 percent worldwide today. It is involved in regulating neuroblast proliferation and its changes alter the rate of division and/or differentiation of neuroblasts during the neurogenic phase of embriogenesis, which could alter the size and structure of the resulting brain. At the time of introgression, images had already been painted on the walls of caves and speech has been in use for over 100,000 years, as had been abstract thinking. Like today, reasoning and thinking were the primary faculties of individuals. Homo erectus either did not possess those faculties or was markedly inferior to Homo sapiens in them. Its brain was smaller and the cortex was apparently less convoluted. Thus, introgressed microcephalin allele directed neurogenesis evolutionary back to less complicated brain structure typical for our evolutionary forefathers, slightly decreasing the level of complexity already achieved by Homo sapiens 37,000 years ago. Despite that, it proliferated at a rapid pace. It yields a supposition: 37,000 years ago the brains of Homo sapiens were too big and too complicated for the kind of thinking needed for the highest fitness of individuals. Since adaptation cannot by definition surpass selection requirements, the volume and complication of the human brain did not originate under selective pressure to improve effective thinking and they cannot be explained in terms of such selection. A proposal to solve this quandary is presented, claiming that Homo sapiens originated just by chance. Endurance running led to the emergence of Homo sapiens. The human mind and larynx used for speech are side-effects of more than a million years of endurance running by pre-human hunters.