Recent research from the University of Sweden demonstrates that a larger brain is expensive for biological entities and it compromises the size of the gut and reproductive ability. Using a study on guppies, they show that the increased cognitive abilities of a large brain comes with significant energy demand. Similarly, In humans,the brain that makes up only 2% of the total mass consumes over 20% of the available energy, starving other organs.
Given this data, it is unclear why evolution will push in the direction of aiding the growth of the most inefficient organ in the body, especially if it substantially affects the ability to reproduce. Large brains have unambiguously pushed humans, other primates, dolphins and whales to smaller family structures, increasing the risk of extinction. Humans, for example, was nearly wiped out with less than a few thousand samples surviving through an incredible bottleneck.
Many have considered natural selection, an effective way to optimize systems. Data show this may not be true. Optimizing locally, allowing certain participants to gain advantage over others in short time horizons have long term deleterious effects on the survival of the species. In the case of humans, the large brain seems to have done further damage as individuals in an attempt at maximizing utility locally and within their life spans have accelerated the destruction of the environment, paving the way to extinction.
Size matters – an evolutionary accident and myopic natural selection processes have aided the development of an inefficient and ineffective organ, that will drive the species to extinction.