The brain has evolved as a very exceptional device that takes in information and tries to optimize prediction of the near, and far, future. Your brain must reduce prediction errors; hence reduce surprise to increase its ability to survive in the world (at least this is one theory of the brain).
However, just like any computing device it costs energy to perform these numerous calculations. And unlike modern computers you cannot keep on adding processors (multi-processors computing) and simply pour more energy into whatever problem one is calculating.
For a typical animal there is a limit of how many calories they can consume and there is an energy cost in the pursuit of food. Even in us modern humans, in the times of abundance we can only efficiently consume so many calories per day (over-consumption in our current environment comes at the cost of general health). In the past if we happened to live in an environment and time of abundance over-consumption would come at the high cost of lower mobility and hence increased likelihood of dying from predators. Therefore, in animals, unlike computers, there are fairly strict energy limits that one can use to think/plan. So like every other economic situation there is a balance, in this case between energy consumption (there is a real world limit to this and it cost energy) and information processing capability.
The human brain had to evolve to maximize information processing per energy input (e.g. calories). Non-primate mammal’s brain consume 4-5% of the bodies total energy intake, while the non-human primate’s brain use 8-10%. However, the human brain consumes 20-25% of the total energy intake (Leonard et. al., 2007). Interestingly, a human brain only accounts for 2% of an adult’s total mass but it consumes 20% of the oxygen and 25% of the glucose (Our gluttonous brain). This suggests that for humans the high energy cost of a large percentage of incoming resources going toward the brain was an advantage in terms of higher level of thinking/planning. Our primary tool is our big but expensive brain.
Obviously there has been great evolution pressure for humans to increase their brain efficiency compared to other species since our niche was the ‘thinking/planning’ ape. Interestingly, researchers think there might have been a cost to these adaptations including some mental illnesses such as schizophrenia.
Bottom line: the human brain high functional ability comes at a cost. Our brain consumes a huge amount of energy compared to its size and must do this to maximize survival by being the best predictor of future outcomes (also see above regarding possible link with mental diseases).
One could break it down to our brain and hence our survival comes down to energy and information. What is the maximal amount of information can we process at a given energy cost. A cold and sobering thought.
see my post on how to improve the brain’s efficiency.