Human Brain Complexity Restrained by Metabolism

From proteins all the way to entire organs, structure nearly always informs function. For example, our hearts are structured in such a way that pumping deoxygenated blood to the lungs, and pumping oxygenated blood systemically are favorable actions. Furthermore, heart size corresponds to body size (2), so that blood can be pumped appropriately. However, there are caveats to the structure dictates function rule.

So what is an exception to the rule? There is new research suggesting that the density of neurons in the human brain is not necessarily ideal for functionality. Some researchers have postulated that increased numbers of neurons would explain the complexity of the human brain, and therefore humans should have extremely dense brains – evidenced by many neurons and numerous synapses (1). As humans, we do have a lot of neurons; however, the number of neurons in a mammalian brain seems to be a product of metabolic constraint (1).

For centuries, neuroscientists have perpetuated this mythical estimate of 100 billion neurons in the human brain – although, no one actually knew where that number came from, or if it mattered relative to other species. Recently, these numbers were quantified as 86 billion neurons in a human brain compared to 33 billion neurons in a gorilla brain (1). If gorillas are larger than humans why is there such a discrepancy? The primate brains require 20% of the daily energy budget; every day each one billion neurons require 6 kcal of energy (1). Consequently, it is costly to have a lot of neurons. One of the differences between us and gorillas is that humans aren’t regularly eating raw foods. Raw foods are generally lower-calorie foods and they require a great deal of energy to find, if you’re a gorilla (1). As humans, we can cook food which means integrating higher energy foods into the diet quickly, with little energy expenditure (1).  In some sense, evolution may have influenced the “structure dictates function” rule. Whereby the “function” of energy metabolism has limited the structure, or the number of neurons we are able to have.

1. Herculano-Houzel S. The remarkable, yet not extraordinary, human brain as

a scaled-up primate brain and its associated cost. PNAs. 2012. 126: 10661-10668.

2. Seo JS, Lee SY, Won KJ, Kim DJ, Sohn DS, Yang KM, Cho SH, Park JD, Lee KH, Kim HD. Relationship between normal heart size and body indices in Korean. J Korean Med Sci. 2000 Dec;15(6):641-655.