How the expensive brain hypothesis explains your big head.
For an organ composing 2% of your total body weight, the brain is one metabolically expensive organ. How expensive? The brain consumes about 20% of someone’s caloric intake. Meaning, someone needing 1800 calories daily allocates 360 calories for the squishy-thing.
Can we blame [brain] it?
Brains are big... size matters and that’s just university and evolution.
Evolutionary trade-offs exist. Kind of like,
“The bigger you are, the harder you fall”.
Except fall gets replaced with evolve. So,
“The bigger you are, the harder you evolve.”
Sounds cool, right? Well, I came up with it.
My favorite example being the aging hypothesis. Why do we age?
The hypothesis to “why” ranges to many factors, one being because of Natural Selection rewards organisms who are the most adapted to survive in a given environment. Most environments on Earth require that an animal grow and evolve the quickest, and oftentimes, the biggest of these animals compete with other suitors and protect their mates/offspring a lot better.
Some famous examples from the HMS Beagle: (that’s the ship our boy Charles Darwin be sailing on back in 1839).
- Big jaws = not so roomy for a brain = smaller cranium.
- Island full of termites = narrower beaks = all you can eat buffet.
- Good swimmer = poor runner = don’t get caught out of water or you’re bird-food during hatching season.
- Beautiful feathers = less aerodynamically efficient = you’re a useless showboating peacock.
Everyone knows that there’s evolutionary trade-offs. For Homo sapiens (humans), we are apart of the Great Ape family (Hominidae), which falls under the scientific classification system known as Primates, the trade-off was a bigger brain.
To be the best, you have to think you’re the best.
Primates similar in size to humans share similar basal metabolic rates (BMR)s. Primates, like the great apes, all burn about the same amount of calories as a human of the same weight.
Turns out, it’s more confusing than that. A paradox exists in physiology and neurology called The Expensive Tissue Hypothesis. Besides the Great Apes, not a lot of other mammals can stomach that news. Literally.
That paradox is that gastrointestinal tract size is inversely proportional to brain size. English: the bigger the brain gets, the smaller the gut gets. This trend can be found among bigger-brained animals. More specifically this trend is found the best in Great Apes. Humans being the most gifted with their bigness by having brains approximately 3 times the size of Chimpanzees.
Burn the same amount of calories, have a bigger brain, have a smaller gut. This might explain why many Anthropologists and Biologists argue that the advent of fire i.e. food-cooking allowed us to evolve so quickly. Most foods, when cooked, are better absorbed than raw foods.
An organism has to find an optimal balance between BMR and size. That’s because of many reasons which I’ll get into later. We forget it in this age but storing energy was once a very important part to our survival. It’s easier to eat 1000 calories than it is to metabolize (about a 9.5 mile run for a person weighing 100 pounds). Balancing BMR and physiology isn’t a matter of looking good for beach season, for many animals on this planet, it’s a matter of life and death.
Cancer, the most extreme case of BMR gone array.
Cancer is defined as:
a disease caused by an uncontrolled division of abnormal cells in a part of the body.
We see people with advance stages of cancer being very emaciated. This is not because of the treatments or therapies, although they play a small role in appetite. This is because cancer cells that divide uncontrollably require calories for energy. Nothing tells them to stop. Eventually, the population of cancer cells is so large that it will starve an organism no matter how much that organism eats to replaced the metabolized calories.
There’s so much more that can be written about this subject, but science is learned best in bite-sized chunks. I’m leaving an article here for further elucidation.
Articles and References:
Pontzer H, Raichlen DA, Gordon AD, Schroepfer-Walker KK, Hare B, O’Neill MC, Muldoon KM, Dunsworth HM, Wood BM, Isler K, Burkart J, Irwin M, Shumaker RW, Lonsdorf EV, Ross SR. Primate energy expenditure and life history. Proc Natl Acad Sci U S A. 2014 Jan 28;111(4):1433–7. doi: 10.1073/pnas.1316940111. Epub 2014 Jan 13. PubMed PMID: 24474770; PubMed Central PMCID: PMC3910615.
URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3910615/
J Hum Evol. 2009 Oct;57(4):392–400. doi: 10.1016/j.jhevol.2009.04.009. Epub 2009 Sep 3.
