Thursday, November 22, 2007

Brains are Expensive

A while back, Larry Moran over at the Sandwalk wrote about previous expectations of scientist about the number of genes in the human genome. (See also his discussion of Pennisi 2005) The guesses ranged from 140,000 to 15,000 genes. Reading this I was reminded of a statement from Thompson (2001) which I quoted in a term paper about language evolution: 30,000 to 50,000 genes of the human DNA are present in all cells, but only activated in brain cells. This would be a quite extraordinary feat, given that there seem to be only about 20,488 genes at all in the human genome. (Pennisi 2007). But there are other statements that characterize the ‘hyperastronomical’ dimensions (Quine 1987) of the brain quite well. (Quine used the word when describing the vastness of Borges’ fictional Library of Babel, a universe-sized library which contains books with all possible combinations of characters there are, making it impossible to find even one comprehensible sentence. But as a complete layman, I sure sometimes feel the same when reading an article about neuroscience… or anything else for that matter)

The Brain consists of an incredible 1010 neurons and 1013 synapses (Gegenfurtner 2005) and the number of possible connectional combinations between is sometimes estimated to be greater than the number of molecules in the universe (Ramachandran) no wonder it is sometimes called the most complex structure in the know universe. Further, with its about 1251.8 cubic centimeters (cc) (compared to the 316.7 cc of greater apes) (Rilling 2006),our brain is about three times bigger than that of an ape would be, given the same body-size (Lewin 2005).
Just as remarkable is the amount of energy the brain consumes. Although the human brain weighs only 2% of an adult’s body weight, it accounts for 20-25% of its resting energy intake. As a comparison, a chimpanzee’s brain only consumes about 8-9% of its resting metabolism. Thus, the brain uses energy at the same rate consumed by leg muscles of a marathon runner when running (Attwell & Laughlin 2001. It seems like that one could make a good joke out of this fact, but sadly I can’t really imagine how.) The costs to run a human brain are, per unit mass, about 8 to 10 times higher than those for skeletal muscles. In the energy-consuming business, it is only topped by the heart. (Dunbar & Shultz 2007).

At the moment there doesn’t seem to be enough data to estimate which brain areas consume how much energy, but there are two promising candidates for the title of ‘hungriest brain area’. 50% to 60% of the human cerebral cortex are devoted to the perception and interpretation of visual stimuli and the reactions to it. Given that, perception is an incredibly complicated and complex task, this seems quite understandable: In sum, the brain receives about two gigabyte of data per second from approximately 200 million photo receptors in the eyes (Gegenfurtner 2005). Another aspirant for the title are the auditory areas, where metabolic rates seem to be 40% greater than in other parts of the brain (Attwell & Laughlin 2001).
Brains really are made out of “expensive tissue” (Aiello & Wheeler 1995). Because of this, specialized intelligence is far more likely to be created by natural selection than general intelligence, or a brain that is ‘just large’ due to the sheer cost of evolving and maintaining such a thing. (Cheney & Seyfarth 2007: 11) (Ha! Suck on this Jean Piaget! ;) ) As Robin Dunbar puts it: “Because the cost of maintaining a large brain is so great, it is intrinsically unlikely that large brains will evolve merely because they can. Large brains will evolve only when the selection factor in their favor is sufficient to overcome the steep cost gradient“ (Dunbar 1998: 179). In my next post I will consider some of the proposals why we do have such damn hungry and huge brains.


Aiello L.C. and P. Wheeler 1995. ”The expensive tissue hypothesis: the brain and the digestive system in human and primate evolution.” Current Anthropology 36:199–221

Attwell, David and Simon B. Laughlin. 2001. “An Energy Budget for Signaling in the Grey Matter of the Brain.” Journal of Cerebral Blood Flow and Metabolism 21:1133–1145.

Dunbar, Robin I.M.1998.“The Social Brain Hypothesis” Evolutionary Anthropology 6: 178-190.

Dunbar, R.I.M. and Susanne Shultz. 2007. .“Evolution in the Social Brain” Science 317: 1344-1347

Lewin, Roger. 2005. Human Evolution: An Illustrated Introduction. Fifth Edition. Suffolk: Blackwell.

Quine, W. V. 1987. Quiddities: An Intermittently Philosophical Dictionary. Cambridge, MA: Belknap Press

Pennisis, Elizabeth. 2005. "Why do Humans have so Few Genes?" Science 309: 80.

Pennisis, Elizabeth. 2007. "Working the (Gene Count) Numbers: Finally, a Firm Answer?" Science 316: 1113.

Rilling, James K. 2006. Human and NonHuman Primate Brains: Are They Allometrically Scaled Versions of the Same Design? Evolutionary Anthropology 15: 67-77.

Thompson, Richard F. 2000. The Brain: A Neuroscience Primer. Third Edition. New York: Worth Publishers.

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