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Reference topic 3: The Expensive Organ

Why human brains got so big, and what they need

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.79 sources7 graded recommendationsTopic hub: Food

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On a cool autumn evening in 1997, the primatologist Richard Wrangham was gazing into his fireplace in Cambridge, Massachusetts, turning over the question of what had stimulated human evolution.1 He found himself remembering the food of the wild chimpanzees he studied in western Uganda. He had tasted their fruits, and later described them without affection: "Fibrous, quite bitter. Not a tremendous amount of sugar. Some make your stomach heave."1

"I tend to think about human evolution through the lens of chimps," he said. "What would it take to convert a chimpanzeelike ancestor into a human?"1 That evening the answer was in front of him. "I realized what a ridiculously large difference cooking would make."1

Mismatch card

The system

The brain and the mental habits it runs.

What it expects

Cooked, varied food; sleep; a band of about 30 people inside a wider network; daily physical and mental challenge.

What it gets

Long hours seated (Topic 6), isolation for some, and prices, deadlines and risks its defaults never met.

What happens in the gap

Defaults misfire as worry, the planning fallacy and reluctance to part with what you own; isolation and inactivity are associated with smaller brain structures and higher dementia risk.

The handrail

Sleep, people, brisk walking, hard new skills, and fish rather than supplements. (●●○)

Strength of evidence

●●○ The brain's cost is measured directly; how it was paid for and why it grew are contested, and most health links are observational.

What this topic covers

  • ·The Bill
  • ·Who Paid
  • ·The First Turn
  • ·Why It Grew
  • ·The Toolkit and Its Misfires
  • ·What the Organ Runs On

The practical version is in the book: Chapter 5, Eat.

The handrail

Part Eight (Topics 40-42) holds the full program. For the brain:

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Sleep 7 or more hours a night on a regular basis

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    the consensus of American sleep medicine.74 ●●○ Loud snoring, gasping or daytime sleepiness warrant a sleep assessment. See Topics 23 and 41.

  2. Keep people in your week

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Social isolation is one of 14 modifiable risk factors; addressing all of them might prevent about 45% of dementia.75 ●●○ (observational) See Topics 29 and 42.

  3. Walk briskly, and build up gradually

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    In a trial of 120 older adults, walking 40 minutes three times a week for a year, after building up from 10 minutes over seven weeks, enlarged the anterior (front) hippocampus by 2%. That reversed one to two years of age-related loss. Pooled trials found no clear effect on total hippocampal volume but a protective effect on the left hippocampus, mainly by preventing shrinkage.76 ●●○

  4. Learn something hard and new

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    Adults aged 60-90 who spent 15 or more hours a week (16.5 on average) for three months learning digital photography or quilting improved episodic memory more than those doing social or easy activities.77 ●○○ Be wary of brain games: in 2016 Lumosity's maker paid $2 million to settle Federal Trade Commission (FTC) charges, with the FTC's Jessica Rich saying the company "simply did not have the science to back up its ads."78

  5. Get DHA from fish, not pills

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Adults: at least 225 g (8 ounces) of seafood a week. If pregnant or breastfeeding, eat 225-340 g (8-12 ounces) of lower-mercury choices and avoid king mackerel, marlin, orange roughy, shark, swordfish, Gulf of Mexico tilefish and bigeye tuna.79 ●●○ In three trials with 4,080 cognitively healthy older adults, omega-3 supplements did not improve cognition.80 ●●● for no benefit

  6. Cook

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Strict raw-food diets are associated with underweight and lost periods.4 ●●○

  7. Put worries on trial

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    Write a worry down, then check what happened.45 ●○○ It complements care rather than replacing it: persistent anxiety is treatable (Topic 31).

The skeptic's corner

The case against this topic, from the research itself.

The gut-for-brain swap is the weakest link in this story; the measured rise in energy use stands on firmer ground.1517 Cooking's timing is also unresolved, and meat is one alternative. From chewing experiments, Zink and Lieberman estimated that a diet one-third meat would save nearly two million chewing cycles a year, and slicing and pounding more. Cooking, they noted, "was apparently uncommon until 500,000 years ago".66 Yet a synthesis that corrected for uneven sampling found no sustained rise in evidence of meat-eating in eastern Africa after Homo erectus appeared.67 And at two French sites, Neanderthals left hearths in warm periods but almost none in cold ones.68 How early Homo paid its bill remains open.

Is the brain shrinking? An analysis of nearly a thousand skulls dated a decrease to the last 3,000 years. Its authors suggested the "externalization of knowledge", storing what a group knows outside any one head, as a cause (Topic 44).69 A reanalysis the next year found the sample dominated by modern skulls, and no detectable decline once that was corrected.70 Either way, brain volume explains only about 6% of the variation in IQ between people, so a smaller brain would not necessarily mean a duller one.71

The biases are softer than pop psychology suggests. Critics called optimistic updating "a statistical artifact", and Sharot's group replied that it survives correction.72 Capuchin monkeys trading tokens seemed to show loss aversion, until a later study blamed delays in reward.73 The evolutionary stories behind these mental habits are hypotheses: easy to tell, hard to test.

Terms this topic defines

Related topics

Sources cited above

  1. 1.Gorman, R.M. (2008). "Cooking up bigger brains." Scientific American, 298(1), January 2008. https://www.scientificamerican.com/article/cooking-up-bigger-brains/
  2. 2.Organ, C., Nunn, C.L., Machanda, Z., and Wrangham, R.W. (2011). "Phylogenetic rate shifts in feeding time during the evolution of Homo." PNAS, 108(35), 14555-14559. https://doi.org/10.1073/pnas.1107806108
  3. 3.Fonseca-Azevedo, K., and Herculano-Houzel, S. (2012). "Metabolic constraint imposes tradeoff between body size and number of brain neurons in human evolution." PNAS, 109(45), 18571-18576. https://doi.org/10.1073/pnas.1206390109 (Great apes' reported feeding times range from about 7 to 10 hours a day.) (great apes feed for about 8 hours a day)
  4. 4.Koebnick, C., Strassner, C., Hoffmann, I., and Leitzmann, C. (1999). "Consequences of a long-term raw food diet on body weight and menstruation: results of a questionnaire survey." Annals of Nutrition and Metabolism, 43(2), 69-79. https://doi.org/10.1159/000012770 (Cross-sectional questionnaire; 216 men and 297 women; 25.0% of women had a body-mass index below 18.5.) (513 respondents, on the diet for an average of 3.7 years; men reported losing 9.9 kg on average since starting and women 12 kg; about 30% of women under 45 had partly or completely stopped menstruating)
  5. 5.Carmody, R.N., and Wrangham, R.W. (2009). "The energetic significance of cooking." Journal of Human Evolution, 57(4), 379-391. https://doi.org/10.1016/j.jhevol.2009.02.011 (Estimated gains: oats 12.1%, wheat 14.5%, plantain 30.2%, potato 30.5%, green banana 35.0%. From the Koebnick survey data, menstruation was absent in 23% of women of childbearing age eating at least 70% raw food and in 50% of those eating 100% raw.)
  6. 15.Navarrete, A., van Schaik, C.P., and Isler, K. (2011). "Energetics and the evolution of human brain size." Nature, 480(7375), 91-93. https://doi.org/10.1038/nature10629
  7. 17.Pontzer, H., Brown, M.H., Raichlen, D.A., Dunsworth, H., et al. (2016). "Metabolic acceleration and the evolution of human brain size and life history." Nature, 533(7603), 390-392. https://doi.org/10.1038/nature17654 (27 chimpanzees, 8 bonobos, 10 gorillas and 11 orangutans; measurements over 7-10 days. The human women in the sample averaged 80 kg, so their body-fat figure describes this sample, not humans in general.) (141 people and 56 apes living in American zoos and in sanctuaries in the Democratic Republic of Congo; after allowing for body size and activity, humans burned about 635 kcal a day more than gorillas and 820 more than orangutans; body fat averaged 41.1% in the human women and 22.9% in the men, against under 10% in chimpanzees and bonobos)
  8. 66.Zink, K.D., and Lieberman, D.E. (2016). "Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans." Nature, 531(7595), 500-503. https://doi.org/10.1038/nature16990 (Meat at one-third of the diet cut chewing cycles by 13% and chewing force by 15%; slicing meat and pounding tubers saved a further 5% and 12%.)
  9. 67.Barr, W.A., Pobiner, B., Rowan, J., Du, A., et al. (2022). "No sustained increase in zooarchaeological evidence for carnivory after the appearance of Homo erectus." PNAS, 119(5), e2115540119. https://doi.org/10.1073/pnas.2115540119
  10. 68.Sandgathe, D.M., Dibble, H.L., Goldberg, P., McPherron, S.P., et al. (2011). "Timing of the appearance of habitual fire use." PNAS, 108(29), E298. https://doi.org/10.1073/pnas.1106759108 (Pech de l'Azé IV and Roc de Marsal.)
  11. 69.DeSilva, J.M., Traniello, J.F.A., Claxton, A.G., and Fannin, L.D. (2021). "When and why did human brains decrease in size? A new change-point analysis and insights from brain evolution in ants." Frontiers in Ecology and Evolution, 9, 742639. https://doi.org/10.3389/fevo.2021.742639 (985 crania)
  12. 70.Villmoare, B., and Grabowski, M. (2022). "Did the transition to complex societies in the Holocene drive a reduction in brain size? A reassessment of the DeSilva et al. (2021) hypothesis." Frontiers in Ecology and Evolution, 10, 963568. https://doi.org/10.3389/fevo.2022.963568 (published 2022)
  13. 71.Pietschnig, J., Penke, L., Wicherts, J.M., Zeiler, M., et al. (2015). "Meta-analysis of associations between human brain volume and intelligence differences: how strong are they and what do they mean?" Neuroscience & Biobehavioral Reviews, 57, 411-432. https://doi.org/10.1016/j.neubiorev.2015.09.017 (88 studies, more than 8,000 people; r = .24.)
  14. 72.Shah, P., Harris, A.J.L., Bird, G., Catmur, C., et al. (2016). "A pessimistic view of optimistic belief updating." Cognitive Psychology, 90, 71-127. https://doi.org/10.1016/j.cogpsych.2016.05.004 ; Garrett, N., and Sharot, T. (2017). "Optimistic update bias holds firm: three tests of robustness following Shah et al." Consciousness and Cognition, 50, 12-22. https://doi.org/10.1016/j.concog.2016.10.013
  15. 73.Chen, M.K., Lakshminarayanan, V., and Santos, L.R. (2006). "How basic are behavioral biases? Evidence from capuchin monkey trading behavior." Journal of Political Economy, 114(3), 517-537. https://doi.org/10.1086/503550 ; Silberberg, A., Roma, P.G., Huntsberry, M.E., Warren-Boulton, F.R., et al. (2008). "On loss aversion in capuchin monkeys." Journal of the Experimental Analysis of Behavior, 89(2), 145-155. https://doi.org/10.1901/jeab.2008.89-145
  16. 74.Watson, N.F., Badr, M.S., Belenky, G., Bliwise, D.L., et al. (2015). "Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society." Sleep, 38(6), 843-844. https://doi.org/10.5665/sleep.4716
  17. 75.Livingston, G., Huntley, J., Liu, K.Y., Costafreda, S.G., et al. (2024). "Dementia prevention, intervention, and care: 2024 report of the Lancet standing Commission." The Lancet, 404(10452), 572-628. https://doi.org/10.1016/S0140-6736(24)01296-0
  18. 76.Erickson, K.I., Voss, M.W., Prakash, R.S., Basak, C., et al. (2011). "Exercise training increases size of hippocampus and improves memory." PNAS, 108(7), 3017-3022. https://doi.org/10.1073/pnas.1015950108 ; Firth, J., Stubbs, B., Vancampfort, D., Schuch, F., et al. (2018). "Effect of aerobic exercise on hippocampal volume in humans: a systematic review and meta-analysis." NeuroImage, 166, 230-238. https://doi.org/10.1016/j.neuroimage.2017.11.007 (Erickson: supervised walking at 60-75% of heart rate reserve after the build-up, against a stretching and toning control. Firth: 14 trials, 737 participants.)
  19. 77.Park, D.C., Lodi-Smith, J., Drew, L., Haber, S., et al. (2014). "The impact of sustained engagement on cognitive function in older adults: the Synapse Project." Psychological Science, 25(1), 103-112. https://doi.org/10.1177/0956797613499592 (259 enrolled, 221 completed; 15 hours a week assigned, 16.51 on average.)
  20. 78.Federal Trade Commission (2016). Lumosity to Pay $2 Million to Settle FTC Deceptive Advertising Charges for Its "Brain Training" Program. 5 January 2016. https://www.ftc.gov/news-events/news/press-releases/2016/01/lumosity-pay-2-million-settle-ftc-deceptive-advertising-charges-its-brain-training-program (A further $50 million judgment was suspended.)
  21. 79.US Food and Drug Administration (2024). Advice about Eating Fish. Content current as of 5 March 2024. https://www.fda.gov/food/consumers/advice-about-eating-fish
  22. 80.Sydenham, E., Dangour, A.D., and Lim, W.S. (2012). "Omega 3 fatty acid for the prevention of cognitive decline and dementia." Cochrane Database of Systematic Reviews, 2012(6), CD005379. https://doi.org/10.1002/14651858.CD005379.pub3 (Three trials, 4,080 participants randomized, 6 to 40 months of supplementation; no trial measured incident dementia.)
  23. 45.LaFreniere, L.S., and Newman, M.G. (2020). "Exposing worry's deceit: percentage of untrue worries in generalized anxiety disorder treatment." Behavior Therapy, 51(3), 413-423. https://doi.org/10.1016/j.beth.2019.07.003 (29 people with generalized anxiety disorder; worries logged for 10 days and outcomes tracked for a month; 91.4% of worried predictions did not come true)

This topic carries 79 notes in the research manuscript; the 23 above are those behind the material on this page. Full source register.