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How diet affects the brain: evolution & development. Greg Downey Lecture 5.2. Encephalization — ‘Bigger is better’ or something more?. New world Monkey 34.1 cc. Strepsirhines 12.6 cc. Old World monkey 89.1 cc. Lesser ape 97.5 cc. Great ape 316.7 cc. Human 1251.8 cc.

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How diet affects the brain: evolution & development

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relative brain sizes

New world Monkey

34.1 cc


12.6 cc

Old World monkey

89.1 cc

Lesser ape

97.5 cc

Great ape

316.7 cc


1251.8 cc

Relative brain sizes

Most formulae based on body size suggest human should be 600 cc.

Source: James K. Rilling. 2006. Human and NonHuman Primate Brains: Are They Allometrically Scaled Versions of the Same Design? Evolutionary Anthropology 15: 65-77.

brain growth over evolution
Brain growth over evolution

Hominin doesn’t just get bigger, it spikes upward.

Relative brain size

Time, in millions of years ago

homo habilis australopithecus habilis
Homo habilis(Australopithecus habilis?)
  • ‘Habilis’ because of ‘handy man’ (discovered 1960).
  • Remains 2.3-1.6 mya.Overlaps Australopithecenes & Paranthropus.
  • Ape-like body.
  • Skeletal traits variable.H. rudolfensis for robust variant.
  • 600-700 cc. brain.Is the big jump with H./A. habilis or with H. erectus?
  • May have made stone tools.

Homo habilis (skull OH 24)


homo ergaster
Homo ergaster
  • ‘Work man’ (1976).
  • 1.8 mya to .6 mya.
  • Larger body than earlier hominins with modern proportions (savanna populations).
  • Human-like traits:Left Africa for Eurasia (range).Diet included meat (cooking?).Tool use.Brain size around 800 cc.
  • Used to be called H. erectus, but now name is reserved for East Asian remains.

‘Nariokotome Boy’

KNM-WT 15000

Remains found at Lake Turkana

Photo by Kenneth Garrett/National Geographic

comparative neurology
Comparative neurology
  • Human brain not simply quantitatively different (bigger).
  • Qualitative differences are crucial.
  • Terrence Deacon: searching for special ‘language’ part of human brain.
relative brain sizes18
Relative brain sizes

Cerebral cortex much larger.

Rat’s cortex = postage stamp

Monkey’s = post card

Human’s = four pages

Chimpanzee’s = page of printer paper

relative brain size
Relative brain size

or... the neocortex is about the size of two large pizzas.

Thanks to Paul Mason for this slide!


Primordial plexiform layer (first & oldest); becomes I & subplate (SP) in human.

Cortical plate divides PPL & forms II-VI. — see also

brain areas that grew
Brain areas that grew
  • Frontal lobe, associated with synthesizing information from other areas and inhibiting action.
  • Volume of white matter, brain interconnections, grows faster than neocortex, eventually constituting 34% of human brain.
  • Differentiation of tissue (but only through development).

Graphic from Getty Images

social brain hypothesis
Social brain hypothesis
  • Average group size correlates with the ration of neocortex to the rest of the brain.

R. I. M. Dunbar, et al. 2007. Evolution in the Social Brain. Science 317, 1344-1347. DOI: 10.1126/science.1145463


Possibility: dietary change?

Social brain hypothesisProblem: What kicks off the process?Larger brain only adaptive once social life complex.
is intelligence all in the brain
Is intelligence all ‘in the brain’?
  • Human intellectual abilities, however, are not carried entirely by genes.
    • “Feral children,” for example…
  • Human company influences intellect.
    • Other primates raised in human environments develop greater intelligence.
    • E.g. tool use in “encultured” chimps & orangutans.
    • Carel van Schaik: gregarious adult social life key.
  • “Human” is especially immature at birth.
    • Brain less developed at same age to open wider learning window.
extended brain
‘Extended brain’
  • Won’t deal with it today (during week on Language)...
  • Language, culture, symbolic systems and other devices both create external supports for cognitive abilities, and...
  • Generate developmental environments that shape the biological unfolding of humans.
  • Human brains are shot through with culture.

Meat and Livestock Australia ad campaign:

‘Red Meat. We were meant to eat it.’

Downloaded from to website for functioning video link.)

problems with a big brain
Problems with a big brain
  • Why doesn’t every animal want one?

Graphic from Getty Images

expensive tissue hypothesis
‘Expensive tissue’ hypothesis
  • Large brain is energy hungry — human brain consumes 25% of our energy when resting.(See readings!)
  • Brain tissue expends 9x body tissue average.
  • In infants, 75% of body’s energy!Only human babies fat (15%).
  • Need for energy-rich food.

Graphic from Getty Images

human diet
Human diet
  • Richard Wrangham argued that human could not eat enough food on ‘chimp diet’ to survive.(Besides, he found the fruit ‘very unpleasant.’)
  • H. ergaster brain size increasing while teeth are growing smaller.Would need more than 5 kilos/day in raw plant food.Around 6 hours/day chewing.
  • Wrangham argues that cooking would be necessary.

Graphic from Getty Images

cost of bigger brain
Cost of bigger brain
  • Examining energy demanding organs.
  • Human gut, especially, is significantly smaller than predicted by patterns in other species.
  • Makes digestive tract less efficient.
comparative gi tract
Comparative GI tract
  • Humans have comparatively long intestine & shortened colon.
  • Resembles other primates (such as capuchin monkeys) who process food in hands.
frugivore gut
Frugivore gut
  • In spite of these challenges, humans clearly have frugivore-derived gut (vegetarian).
  • Pouch in colon to ferment plant foods.
  • Intestine is expandable and quite long (compared to carnivore) & stomach small.
  • Gut transit time in humans: 38-48 hours.Carnivores: 2.5 to 26 hours.
the radiator hypothesis
The Radiator Hypothesis
  • In hot environment, brain temperature may be the one biggest limit on survival (and human brains generate energy).
  • A. afarensis began to develop openings in the skull (emissary foramina) through which blood could flow out to cool the brain.
  • Brain temperature was constraint; ‘radiator’ released this constraint.

Lower photo from Wolfgang Zenker and Stefan Kubik (1996:4); illustrations from

Images and discussion from Dean Falk,

the radiator hypothesis38
The Radiator Hypothesis

Cranial capacity

Mastoid foramina

Parietal foramina

how to afford your brain
How to afford your brain
  • Evolutionary strategies

Graphic from Getty Images


man the hunter hypothesis
‘Man the hunter’ hypothesis
  • Did hunting drive human evolution by fueling hungry brain?
  • Evidence of butchering in stone marks on bones & refuse piles.
  • Evidence from parasites.
  • Modern foragers get 50% of calories from meat (chimps <3%, who don’t host tapeworms).
man the hunter hypothesis41
‘Man the hunter’ hypothesis
  • Might seek especially rich foods (like brains or marrow).
  • Would also help explain expanding range of H. ergaster (out of Africa).
  • But data and jaw suggests small animal hunting (not romantic image of big game hunting).

Graphic from Getty Images

something fishy about the brain
Something fishy about the brain?
  • Shoreline foraging provided high protein frogs, clams, fish, & bird eggs (fish bones with H. habilis).
  • Shore rather than savannah as the crucial niche.
  • Evidence: Iodine deficiency.
  • Possible, but still theoretical…
recent changes in the brain

Graphic from Getty Images

Recent changes in the brain
  • In last 35,000 years, brain size has shrunk 11%.
  • In last 10,000 years, brain size has shrunk 8%.
  • Are domesticated food sources adequate?

Ruff, Trinkaus & Holliday 1997.

diet of early homo
Diet of early Homo?
  • Evidence suggests no single pattern (‘unspecialized frugivore’); tooth-wear patterns, for example, vary.
  • Perhaps the best evidence of dietary versatility and ability to inhabit variety of ecological niches (like the versatile lower body for locomotion).
  • The most interesting thing is the ability to meet energy demands from varied niches with underdeveloped guts (debated), jaws and teeth.
  • Humans likely omnivores for a very long time; clearly occupied a different niche from other living Great Apes (see also evidence from tapeworms).
  • Modern health problems are not because we are eating the wrong food; the problem is the lack of activity and surplus of calories.
sex and reproduction
Sex and reproduction
  • Week Six

Graphic from Getty Images

select references see unit outline for more
Select References (see unit outline for more)
  • Aiello, L. C., and P. Wheeler. 1995. The expensive-tissue hypothesis: The brain and the digestive system in humans and primate evolution. Current Anthropology 36:199-221.
  • Bradbury, Jane. 2005. Molecular Insights into Human Brain Evolution. PLoS Biology Biology 3(3): e50. DOI: 10.1371/journal.pbio.0030050
  • Dunbar, R. I. M., et al. 2007. Evolution in the Social Brain. Science 317, 1344-1347. DOI: 10.1126/science.1145463
  • Hladik, C. M., D. J. Chivers, and P. Pasquet (et al.). 1999. On Diet and Gut Size in Non-Human Primates and Humans: Is There a Relationship to Brain Size? (and commentary) Current Anthropology 40(5): 695-698. (pdf available)
  • Hladik, C. M., and P. Pasquet. 2002. The human adaptations to meat eating: a reappraisal. Human Evolution 17(3-4):199-206. (pdf available)
  • Rilling, James K. 2006. Human and NonHuman Primate Brains: Are They Allometrically Scaled Versions of the Same Design? Evolutionary Anthropology 15: 65-77. (pdf available)
  • Ruff, Christopher B., Erik Trinkaus and Trenton W. Holliday. 1997. Body mass and encephalization in Pleistocene Homo. Nature 387: 173-176.
  • Ungar, Peter S., Frederick E. Grine, and Mark F. Teaford. 2006. Diet in Early Homo: A Review of the Evidence and a New Model of Adaptive Versatility. Annual Review of Anthropology 35:209-228.
  • Wrangham, R. W., J. H. Jones, G. Laden, D. Pilbeam and N. L. Conklin-Brittain. 1999. The raw and the stolen: Cooking and the Ecology of Human Origins. Current Anthropology 40:567-594.
  • Diet diagrams from Aiello and Wheeler. 1995. Current Anthropology. Reproduced in