How diet affects the brain: evolution & development Greg Downey Lecture 2

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How diet affects the brain: evolution & development

  • Greg Downey

  • Lecture 5.2

Encephalization — ‘Bigger is better’ or something more?

Relative brain sizes

Encephalization: ‘expected’ brain size

Encephalization — Evolutionary trends

Encephalization among hominins

Brain growth over evolution

The first of our genus: Early Homo

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 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.

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 sizes

  • Cerebral cortex much larger.

Relative brain size

Neocortex shape (sulchi, fissures, gyri)

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).

Social brain hypothesis

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

Social brain hypothesis Problem: What kicks off the process? Larger brain only adaptive once social life complex.

Is intelligence all ‘in the brain’?

  • Human intellectual abilities, however, are not carried entirely by genes.

  • 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.

‘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.

Diet and brain: What were we ‘meant’ to eat?

  • Hominin dietary patterns

Problems with a big brain

  • Why doesn’t every animal want one?

‘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.

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.

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

  • Humans have comparatively long intestine & shortened colon.

  • Resembles other primates (such as capuchin monkeys) who process food in hands.

Diet, selection and reduced pressure

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

  • 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.

The Radiator Hypothesis

How to afford your brain

‘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’ 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).

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…

H. sapiens and P. boisei teeth

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?

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

  • Week Six

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

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