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

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.

• “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’

• 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

• Evolutionary strategies

‘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 www.beyondveg.com.