1. 2 3 How diet affects the brain: evolution & development Greg Downey Lecture 5.2

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How diet affects the brain: evolution &

development

Greg DowneyLecture 5.2

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Encephalization — ‘Bigger is better’ or something more?

Relative brain sizes

Old World monkey89.1 cc

Lesser ape97.5 cc

Great ape316.7 cc

Human1251.8 cc

New world Monkey34.1 cc

Strepsirhines12.6 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.

Most formulae based on body size suggest

human should be 600 cc.

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Encephalization: ‘expected’ brain size

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How big is the

human brain?

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Encephalization — Evolutionary trends

Graphic from Getty Images

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Encephalization among hominins

Brain growth over evolution

Time, in millions of years ago

Rela

tive

bra

in s

ize

http://www.talkorigins.org/faqs/homs/a_brains.html

Hominin doesn’t just get bigger, it

spikes upward.

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How big does a brain have to be?

750cc - Author Keith.

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The first of our genus: Early Homo

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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)Source:

http://www.mnh.si.edu/anthro/humanorigins/ha/a_tree.html

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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 15000Remains found at Lake

TurkanaPhoto by Kenneth Garrett/National

Geographic

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Comparative

neurology

Human brain not simply quantitatively different

(bigger).

Qualitative differences are crucial.

Terrence Deacon: searching for special ‘language’ part of

human brain.

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Relative brain sizes Cerebral cortex much larger.

Rat’s cortex = postage stamp

Monkey’s = post card

Chimpanzee’s = page of printer paper

Human’s = four pages

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Relative brain sizeor... the neocortex is about the size of two

large pizzas.

Thanks to Paul Mason for this slide!

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Neocortex shape (sulchi, fissures, gyri)

http://www.nature.com/nature/journal/v437/n7055/fig_tab/nature04103_F1.html — see also www.brainmuseum.org

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

Cortical plate divides PPL & forms II-VI.

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• 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).

Brain areas that grew

Graphic from Getty Images

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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

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Social brain hypothesis

Problem: What kicks off the process?Larger brain only adaptive once social

life complex.

Possibility: dietary change?

•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.

Is intelligence all ‘in the brain’?

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‘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.

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Diet and brain:What were we ‘meant’ to

eat?Hominin dietary patterns

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QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Meat and Livestock Australia ad campaign:‘Red Meat. We were meant to eat it.’Downloaded from www.mla.com.au (Go to website for functioning video link.)

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Problems with a big brain

Why doesn’t every animal want one?

Graphic from Getty Images

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‘Expensive tissue’

hypothesisLarge 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

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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

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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.

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Comparative GI tract

Humans have comparatively long intestine & shortened

colon.

Resembles other primates (such as capuchin monkeys)

who process food in hands.

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Diet, selection and reduced pressure

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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.

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• 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

Lower photo from Wolfgang Zenker and Stefan Kubik (1996:4); illustrations from http://www.anthro.fsu.edu/research/falk/concepts.html

Images and discussion from Dean Falk, http://www.albany.edu/braindance/Theories.htm.

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The Radiator Hypothesis

Cranial capacity

Parietal foramina

Mastoid foramina

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How to afford your brain

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Evolutionary strategies

Graphic from Getty Images

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‘Man the hunter’

hypothesisDid 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).

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‘Man the hunter’

hypothesisMight 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

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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

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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.

Graphic from Getty Images

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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.

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Sex and reproduction

Week Six

Graphic from Getty Images

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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.