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Submitted by

Holger Skorupa

Date of submission

20th of October, 2008

Course Information

Essay 1: Outline and evaluate the expensive-tissue

hypothesis proposed by Aiello/Wheeler (1995).

"Diet, Evolution and Culture", 200809-ALGY 387. Supervisor:

Jessica Pearson, School of Archaeology, Classics and

Egyptology

Status

Erasmus & Exchange Students Program

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1. Outline and evaluate the expensive-tissue hypothesis proposed by

Aiello/Wheeler (1995).

Hominid evolution has been a widely discussed aspect concerning its effects on ecological,

physiological, and behavioral as wel as reproductive, and metabolic development of humans

and non-human primates for plenty of years. During the last decade, a huge amount of

investigations regarding large brain size in humans have tried to figure out whether the

observation, that humans have a relatively larger brain size than other primates or non-

primate mammals, is based on either the correlated decrease of other tissuesi or significant

foraging strategiesii, or even both.iii Although it is common, that an enlargement of the brain ­

being a high expensive metabolic organiv ­ has to lead in dietary changes or that it was

influenced by nutritional variations during the evolution of humans and non-human primatesv,

there are different possibilities to explain this evolutionary progress. In 1995, Aiello/Wheeler

published their expensive-tissue hypothesisvi regarding the relationship between large brain

sizes in humans and high-quality diets. The authors attempt to confirm the parallel between

the increase of hominid brains, the obviously correlated decrease of the gastrointestinal tract

and dietary changes based on animal protein. They constitute the large brain sizes in human

and non-human primates and its connection to nutritional varies to be one of the most

significant prime releasers of brain evolution.vii The submitted essay portrays the main

arguments of Aiello/Wheeler. By outlining the authors′ intentions concerning the energy

balance in humans and non-human primates, answering the linkage between the basal

metabolic rate and the relative brain size of humans in the negative, and ascertaining both

the lack of data based on human and non-human primates′ evolutionary development, and

the weakness of recent isolated studies, the essay wil make an appraisal of the usefulness

of the expensive-tissue hypothesis. Therefore, several papers of scientists facing the prime

mover theories wil be introduced.viii This pattern appears to be the best to indicate the

advantages and weaknesses in the line of Aiello′s/Wheeler′s arguments. The essential

question wil be, how early humans are able to fulfil the high energetic costs of their large

brains ­ in the following named as cost question.ix

The expensive-tissue hypothesis suggests an important and therefore essential y notable

relation between the brain size, the relatively small size of gut of early humans in

comparison to non-human primates and the hominid dietary habits.x The decrease of early

human′s gastrointestinal tract was probably influenced by embedding higher-quality food as

wel as the introduction of cooking to make food more digestible.xi Aiello/Wheeler argue, that

the change of diet to a qualitatively higher food preparation could have been the most

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significant possibility on the subject of the enlargement of the brain size during hominid

evolution. They point out, that mankind has to expend a larger sum of their energetic budget

on brain metabolism.xii These increased demands are compensated by both the reduction of

the gut size and an associated change of early

Homo′s

dietary behaviour during the two

major periods of brain enlargement. The appearance of the genus

Homo

meant the first

dramatic development of hominid brain size two mil ion years ago. It was followed by the

outcoming of archaic

Homo sapiens

in the latter half of the Middle Pleistocene.xiii In addition

to Aiello/Wheeler several ecological, biological, and archaeological scientists are claiming

that there had to be much more aspects for the overall reduction from the externalization of

organic functions during the evolution. While Fish/Lockwood suggest a variation of

associated factors like social behaviourxiv and foraging strategyxv, other critics of the

expensive-tissue hypothesis predict assorted biological settings.xvi Although especially Aiel o

recognizes these notable conclusions in several articlesxvii, the authors underline the obvious

problems of only little scientific verification on the theme and therefore ascertain the

absences of adequate data.xviii

Additionally Aiello/Wheeler criticize the attempt of answering the cost question by comparing

the enlargement of brain size with a decrease of the relative mass of other organs having

relatively high mass-specific metabolic rates. They argue that these scrutinises result in

arithmetic rather than metabolic research. However, Henneberg is highly critical of the

expensive-tissue hypothesis in this mathematic case.xix On the other hand Fish/Lockwood

prove Aiello′s/Wheeler′s observations by comparative studiesxx. Therefore the expensive-

tissue hypothesis seems to offer a large space of scientific controversies. The authors

defence themselves by pointing out to attempt, how the metabolic cost of relatively large

brains was possible without both an enlargement of the basal metabolic rate and the mass-

decrease of hominid gut.xxi Emphasizing their arguments the authors portray the

observations with an il ustration.xxii

Furthermore Aiello/Wheeler name cows as animals having relatively large guts that are

characterized by extensive and complex fermenting chambers like the gut.xxiii Regarding

cows as non-human mammals being typical for large quantities of food of low assimilation

the authors predict the dependence of gut size on both the bulk and the digestibility of food.

Thus they recommend that diets characterized by smaller aggregates of high-digestible food

entail smaller guts, a simple stomach, and long small paunches.xxiv The criticism of

Armstrongxxv, Falkxxvi, and Wrangham/Holland Jones/Leightonxxvii of simply observing primate

data ­ although a very short investigation of cows′ gastrointestinal tract is given ­ without

presenting differences between primates′ encephalization and other taxonomic groups like

birds or bats is faced by Aiello/Wheeler. They admit that such data would be very useful if

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adequate statistics would be provided which always has to be a significant factor of objective

research.

However several scientists and scholars express their opinions about the first part of the

expensive-tissue hypothesis by Aiello/Wheeler. Especially the authors′ cost question seems

to be an immediately discussed theme. Falk, for example, points out that the expensive-

tissue hypothesis is likely to be in contrast to most "prime-mover" theories calling

Aiello′s/Wheeler′s research as a "prime-releaser" theory.xxviii He supports the authors′

physiological and anatomical study, although adding his "radiator" theory as a useful term

concerning further research.xxix Holloway and Henneberg, by contrast, examine

Aiello′s/Wheeler′s cost question critically. While Holloway names the complexity of the brain

including plenty of behavioral functions that can affect all aspects of human′s lifexxx,

Henneberg criticizes the obvious reduction of the evolutionary development of

Homo

regarding only two periods of changes in body mass as done by Aiello/Wheeler.xxxi

Therefore, both critics seem to be representatives of the "prime-mover" theory. Moreover,

Henneberg faces much more examples showing the overall reduction from externalization of

functions like the muscle decrease or the size of the viscera servicing the body during

hominid evolution.xxxii Both Holloway and Henneberg come to the conclusion, that

Aiello′s/Wheeler′ first part of the cost question has to be enlarged by examining why humans

need larger brains and if a bigger acumen was actually better. By disclosing that point, they

argue for the development of foraging strategies and social behaviour of early

Homo

being

significant factors of the enlargement of the brain mass, too. However, Aiello/Wheelerxxxiii ­

and in further studies especially Aiel o ­ constitute that it would be false to do remarkable

research only on a high-quality diet as the most obvious prime releaserxxxiv, they answer the

criticism shown above in the negative by not affecting the expensive-tissue′s main issue

directly.

After explaining the relation between the brain size, the relatively small size of the gut, and

the changes in food quality, Aiel o/Wheeler provide a detailed attempt to investigate this

connection within human and non-human primates. Several examples are given to follow the

intention, that there is no obvious linkage between the relative basal metabolic rate and the

development of hominid brain size during the evolution.xxxv Aiel o/Wheeler use

Presbytis
rubicunda

in comparison with

Presbytis cristatus

, for example, to contrast different diet

qualities.xxxvi While

P. cristatus

is known to be aware of poor-quality food regarding their

energies,

P. rubicunda

had a high-quality diet. Giving another instance the authors name the

Hylobatidae

by accenting

Hylobates lar′s

feeding on fruits rather than on leaves with

Hylobates syndactylus

that is conscious to eat leaves rather than fruits.xxxvii However, these

observations show a positive correlation between relative brain size and the size of the

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