i
Submitted by
Holger Skorupa
Date of submission
20
th
of October, 2008
Course Information
Essay 1: Outline and evaluate the expensive-tissue
hypothesis proposed by Aiello/Wheeler (1995).
³'LHW(YROXWLRQDQG&XOWXUH´-ALGY 387. Supervisor:
Jessica Pearson, School of Archaeology, Classics and
Egyptology
Status
Erasmus & Exchange Students Program
ii
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 well 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 tissues
i
or significant
foraging strategies
ii
, or even both.
iii
Although it is common, that an enlargement of the brain ±
being a high expensive metabolic organ
iv
± has to lead in dietary changes or that it was
influenced by nutritional variations during the evolution of humans and non-human primates
v
,
there are different possibilities to explain this evolutionary progress. In 1995, Aiello/Wheeler
published their expensive-tissue hypothesis
vi
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. %\ RXWOLQLQJ WKH DXWKRUV¶ LQWHQWLRQV FRQFHUQLQJ WKH HQHUJ\
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-KXPDQSULPDWHV¶HYROXWLRQDU\GHYHORSPHQW, and
the weakness of recent isolated studies, the essay will make an appraisal of the usefulness
of the expensive-tissue hypothesis. Therefore, several papers of scientists facing the prime
mover theories will be introduced.
viii
This pattern appears to be the best to indicate the
DGYDQWDJHV DQG ZHDNQHVVHV LQ WKH OLQH RI $LHOOR¶V:KHHOHU¶V DUJXPHQWV 7KH HVVHQWLDO
question will 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 essentially 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
KXPDQ¶VJDVWURLQWHVWLQDOWUDFWZDVSUREDEO\LQIOXHQFHG by embedding higher-quality food as
well 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
iii
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 +RPR¶V 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 million 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 behaviour
xiv
and foraging strategy
xv
, other critics of the
expensive-tissue hypothesis predict assorted biological settings.
xvi
Although especially Aiello
recognizes these notable conclusions in several articles
xvii
, 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
SURYH $LHOOR¶V:KHHOHU¶V REVHUYDWLRQV E\ FRPSDUDWLYH VWXGLHV
xx
. 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 illustration.
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
Armstrong
xxv
, Falk
xxvi
, and Wrangham/Holland Jones/Leighton
xxvii
of simply observing primate
data ± although a YHU\ VKRUW LQYHVWLJDWLRQ RI FRZV¶ JDVWURLQWHVWLQDO WUDFW LV JLYHQ ± without
SUHVHQWLQJ GLIIHUHQFHV EHWZHHQ SULPDWHV¶ HQFHSKDOL]DWLRQ DQG RWKHU WD[RQRPLF JURXSV OLNH
birds or bats is faced by Aiello/Wheeler. They admit that such data would be very useful if
iv
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. EVSHFLDOO\WKHDXWKRUV¶FRVWTXHVWLRQVHHPV
to be an immediately discussed theme. Falk, for example, points out that the expensive-
WLVVXH K\SRWKHVLV LV OLNHO\ WR EH LQ FRQWUDVW WR PRVW ³SULPH-PRYHU´ WKHRULHV FDOOLQJ
$LHOOR¶V:KHHOHU¶V UHVHDUFK DV D ³SULPH-rHOHDVHU´ WKHRU\
xxviii
+H VXSSRUWV WKH DXWKRUV¶
SK\VLRORJLFDO DQG DQDWRPLFDO VWXG\ DOWKRXJK DGGLQJ KLV ³UDGLDWRU´ WKHRU\ DV D XVHIXO WHUP
concerning further research.
xxix
Holloway and Henneberg, by contrast, examine
$LHOOR¶V:KHHOHU¶VFRVWTXHVWLRQFULWLFDOO\:Kile Holloway names the complexity of the brain
LQFOXGLQJ SOHQW\ RI EHKDYLRUDO IXQFWLRQV WKDW FDQ DIIHFW DOO DVSHFWV RI KXPDQ¶V life
xxx
,
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, ERWK FULWLFV VHHP WR EH UHSUHVHQWDWLYHV RI WKH ³SULPH-PRYHU´ WKHRU\ 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
$LHOOR¶V:KHHOHU¶ILUVWSDUWRIWKHFRVWTXHVWLRQKDVWREHHQODUJHGE\H[DPLQLQJZK\KXPDQV
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/Wheeler
xxxiii
±
and in further studies especially Aiello ± constitute that it would be false to do remarkable
research only on a high-quality diet as the most obvious prime releaser
xxxiv
, they answer the
criticism shown above in the negative by not affecting the expensive-WLVVXH¶V PDLQ LVVXH
directly.
After explaining the relation between the brain size, the relatively small size of the gut, and
the changes in food quality, Aiello/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
Aiello/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 +\OREDWHV ODU¶V 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
v
gastrointestinal tract of the
Old World Colobinae instead of giving an evidence for a higher
mass-specific metabolic rate. At that point of investigation Aiello/Wheeler criticize
comparisons suggesting that
Old World Colobinae were significant examples for having
lower relative basal metabolic rates or guts with higher mass-specific metabolic rates
indicating a larger energetic demand. Although both authors do not deny this theory, they
refer to the weakness of such arguments. They point out the necessity of also observing
several data such as brain size and the mass of the gastrointestinal tract to receive objective
results. They follow the prediction of Ruff/Walker, here.
xxxviii
There are only few adequate
data with reference to the metabolic rates of early human and non-human primates
available.
xxxix
Furthermore, different individuals are investigated to receive necessary
information. Therefore a strengthen comparison is very problematic and highly speculative.
Apart from such criticisms
xl
, Aiello/Wheeler present their observations by illustrating the
relative brain mass and the relative gut mass in primates on the basis of both their observed
and expected sizes.
xli
The authors also predict, that their expensive-tissue hypothesis might give a new approach
FRQFHUQLQJDQWKURSRLGSULPDWHV¶UHODWLYHO\ODUJHUEUDLQVWKDWZHUHQRWOLQNHGWRDKLJKEDVDO
metabolic rate.
xlii
But ± as an appreciable result of their research ± they come to the
conclusion that the significant parallel between the relative brain size and dietary habits of
primates is a positive correlation between the relative brain size and the size of the
gastrointestinal tract.
xliii
By illustrating the connections of a high-quality diet and an increased
encephalization Aiello/Wheeler underline this aspect.
xliv
Besides, Fish/Lockwood have tested
WKH DXWKRUV¶ REVHUYDWLRQ 7KH\ FRPH WR $LHOOR¶V:KHHOHU¶V SRLQW RI YLHZ DQG JLYH WKHLU
support
to
the
expensive-tissue
hypothesis,
although
it
is
added
by
Leonard/Robertson/Snodgrass, that several testings of differences in gastrointestinal size
between primates and non-primate mammals have failed.
xlv
Nevertheless, the arguments of
these scientists are not weak since it is clear, that humans have a relatively small gut size
with reference to their body mass. But there are concerns remaining, however, asking how
the decrease of the gut might help to balance the enlargement of energetic costs that
became necessary by the increase of brain size during hominid evolution.
xlvi
Thus
Aiello/Wheeler supplementary answer critics Henneberg
xlvii
in the negative by maintaining
that it was definitely clear that the totally available energy was significant for surviving and
hominid reproduction.
xlviii
Additionally the expensive-WLVVXH K\SRWKHVLV¶ DXWKRUV DVFHUWDLQ IHZ DUJXPHQWV ZLWK OHVV
reputation. Being followed by the further examination of Leonard/Robertson/Snodgrass
regarding the decrease of hominid tooth surface area in comparison to the
Australopithecines
xlix
, Aiello/Wheeler do not argue against other aspects which seem to be
0 comments