HUMAN EVOLUTION

Vol.13 - N. 3-4 (229-234) - 1998

M. Henneberg,

Department of Anatomical Sciences,
University of Adelaide, Adelaide

5005,

Australia

V. Sarafis

Centre for Microscopy and
Microanalysis,
The University of Queensland,
Brisbane 4072, Australia

Human adaptations to meat eating

It is argued that Homo sapiens is a habitual rather than a facultative
meat eater. Quantitative similarity of human gut morphology to
guts of carnivorous mammals, preferential absorption of haem
rather than iron of plant origin, and the exclusive use of humans
as the definitive host by Taenia saginata and the almost complete
human specificity of T. solium are used to support the argument.

K. Mathers

Department of Anthropology
University of California, Berkeley,
94720 USA

Keywords: australopithecinae,
Tacniods, parasites, hominids

Introduction

Currently meat of various animals forms a substantial component of the human diet. The

origin and role of meat eating in hominid evolution have been widely debated. There is,
however, no consensus yet in the palaeoanthropological literature on when habitual meat eating
originated nor whether it started as hunting or scavenging (Rose and Marshall 1996 and
following commentary). Gut macro- and microscopic morphology has been seen as reflecting
diet in mammals (Chivers and Hladik 1980). Alas, guts do not fossilise and therefore any direct
evidence for their evolution in hominids is not available. Indirectly, however, it can be
deduced from reconstructions of the skeleton that abdominal contents of australopithecines
were larger, in proportion to their body size, than those of early and modern humans (Aiello
and Dean 1990, Aiello and Wheeler 1995). Inference of meat eating from the Plio/Pleistocene
archaeological record is difficult and results are open to debate. Hominid dentition does not
provide clear indication as to meat eating (Lucas et al. 1985). Trace element (Sillen 1986,
1992) and isotope analyses (Lee-Thorp 1989, Lee-Thorp and Van der Merwe 1993) were used
to determine diet of hominids directly from the fossils. Results may be interpreted as indicating
substantial amounts of animal protein in Pilo/ Pleistocene hominid diet, but they do not agree
well with accepted interpretation of dietary differences between robust australopithecines and
early hominines. Chemical changes taking place in a fossil (diagenesis) also create some
problems (Sillen 1992).

Comparative studies of living humans and animals together with their intestinal parasites

seem to be an appropriate avenue to gain insight into human biological adaptation to meat
eating.

230

HENNEBERG, SARAFIS, MATHERS

Modern human energy intake and gastrointestinal tract morphology will be compared with

those of other extant animals in order to ascertain whether human digestive physiology differs
from that of other primates. Also the evidence from intestinal parasites acquired from food
ingestion and unique to mankind will be discussed in relation to meat eating.

Efficiency of digestion and absorption

Energy intakes per day per kg of body weight for a range of extant animals are shown in

Figure 1. Carnivore energy requirements were based on food fed to lions and leopards in the
Johannesburg ZOO (N Roux, pers. comm.) and the Krugersdorp Game reserve (M Friedrichs,
pers comm.). Energy intake of baboons and chimpanzees was obtained from Nicolosi and Hunt
(1979); ungulate requirements were given by Belovsky (1987) while FAO standards were used
for humans. Energy intake, in kJ per day per kilogram body mass is the highest for ungulates
(herbivores) and the lowest for carnivores.

Figure 1. Ranges of energy intake (kJ/day/kg) for various extant animals and for humans.

Figure 2. Comparison of the coefficient of gut differentiation (CGD) for various extant animals and for humans. Data for animals
from Chivers and Hladik (1980).

HUMAN ADAPTATIONS TO MEAT EATING

231

TABLE

Quantitative comparison of gastrointestinal tracts of some animals and humans. Data sources described in the text.

Animal intestinal

length

to body length

gastrointestinal surface area
to body surface area

Cattle

20:1

3.0:1

Horse 12:1

2.2:1

Baboon

8:1

1.1:1

Dog 6:1

0.6:1

Human

5:1

0.8:1

Cat 4:1

0.6:1

Chimpanzees and baboons as predominantly plant-eating primates fall between these

extremes. Total food , intake in humans tends to be the lowest of the three primates and closest
to that of carnivores.

Standardised by body mass energy intake is a measure of the efficiency with which the gut

extracts nutrients from the ingested food mass since basic metabolic requirements of all
mammals are similar. Carnivores eat less per kilogram body weight because they are
physiologically and anatomically adapted to a nutrient rich diet.

Physiologically, haem and other porphyrine-iron compounds derived only from meat are

absorbed by humans in preference to ionic iron, whereas herbivorous animals cannot absorb
these compounds and rely on absorption of ionic iron (Bothwell and Charlton 1982). We do not
know whether the other primates absorb haem as selectively as humans do but further
comparisons of absorption in the gut of various primates could be useful to elucidate the issue.

Quantitative morphology of the gastrointestinal tract

McNeil Alexander (1991) has suggested a generalisable model which allows predictions of

gross gut morphology from the size of an animal and its diet. This model suggests that relative
to the body size the gut size should reduce with the shift to a richer diet. In particular, parts of the
digestive system in which processing and digestion of cellulose take place will be reduced. This
model can be applied to hominoids. Quantitative comparisons of gastrointestinal tract
morphology of extant animals confirm differences between carnivores and herbivores and
underline the fact that human gut morphology resembles that of carnivores. Numerical
information on average dimensions of various parts of the human gastrointestinal tract can be
found in more complete textbooks of anatomy (e.g. Williams et al.1995). Data for domesticated
animals are available in the literature (Church and Pond 1988). The gut of a baboon

(Papio

ursinus)

was assessed by our own (MH) dissection of an adult female. The ratio of intestinal

length to body length and the ratio of gastrointestinal surface area to body surface area show
that the human pattern fits between the carnivores (dog and cat) and a baboon (Table 1).
Comparison of the coefficient of gut differentiation as defined by Chivers and Hladik (1980)
echoes the same pattern (Figure 2). The coefficient is defined as a ratio of a sum of the surface
areas of the stomach and the large intestine to the surface area of small intestine. Herbivores -
horse, gorilla and sheep - fall at one extreme while carnivores - leopard, dog and cat - occupy
the other. Gut morphology places humans midway between other primates and carnivores. A
similar differentiation between humans and other primates can be seen in a study of
gastrointestinal allometry in primates (Martin et al. 1985).

2 3 2

HENNEBERG, SARAFIS, MATHERS

Taenioid parasites

Cestodes of the family Taeniidae are parasites of carnivores (Okamoto et al. 1995).

Cestode parasite Taenia saginata uses humans as the only definitive host and T. solium almost
exclusively uses human hosts, although it is sporadically found in monkeys (Grove 1990).
These two taenioids are very closely related species that differ strongly from some other
species of Taenia: pisiformis, ovis, taeniaformis on molecular biology and monoclonal antibody
tests (Harrison and Parkhouse 1989). All human-specific taenioids are exclusively transmitted
through the eating of meat. Intermediate hosts preferred by T. saginata are bovids, ovids, and
caprids, while T solium preferential intermediate hosts are suids, mainly the domestic pig
(Miyazaki 1991). It is strongly suggested that mammalian hosts and their parasites undergo
close co-evolution and that speciation in the parasites lags behind speciation in hosts (Hafner and
Nadler 1988). C Joyeux and J-G Baer (1961) in their review of cestodes mention meat eating by
humans as the cause of infection by these two Taenia species. This indicates the habitual eating
of meat by humans and differentiates humans from chimpanzees who also hunt animals and eat
meat. They also draw attention to the stricter definitive host status of humans for Taenia
saginata than for T. solium. A third form of Taenia has recently been claimed as having humans
in Asia as its only definitive host. It is still uncertain whether it is a separate species or a
subspecies of T. saginata (Garcia and Bruckner 1997), but it may be of considerable antiquity .

Taenia species parasitising humans are related to those parasitising dogs. The +100

thousand years of association between humans and dogs recently described by Vila et al. (1997)
may suggest that humans acquired taenioid parasites initially from dogs sufficiently early to
allow their speciation.

Pongids are not definitive host for any taenioid cestode. This suggests that the three

specifically human taenioids have evolved after the separation of hominids from the common
ancestor. Meat eating in chimpanzees has recently been suggested to be a regular occurrence
(Teleki 1973, Wrangham and Van Zinncq Bergmann Riss 1990, McGrew 1992). The lack of
meat-transmitted parasites in pongids, however, suggests that the frequency of meat ingestion by
pongids is so low that it cannot support reproductive success of a muscle dwelling cysticercoid.

It is well known that because hominid dentition cannot penetrate the hide of larger

mammals, meat eating had to be coupled with the use of weapons and tools. Cooking may have
also been of importance. Human tenioid cysticerci occur with the highest frequency in animal
masseter muscles and in the heart, while their frequency is lowest in trunk muscles (Sprehn
1932, Miyazaki 1991). Since thorough cooking

supposed to kill cysticerci it may be

suggested that selection caused their preferential placement in those parts of the carcass that are
relatively less likely to be thoroughly cooked. T. saginata cysticerci have been shown to survive
in about 2 % of beef cuts used for cooking as "suya" and consumed at a stage considered
normal for human consumption in Nigeria (Mosimabale and Belino 1980).

Conclusions

The comparative anatomy and physiology pattern indicates that modern humans are well-

evolved meat eaters but we do not know the time scale necessary for such changes to have
occurred in human biology. Some evidence that meat eating has a long history in the human
lineage is provided by comparisons of Sr/Ca ratios and carbon isotope ratios from

HUMAN ADAPTATIONS TO MEAT EATING

233

hominid fossils (Sillen 1986, 1992, Lee-Thorp 1989, Lee-Thorp and Van der Merwe 1993). It
seems that on these indicators robust australopithecines and early

Homo

differ from baboons and

fall in between the herbivore and carnivore ratios. This may point to significant meat ingestion as
part of an omnivorous diet already in robust australopithecines and early Homo. Therefore we
may postulate that physiological, anatomical and behavioural adaptations to habitual reliance on
meat eating occurred in the hominid lineage at the australopithecine stage.

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