Are Humans "Omnivores"? 2nd Ed.
John Coleman, 2008
A
review of biological factors for determining whether human dietary adaptations
conform to the concept of “omnivorism”.
Introduction
It is not unusual to hear the claim that humans are
"omnivores", even authoritative text books and research papers use
this term. Unfortunately, although sounding scientific, the meaning of this
term is somewhat vague. On the one hand, the term omnivore can simply be used
to describe animals that eat plant or animal foods, and on the other, it may be
used to infer that an animal is biologically adapted to consume both plant and
animal foods, and perhaps, that it is supposed to do so. A broader, and perhaps
more appropriate definition of an omnivore, would refer to animals than can
consume all kinds of food. This is because omni derives from the latin omnis
meaning all.
Therefore, in common use, the term omnivore may be used to
describe both a) what an animal does and b) what an animal is.
Clearly humans can be said to be omnivores in the sense that they do eat both
plant and animal foods, but this is just a truism. As an example of the
contrast between is and does, cattle consume animal remains in contemporary
farming practices, yet cattle are still considered to be herbivores - though
they could be said to be omnivores in that they can eat an omnivorous diet.
This article addresses the suggestion that humans are
biologically omnivores, because this is what people infer when they say
"humans are omnivores".
Establishing a testable criteria for
omnivorism
There seems to be no scientific procedure by which to
establish that an animal is biologically omnivorous, and it is because of this
that debate is possible. A reasonable proposition is an omnivore should be able
to make both plant and animal foods a significant part of the diet without
detrimental effects. To really make a convincing case for an animal being
omnivorous we would like to see both biological adaptations to plant and animal
foods. Without testable truth claims and concrete evidence, it is spurious to
use the term omnivore. Even so, there is quite clear biological evidence, as we
shall see, that could confirm that a species is adapted to a diet of both plant
and animal foods.
This article is the result of a broad study of scientific
evidence to identify human dietary adaptations, and presents data which
together suggest that humans meet the criteria for being a specialist
frugivore, and furthermore we see no compelling evidence that humans meet
the omnivorous criteria set out above. Specialist frugivores could be defined
as animals that have specialised adaptation to a diet high in fruit. As with
other frugivorous animals, this classification is general, and would not
necessarily exclude consumption of other kinds of plant matter, i.e. non-fruit
plant foods, or even animal matter as lesser constituents of the diet. A
specialist frugivore is therefore to be seen as distinct from an omnivore.
An omnivore should be capable of eating significant amounts
of animal matter, without detrimental effects, and would have clear adaptations
to such a diet in order to make the claim convincing.
In this article I assume that the dog or pig is an archetype
for the omnivore, because we know they well tolerate both plant and animal
matter as a regular and significant part of their diet, and because they are
well known to eat just about anything that is edible. However, many non-human
primates include significant amounts of animal matter in their diet, and so
might make a better comparison. Unfortunately, little is known and documented
about how well the great apes, the group of animals to which humans are most
closely biologically related, tolerate significant intakes of animal matter,
and furthermore, primates seem far more selective with their food choices than
the archetype omnivores suggested.
There are some confounding issues with studying primates.
The non-human primates, like humans, are capable of learning new eating
behaviours and forming localised food cultures, such that it may not be easy to
generalise about their diets. Therefore, when we see one group of a species of
primates consuming a diet that is mostly plant based, and another group of the
same species that is consuming much more animal products, it doesn’t follow
that the primate is omnivorous by biological adaptation. We may have a case of does
an omnivore diet, rather than is an omnivore biologically. Thus we
discover exactly the same issues that occur in calling humans omnivores.
Terms
The Flawed Philosophy
Before examining the scientific evidence, there are serious
problems with the proposition that humans are omnivores. These philosophical
issues fall into 2 distinct camps. Both of these issues seem to make the topic
unsuited to scientific discourse.
1) Lack of a testable proposition
Claims that humans are biological omnivores are hampered by the
lack of any clear set of rules (i.e. a testable hypothesis) for establishing
what an omnivore is. Some authors describe an omnivore as an animal that is
neither a herbivore nor a carnivore. However, a definition of what something is
not, lacks cognitive content, and is not a classification - in this case
omnivores are said to be "non-specialists", able to eat either plant
or animal matter. But these kinds of poorly constructed pseudo-definitions are
weak, because under conditions of domestication where technology is applied to
foods, even unequivable herbivores can consume processed animal remains.
2) Confusion of similarities and
equivalences
Food processing technology can bypass adaptations, allowing
animals to consume things they would naturally be incapable of acquiring or
consuming. Indeed the use of technology in preparing foods would tend to
indicate a lack of adaptation. Specifically, the applying of food processing
technology, i.e. hunting with weapons or trapping, cutting, cooking and tenderising,
to render animal matter edible, seems to contradict the notion of humans being
adapted to capturing and consuming animal matter. There is clearly a world of
difference between wild animals, which naturally procure and consume animal
foods, and civilised humans who consume meat, which is technologically
processed animal matter. Similar arguments can be made regarding other foods
that humans must process using technology before consumption.
As has been demonstrated, while there is clearly a
similarity in that both humans and non-human animals exist that eat practically
anything edible, the behaviours are in fact not equivalents. Wild animals
procure and consume their food by using innate biological systems, whereas
humans obtain and consume a wide range of foods as a result of technology.
If we are forced to concede that humans can be called
omnivores, even though this results from the use of technology, then we must
also accept that humans can fairly be called birds as a result of using flying
machines, and fish as a result of using underwater survival technology. Such
positions are unacceptable, and if pursued, only convince us that the claim
that humans are omnivores is not based on any form of natural equivalence, as
proponents intend when they claim that humans are omnivores.
Classification Confusion
Because the term omnivore is vague, it is not surprising
that authors often differ in their classifications of the dietary status of
animals. Pilbeam(9) describes apes as very broadly "herbivores", as do
Yerkes and Yerkes(4), whereas Maier(2) says primates should be considered to be
"omnivores". Opinions on how to classify primates in general, and
chimpanzees, our closest genetic relatives, seem to be at variance.
In order to classify digestive systems, Chivers has
performed some of the most extensive study of mammal digestive system anatomy,
yet his research on humans is inconclusive. Summing up in 'Diet and Guts'(1) he
states that human gut anatomy is characteristic of meat-eating, or some
other rapidly digested foods. However, his plots show the human digestive
anatomy is at the edge of the "carnivore" cluster. Even more
critically, at the centre of the "carnivore" cluster is Cebus
capucinus (the white-fronted capuchin). According to The Pictorial Guide To The
Living Primates, Cebus capucinus eats 95 types of fruit that make up 65% of its
diet, while leaves make up 15%. The remainder of the diet consists of berries,
nuts, seeds, shoots, buds, flowers, gums, bark and animal matter including
insects, in that order. Cebus capucinus is primarily a foli-frugivore, not a
carnivore, but they have also been called omnivores, because they consume a
range of animal matter. Cebus capucinus is rumoured to have a digestive system
somewhat like humans, and Chivers chart demonstrates some similarity, but his
carnivore data set seems to be misleading, and is not objectively defined. We
should be careful with using concepts such as "similarities", because
they are subjectively formed.
Of the few examples of other species categorised as
omnivores, none seem to closely resemble humans in their anatomy, unless
perhaps the chimpanzee is to be categorised as an omnivore. However, the
chimpanzee is often described as a frugivore, or foli-frugivore although others
call it an omnivore and as we shall see, human anatomy is distinct from the
chimps. Presently, there is no precise system for classifying a species diet
based on either its anatomy or behaviour. Anatomical observations can be
misleading, for example Milton(7) points out that the panda bear is said to
have a digestive system that resembles a carnivore(p. 14), yet it normally eats
a herbivorous diet - is it an omnivore? Furthermore within the order Carnivora
species that all share anatomical traits (by definition) have diets that vary
from pure carnivory, through omnivory to frugivory(p. 14).
Chimpanzees favour a high fruit diet when fruit is in season
but diversify, and may include more foliage and animal matter when fruit supply
is sparse. Furthermore, chimpanzees also have food cultures and procure foods
using primitive "tools" (not real technology), so that as with
humans, their diet may not reflect their adaptations as much as local habitat,
traditions and learnt behaviour to deal with food shortages.
We might accept this as a good example of omnivores, but
even rabbits engage in mild cannibalism (they will nibble dead rabbits), and
many other "herbivores" are known to eat their placentas. Herbivorous
species will also eat animal matter under captive conditions, and most
"herbivores" will ingest incidental insect matter along with foliage,
but this doesn’t seem to allow us to call them omnivores.
It seems that an opportunity to eat nutritious food is not
passed over by wild animals, even when it involves a herbivore consuming its
own placenta. On this basis then, we might conclude that all mammals are
"omnivores" - however a catchall definition is not really a category
and such broad categorisation would allow similarities to be passed off as
equivalents. Clearly, there are issues of frequency, quantity and type of
animal matter consumed. These need quantifying before a species can be called
an omnivore in the biological sense. Such clarification needs to separate
animals that infrequently eat flesh in small amounts, or under unnatural
conditions due to domestication or unusual environmental pressures, from those
that eat animal foods more uniformly, and tolerate such a diet without
detriment. Some might suggest that the practice of varying the diet under
environmental pressure is what sets omnivores apart from carnivores and
herbivores, and that such behaviour is evidence of omnivory.
Having studied a significant amount of literature, it
becomes obvious that as yet academics have not produced the kind of systematic
quantified and widely agreed definitions found in and expected of a precise
science. There are significant inconsistencies in classifying primate diets. We
are entitled to demand more cognitive content from those who claim that humans
are omnivores.
Digestive System Anatomy
The digestive system in humans is dominated by intestines
that are relatively larger than in other primates, and a colon that is
relatively smaller. Overall, the human digestive system is also a less
significant portion of the body when compared to other primates. According to Milton(7) the human small intestine makes up greater than 56% of the total gut, whereas
the colon makes up only 17 to 23%. However, in all other apes the colon makes
up greater than 45% of the total, and the intestines from 14 to 29%. This
corroborates Chivers findings, and demonstrates that the human digestive
anatomy is in a class distinct from the other apes. This being the case is
logical to look outside of the apes for a species that might better match our
digestive anatomy, perhaps at monkeys, birds or bats.
In general, larger primates including all of the great apes
are foli-frugivores but eat some animal matter, and the smaller are usually
faunivores (Tarsius sp.) that may also eat fruit (e.g. Galagoides demidoff).
Amongst the primates only Callithrix humeralifer (tassel-eared marmoset) and
Ateles paniscus (black spider monkey) eat more than 80% of their diet as
fruit(11), with the remainder coming mainly from gum or foliage respectively
and then a small percentage from animal matter. The tassel-eared marmoset is
almost totally frugivorous, in that the gums that make up 17% of its diet are
also chemically similar to fruits in being primarily a source of carbohydrate.
The remaining 0.5% of feeding time is spent on ingesting small insects. Strong
frugivory is therefore found in only a couple of species out of the 234 known
primate species.
A study of the literature on functional anatomy reveals that
foliage is digested mainly in highly sacculated stomachs or haustrated colons.
These adaptations dramatically increase the gut volume for a given length, thus
slowing digestion down so that bacterial fermentation can occur. Humans also
have haustrated colons, but the degree is not as great as in the great apes.
The foods which digest mainly in the intestines are animal matter and fruits,
which can be broken down speedily compared to leaves, due to lack of the
indigestible cell walls found in foliage.
Chivers work omits birds and bats, so also omits any highly
frugivorous species. It is only amongst birds and bats that we encounter
animals that live exclusively on fruits such as the totally frugivorous
pteropodid bats. It's worth noting that fruits are often infested with insects,
so that frugivores are incidental insect eaters. Jordano mentions in his
chapter 'Fruits and Frugivory'(5), that a gut dominated by the intestines is
also characteristic of strong frugivores (p.145). For example frugivorous bats
such as Wahlberg's fruit bat are reported to have small intestines that makes
up 94% of the total digestive system(16), although frugivorous bats may spit
the fruit fibres out, ingesting only the juices. Jordano also points out (p.138)
that frugivores require no special adaptations or special digestive processes
for processing fruit, the same claim usually made for "omnivores".
In contrast to Chivers findings(1), Hladik, Chivers and
Pasquet(12) plotted the area of functional mucosa vs. functional body size for
folivores, frugivores and faunivores, and found that humans fitted the
frugivore trend. Each trend line was completely separate in this study. This
technique therefore seems to be somewhat more accurate at prediction than Chivers
methods, yet both researchers basically confirm that human gut anatomy is
effective for speedily digested foods.
In summary, digestive anatomy research shows features of the
human digestive system consistent with a diet of foods digested more rapidly than
tough plant fibres. The surface area to functional body size ratio is
consistent with that of frugivores. In terms of ratio of intestine to colon,
humans fall between the figures found for the foli-frugivorous apes, and the
extreme condition of soft fruit and juice eating bats (see table below). The
digestive system in humans is dominated by the small intestines, a feature
common to frugivores, but also to faunivores and omnivores.
|
|
Great Ape
|
Human
|
Frugivorous Bat
|
|
Intestines
|
14% - 29%
|
56%
|
94%
|
|
Colon
|
45%
|
17% 23%
|
4%
|
|
Diet
|
Foli-frugivore
|
Intermediate?
|
soft fruit/fruit
juices
|
|
Fruit
|
~64%*
|
?
|
~100
|
|
Fibres
|
~27%*
|
?
|
~0 (ejected)
|
|
Animal Matter
|
~4%*
|
?
|
~0
|
Reducing the highly complex digestive system to a few simple
measurements and in the absence of consideration of the chemistry and
physiology is over-reductive. Digestive system anatomy can tend to reflect the
physical properties of the food, rather than the source of food, and as such
cannot determine the fine details of the diet, or may be misleading and is
certainly inconclusive. Even so, there is no reason to exclude humans from
being classed as highly frugivorous based on their digestive system anatomy.
Oral Features
An animal’s mouth is the first part of the digestive system
to process food, and therefore it has to deal with food in its natural
unprocessed state. Therefore the anatomy of the mouth has much to say about the
diet of an animal.
Just as human digestive anatomy does not reflect the trend
found in the great apes, similarly humans have dental and other oral anatomy
that sets them apart from nearly all of the other primates. In the great apes,
prominent canine teeth are the rule, and they play a role in display, defence
and in feeding. The remaining teeth are however strikingly similar to human
teeth, for example, bonobo teeth and human teeth look almost identical, as
pictures in the book 'Bonobo: The Forgotten Ape' reveal(10). This suggests a
very similar diet or dietary strategy and thus evolution. In contrast to the
other great apes, the human canine tooth is no longer prominent and resembles
the size and shape of the incisors. Because of this similarity, human canines
are known as "incisiform" canines, and it has been suggested(8) that
they function as extensions of the incisors and by analogy perform the same
function.
Incisiform canines that are large and spatulate are found in
herbivores. The Dusky Titi monkey whose diet is 54% fruit, 28% leaves and 17%
insect(11) also has incisiform canines(27). According to Pilbeam(8)
"absolutely and relatively large incisors are correlated with food
procurement tasks (what must be done to obtain bite-sized portions), such as
biting into large fruits with hard rinds."
A depiction in Dental Functional Morphology by Peter W.
Lucas (p. 130), shows the functioning of incisors in primates as either
removing the flesh of fruits, or stripping leaves from branches. Furthermore,
humans have in common with animals that regularly suction-feed; a small mouth,
a smooth and vaulted palate, a smooth and round tongue that can be shaped to
fit tightly against the palate, a closed parabolic upper tooth row without long
canines and diastemas, and a descended larynx(27). The human mouth is superbly
adapted to scooping out and pulverising juicy fruits.
The dental and oral anatomy of humans is entirely consistent
with that of a frugivorous great ape, with the addition of canine teeth further
adapted to a biting plus suction fruit diet. Canine teeth still develop from a
structure that is pointed, but juicy fruits have probably played such a
significant role in human evolution that selection in favour of thick enamel
outgrowths has taken place, so that they have evolved to form an incisor like
“canine”.
Terms
Faunivores – animals that eat animal matter which may be
either vertebrate or invertebrate
Sacculated – anatomy consisting of multiple bag like
structures
Haustrated – anatomy consisting of multiple pouch like
structures
Spatulate – anatomical feature shaped like a spatula
Diastemas – gaps between teeth
Digestive Transit
Anyone who has observed the rate at which carnivorous and
herbivorous species chew, cannot fail to see how slow herbivores chew - indeed
many carnivores hardly chew at all before swallowing. According to Lucas
(ibid., p.148) humans chew at a rate of 1.3 per second. In contrast pigs and
dogs (archetype omnivores), chew at a rate of 3.03 and 3.16 per second
respectively, whereas mountain goats chew at a rate of 1.28 per second - a rate
consistent with a range of herbivores of a similar size to humans. Humans seem
to eat too slowly when compared to the archetype omnivores.
According to Jordano(5), In order to receive sufficient
protein from a high fruit diet, strong frugivores must consume large quantities
of fruit which they digest and eliminate rapidly. Chivers concludes that the
human digestive system is adapted for rapidly digested foods(1). Milton's paper(7) claims that the human digestive system is still in its ancestral slower
digesting herbivorous state. If humans really are adapted to meat eating, and
as alleged by Chivers meat is rapidly digested, then why do the subjects in Milton's study only digest slowly like herbivores?
According to Milton, mean transit time for liquid markers in
chimps fed high fibre diets (a more natural scenario) is 35.1±2.3 hours,
whereas in humans the figure is 38.9 to 61.6 hours for high and low fibre diets
respectively (results for particle markers are similar). Evidently, despite the
relative lack of haustration in the human colon, the human does not digest the
cultural "omnivorous" diet faster than a chimp that has to break down
tough leaf matter for a day and a half. According to Burkitts figures(13) only
rural villagers on a high fibre diet have transit times comparable to those of
the chimpanzee. In contrast, those that eat more processed Western diets had
transit times from 42.4 hours for UK vegetarians to 83.4 - 144 hours for naval
persons.
It's worth pointing out that contemporary human
"omnivores" suffer appendicitis, diverticular disease, cancer of the
colon, indeed some 40% of the UK population suffer from constipation(14), while
haemorrhoids affect about a third of the population, and about 2 thirds of the
older population. It is not clear how these medical issues affect comparative
digestive studies that include such humans.
In Miltons study, all of the great apes studied and humans
had a time of first appearance of digestive markers of roughly 24 hours. In the
archetype "omnivore", the dog, mean transit times of 37.4 hours
reduced to 28.7 when more fibre was added to their diet(15) - however time of
first appearance may be much faster. Milton gives figures for some carnivores,
which show that they digest and eliminate faster than humans can on high-fibre
diets.
Research on digestive transit times seems to fit
uncomfortably with the findings on digestive anatomy. Domestic dogs don't seem
to digest meat centred diets faster than chimpanzees fed high fibre diets or
humans fed high fibre diets, although they do so when more fibre is added to
their diet. It may be that mean transit times are more reflective of diet and
body size than anatomy. Reasearch on humans has also found that the addition of
meat to the diet correlates with reduced transit times(17), a risk factor for
cancer (17,18). Research on transit times seems limited to too few species and
observations. However, Miltons study is not consistent with humans being
efficient meat eaters. Furthermore, digestive transit times for meat in humans
may be artificially lower due to artificial extraction of blood, from which the
high iron content promotes constipation.
Biochemical Factors
Following the trend of humans being an outlier from great
ape digestive anatomy, it is perhaps not surprising to find that our nutrient
requirements are also unusual. The nutritional makeup(19) of human breast milk
is shown in the table below along with that of great apes.
|
Primates
|
Total Solids (%)
|
Protein
(%)
|
Fat
(%)
|
Sugar
(%)
|
Ash
(%)
|
|
Human(*1)
|
12.5
|
1.0
|
4.4
|
6.9
|
0.2
|
|
Great Apes(*2)
|
11.5
|
2.8
|
3.0
|
5.5
|
0.2
|
The great apes produce milk that contains nearly 3 times as
much protein as human milk, and slightly less sugar and fat. This is not
surprising because human babies are born in a comparatively immature state of
development because their relatively large head must pass out through the
limited aperture of the cervix, while the body is still immature. The lack of
development of the human infants’ body, and its relative immobility, is
parsimonious with a low protein and energy milk that is consumed frequently.
The characteristic lack of development of the human body
versus head development is called desomatisation, and is a
characteristic of primates in general, as Terrance Deacon explains in his book
'The Symbolic Species'(20).
According to Harper's Biochemistry, 24th Ed., the
average male human body is 17% protein (p. 6), of which most is muscle. Human
muscle is from 18 to 20% protein, whereas brain tissue is only 8% protein(21)
but has double the fat of muscle. The proteins in brain tissue are also far
longer lived than in muscles. Brain tissue is therefore cheap in terms of
protein requirements compared to muscles.
Approximate figures derived from work by Nancy Lou
Conklin-Brittain et al.(22), show that wild fruits eaten by chimpanzees are on
average 0.9% protein, 4% carbohydrate (1% sugar and 3% fibre) and about 0.4%
fat. As adult requirements for nutrients are lower than those required for
growth and development, it is easy to see why chimpanzees can manage to live
off fruits as a high proportion of their diet. By analogy a similar situation
should follow for humans, with a lower protein requirement, and perhaps a greater
requirement for sugars to fuel the brain, as glucose is the primary metabolite
of brain tissue.
Human biochemistry is poor at dealing with high protein
intakes, as one might expect of a species adapted to a high fruit diet. When
dietary protein intake exceeds about 100 to 150 grams of protein per day infant
birth weight has been found to be reduced(23). At higher levels of protein
intake a deadly condition called 'rabbit starvation' is induced(24), although
figures vary from 35% of daily calorie(24) intake to 50% of daily calorie
intake(23). No similar conditions have been reported in faunivores or
unequivable omnivores. In contrast, in dogs that are fed grain based pet foods
they are reputed to develop skin and hair problems.
One idea of many anthropologists is that use of animal
matter has allowed humans to provision themselves with sufficient additional
calories per unit food weight over their original plant based diet, to allow
them to evolve larger brains. But as Deacon points out(20), humans grow smaller
bodies, not larger heads. Furthermore, this energy boost by consumption of high
calorie foods is alleged to be part of a maternal dietary strategy(2). However,
according to Speth(23), during this period women commonly experience aversion
to meat and meat odours, and cravings are for the most part, for carbohydrate
foods. Speth also speculates that high levels of meat during pregnancy may be
deleterious to the foetus.
Humans are also unable to synthesise vitamin C, a
characteristic unique to herbivores, including the great apes. This, and the
fundamental differences in anatomy and physiology described above, should
exclude humans from being grouped along with other omnivores, and fit the
theory of a strong frugivory in humans. Vitamin C has only a 30 minute
half-life in blood plasma, so humans must regularly ingest fresh food in order
to maintain a significant pool(31). Given this fact, it is no surprise that
fruit was found to be an effective remedy against scurvy.
One of the most common diseases associated with the human
consumption of animal products is cardiovascular disease. Atherosclerosis is
the most prevalent of the deadly degenerative diseases. According to Harpers
Biochemcistry, 4th Ed., "The rabbit, pig, monkey and humans are species in
which atherosclerosis can be induced by feeding cholesterol. The rat, dog, and
cat are resistant.", and further that "Diets rich in palmitate
inhibit the conversion of cholesterol to bile acids." Meat is a rich
source of palmitate. On page 281, it is further stated that infrequent large
meals (consistent with meat eating) versus more continuous feeding (consitent
with plant eating), adversely affects cholesterol status.
Another dangerous affect of eating diets high in meat (or
other sulphurous compounds), over plant based diets is the production of
sulphuric acid in the colon. Bacteria in the gut will convert undigested
sulphur amino acids into hydrogen sulphide (the rotten egg smell), this
combines with water and makes sulphuric acid. This is thought to promote a
number of diseases as reported in New Scientist(26). It seems that the human
colon is, after all, better adapted to plant foods than meat.
Terms
Outlier – away from the rest of the population
Herbivory is in the genes
Primary hyperoxaluria type 1 (PH1) is a recessive disease in
which an enzyme, alanine:glyoxylate aminotransferase (AGT), is mistargetted
from the peroxisomes where it functions in the glyoxylate pathway, to the
mitochondia (28) where it is inefficient. It can be caused by defects in at
least 2 glyoxylate-metabolizing enzymes and leads to excessive urine oxalate
excretion resulting in kidney stones and/or calcification of the kidney which
can occur in childhood or adolescence. Patients used to die on average at age
36 (29), however vitamin B12 therapy and dietary changes can help to increase
life span in certain forms of the disorder.
According to Birdsey et al., "One molecular adaptation
to diet that is spread widely across Mammalia is the differential intracellular
targeting of the intermediary metabolic enzyme alanine:glyoxylate
aminotransferase (AGT), which tends to be mitochondrial in carnivores,
peroxisomal in herbivores, and both mitochondrial and peroxisomal in
omnivores."(30)
As we have seen, normal humans express the AGT gene
effectively in their peroxisomes, but when AGT is targeted to the mitchondria
such as in the PH1 mutation, it cannot operate effectively. It can thus be
concluded that humans evolved through a herbivorous lineage, having
peroxisomes, but not mitochondria adapted to effective glyoxylate metabolism.
Terms
Hyperoxaluria - excessive urinary oxalate
Peroxisomes – parts of the cell that help to metabolise
fatty acids and other metabolites
Mitochondia – parts of the cell responsible for producing
energy
Behaviour
The Yerkes spent much of their lives working with primates
and studying primate literature. According to Tuttle(4) Yerkes and Yerkes
eschewed the "facile" use of the term 'instinct' throughout their
book because they had concluded that most apes, particularly infants, will easy
accommodate themselves to a wide range of human foods(p. 55). This is
presumably the same situation in which we find humans, with no particularly
strong instincts to eat particular foods in their natural state. Indeed some of
the few drives genuinely found in humans, and therefore perhaps 'instincts',
are the sweet tooth, a repulsion to bitter substances and the 'Pica'
phenomenon. Humans would certainly not eat the bitter leaves and distasteful
fruits that are part of the chimpanzees diet.
An instinctive attraction to the smell of prey species is
also not a trait that we find in humans, and would expect of a typical
carnivore or omnivore. Fruit eating species locate their foods visually.
Archetypical omnivores such as pigs, dogs or bears have acute smell and can
locate buried food. Of course, prey can be tracked visually, which is a good
method for obtaining some kinds of insects.
According to Chivers(3) humans only make it as omnivores
because the application of food processing technologies(p. 4), these allow
humans to render tough plant and animal matter edible. He even goes further in
stating that omnivorism is impractical because of the inability of any
digestive anatomy to deal with significant amounts of tough plant matter, fruit
and animal matter. Animals tend to focus on 1 or 2 different food types as the
mainstay of their diet.
Without the application of fire and the use of hunting
tools, what would ancient human ancestors have ate? Some of paleoanthropology
literature suggest that human ancestors were frugivorous, although there is
often a suggestion that animal foods were part of the diet, either as scavenged
carcasses or as invertebrate matter. Whatever the truth, we do know that modern
humans are not attracted to the smell of dead animals. Nor do humans typically
hunt down other animals and consume the remains in their bloody raw state, or
in the state of decay in which animal foods are sometimes found in the wild
when eaten by omnivores. One might suggest that this is due primarily to
cultural conditioning, but the facts remain that there are a number of
unpleasant risks one faces in eating uncooked animal matter, including various
parasites and toxins found in necrotic tissue, and the general unpalatable
nature of carrion. Perhaps this is why so much effort is made by humans to
disguise the genuine flavour and appearances of animal foods with herbs and
other; often plant derived seasonings, a situation not parsimonious with a
carnivore or omnivore. In contrast, humans are attracted to the smells of
fruits and flowers, and to sweet flavours.
Confounding Issues
According to Milton(7) in a 1904 publication, the
physiologists Elliot and Barclay-Smith had declared that the human gut was
closer to that of a herbivore than an omnivore. They were either not believed
or were forgotten, or perhaps people thought their conclusions imprecise? In
the following century far more comparative biology has been performed, and
despite (or because of) thousands of dissections, research still seems to be
inconclusive.
There are axiomatic issues facing comparative biologists
that make it impossible for their venture to yield usefully precise results.
The further afield one goes in species gap between man and the subject animal
for comparison, the less the chances that one will discover a species that is
similar enough to man to be of use. The chimpanzee seems to have become a focus
for many anthropologists, but examination of its teeth, faeces and a taste of
the food it eats, should be enough to convince anyone that we don't share the
same diet as chimps. Chimpanzees are simply not human ancestors, and have
followed their own very different evolutionary path for millions of years. In
any case, chimps have their own food cultures and environmental challenges, and
they can learn new behaviours by observing humans, all of which could confound
comparisons.
Perhaps because of the chasm between the great apes, others
have chosen to focus on mans more recent ancestors by examining fossil finds,
or by looking at contemporary hunter-gatherers. Unfortunately as Lewin
commented(25) there is no way of knowing whether fossil finds are our
ancestors, or simply our cousins(p. 59). As each new fossil find is
incorporated into the evolutionary tree, the tree becomes more complex, and the
simple linear view of our alleged ancestry that is presented, has to be
adjusted. Lewontin says that "most fossils of different ages cannot be
connected in a linear sequence, but represent a small sample from a lot of parallel
lines."
Furthermore, since both enculturation and habitat
discordance are confounding influences even in the chimpanzee, what chance have
we of finding a naturalistic diet amongst ancient hominids? In addition, even
if we could, how would we convince people that their diets were actually
healthy with no medical records, or soft tissue to analyse? A similar issue
applies to studies of contemporary hunter-gatherers - are they actually
healthy? Certainly, there are lessons to be learnt, but a wealth of empirical
nutritional research is sometimes ignored in the hope of finding a dietary
ideal in hunter-gatherers or apes.
Before embarking on a mission to create a diet
classification, it is essential to have rigid mutually exclusive quantified
categories defined, and some kind of systematic method. The absence of such a
precise system inevitably led to the confused naming free-for-all that has
occurred. The term omnivore seems to apply to a vast array of different diets
that range from small amounts of insect matter to regular and complete meals of
carrion. Due to this imprecision, scientists would do well to refrain from
using the term, as Chivers (and others) have pointed out.
Where does this leave the human? We can show a substantial
range of adaptations to plant based diets, and in particular to juicy fruits.
We can also show some not very close anatomical similarities to smaller
primates that do make up a significant proportion of their diet from animal
matter. However, there seems to be no compelling evidence of human adaptations
to consume animal foods, and plenty of evidence of a lack of adaptation. This
seems to make a bad case for humans being grouped in along with pigs, dogs or
bears and other irrefutable “omnivores”.
The behavioural evidence is perhaps the least scientific
evidence, and yet in many ways is seems quite compelling. Taken as a whole, the
evidence seems to suggest a foli-frugivous diet as natural for humans, with a
digestive system that can probably tolerate small amounts of animal matter. It
is therefore imprecise to call humans omnivores, but not necessarily entirely
mistaken.
At the beginning of the article a criteria was suggested for
testing whether humans are omnivores based on the health impacts of consuming
significant amounts of animal products. Amongst the world’s populations, it is
in general the Western populations that consume the most animal products. The
epidemiology of these populations has clearly established that diets high in
animal products are directly associated with the epidemics of degenerative
diseases such as cardiovascular disease and cancer.
The problem is that epidemiology is based on statistical
associations which do not reveal causative factors. So far, comparing meat
eaters with vegetarians has failed to produce conclusive evidence(32). Even so,
evidence does seem to suggest that reducing meat intake is associated with
healthier outcomes. Studies that compare meat eaters and vegetarians are
confounded by the fact that vegetarians have lifestyles, which are healthier in
many ways versus the average meat eating person - in any case vegetarians may
still eat significant proportions of animal products. A better study would be
on a population where the level of consumption of animal foods varies, while
other lifestyle factors remain comparable. However, this would still not
overcome the problems inherent in making statistical associations.
Even if it were possible to overcome the problem of using
statistics, it might be suggested that contemporary animal rearing methods
produce unhealthy animal foods. Indeed there is now a popular movement towards
more "natural" rearing of animals, in the belief that such animals
make healthy food. There seems to be little substance to this claim.
Traditional populations that consume large amounts of animal products, from
free roaming animals, have been found to suffer with extensive cardiovascular
disease(33). If it is further proposed that their intakes are excessive, then
this leads to the question of what level of animal food intake is believed to
be free of adverse effects. This then leads onto the question of why humans
should be adversely affected by eating more animal foods if they are supposedly
adapted to such a diet.
Given all of the above information, the evidence for humans
being omnivores is not compelling. Humans have no clear-cut adaptations to
consume animal foods, and regular consumption of animal foods is associated
with unhealthy outcomes. In contrast the evidence presented is completely
consistent with the claim that humans are specialised frugivores.
Terms
Enculturation - process whereby an established culture
teaches an individual by repetition its accepted norms
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