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Are we carnivores? e implication for protein consumption
Keywords
Protein, Paleo Diet, Ethnography, Evolution
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Introduction
The Paleo Diet evolutionary mismatch principle means that the closer we stay to
the diet that we evolved to consume the better chances we have to stay healthy.
In this context, the question of the evolutionary level of protein consumption
during the Paleolithic has never received adequate attention. Since there is
relatively little protein in plants, the answer is derived from the relative amount of
animal food in the human diet. If animal food consumption were relatively high
during the Paleolithic, then relative protein consumption would have also been
high.
Quite a few authors tried to estimate the Plant: Animal ratio (DPA) in the
humans’ Paleolithic diet [1-7]. A wide variation of DPA’s was predicted with
averages ranging between 66% plants and 33% animal to 35% plants and 65%
animal. Alas, because in the archaeological record plants preserve poorly or not at
all, all of the estimates relied to a great extent on the diets of recent hunter-
gatherers groups with a tacit claim for the analogy between the periods. However,
I claim that the hunter-gatherer's ethnographic record should not be used to
predict Paleolithic diets, or indeed even variability in the diet, as the ecologies of
the two periods are so different as to deny any scientific validity to such
prediction. Here I outline a short review of the relevant ecological conditions in
support of my claim. A full paper is in preparation.
Recent hunter-gatherers ethnography is a misleading source
of Paleolithic diet reconstruction
In discussing the use of ethnographic sourced analogies in archaeology, Ascher
(8) summarized his contemporaries, Clack, Willey, and Childes’ opinions thus:
“…the cannon is: seek analogies in cultures which manipulates similar
environments in similar ways.” In other words, the degree of similarity between
the ecological and technological conditions of the known and unknown periods is
the key criteria in judging the validity of ethnographic sourced analogies.
A review of the recent ecological conditions reveals that especially in one crucial
aspect, availability and size of faunal and floral resources, there is a drastic and
unbridgeable gap between the Paleolithic and the recent period.
In a recent paper Smith et al. [9] calculated the mean body weight of non-volant
(not flying) terrestrial mammals during the last 2.5 million years. A drastic
decline from approximately 500 kgs to about ten kgs occurred during this period.
In the same vein, Bibi et al. [10] compared the faunal assemblages of Olduvai
Middle Bed II at 1.7-1.4 million years ago (Mya) to faunal communities in the
present day Serengeti. They concluded that “The sheer diversity of species,
including many large-bodied species, at Neogene and Pleistocene African sites
like Olduvai, is perplexing and makes extant African faunas look depauperate in
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Ben-Dor: Are we carnivores? The implication for protein consumption
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comparison.” Indeed, they present a hypothesis, supported by reduced carnivore
richness in the Early Pleistocene [11], that human predation may have been the
cause of the loss of large herbivores during the Pleistocene.
A significant part of the reduction occurred in the Late Pleistocene and is a global
phenomenon. During the Late Quaternary Megafauna Extinction, about 90 genera
of animals weighing >44 kg became extinct beginning some 50 Kya [12]. The rate
of extinction by body size follows a typical pattern in which the largest size
genera became more completely extinct. In all the continents, apart from Africa
and the Indian sub-continent, all genera exceeding 1000 kg became completely
extinct, and those in the 1000-320 kg category became 50-100% extinct. In
Africa, Some 25% of what was left in the Late Quaternary’s megafauna (>45 kg)
became extinct [13].
In Africa, however, even the few large animals that remained were hardly
available for hunting by HG groups that form the basis for many analogies with
the Paleolithic, the Hadza, and the San. Elephants were hunted by Europeans with
guns in the Hadza and San’s territories for over a hundred years. Evidence for a
drastic decline in the availability of animals as a result of herders and farmers
encroachment abound [14, 15]. The result is that the Hadza no longer hunt the
three largest animals, elephants, rhinos and hippos.
Moreover, the disappearance of large animals, and especially elephants, caused a
critical increase in the availability of plant food sources. Elephants are known to
be a formidable predator of baobab trees [16]. Baobab is the single largest
contributor of calories to the Hadza as well as a home for their most popular
species of honey bees. The same phenomenon occurs in the San (!Kung) territory
where the mongongo tree, their main staple food source, was subject to partial
destruction and growth retardation when elephants were present in its vicinity
[17:312].
In summary, the differences in the relative availability of plants and animals and
especially big animals, between the Paleolithic and the recent period are so
critical that they prevent any prediction from the recent HG DPA and Paleolithic
DPA, including any conclusion regarding the degree of DPA variability during
the Paleolithic.
So, if not Ethnography and not Archaeology, are there other fields of knowledge
we can explore? As it turns out, physiology can be a trove of information for
evolutionary DPA, as adaptations to one DPA or another are stored in our body in
the forms of genetics, morphology, metabolism, and sensitivity to pathogens.
Reconstruction of the Paleolithic diet based on human
physiology
A more detailed reconstruction which was performed as a part of my Ph.D. thesis
and is in preparation for publication. What follows is a short review of some of
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the physiological adaptations or lack thereof that provide evidence for the nature
of our past diet. The first three adaptations are unique in that the authors
themselves point out (maybe to their surprise) that according to their findings
humans were carnivores.
Weaning like a carnivore
Life history (length of gestation, weaning, mating, and death) is strongly defined
in a species. Psouni et al. [18] compared the difference in the age of weaning
between carnivores and other animals. Humans were found to be in the
carnivores’ group while chimpanzees and other primates with the non-carnivores.
The explanation that they provide is that the external food of carnivores is much
more similar to the milk of the mothers, so a shorter adjustment time suffice. They
conclude: "Our findings highlight the emergence of carnivory as a process
fundamentally determining human evolution."
Many smaller fat cells like all carnivores
Pond and Mattacks (19) compared the structure of fat cells in various types of
animals. Carnivores were found to have a higher number of smaller fat cells and
omnivores a smaller number of larger fat cells. Humans were found to be at the
top of the carnivorous pattern. Pond and Mattacks conclude: “These figures
suggest that the energy metabolism of humans is adapted to a diet in which lipids
and proteins rather than carbohydrates, make a major contribution to the energy
supply.”
Stomach acidity of a unique carnivore
Beasley et al. [20] found that carnivores’ stomachs are more acidic than
omnivores’ stomachs. Moreover, scavengers, which have to deal with the highest
load of pathogens, have the highest level of stomach acidity. Humans had the
highest level of acidity among all the 54 reviewed animals. Producing acidity, and
retaining the stomach walls to contain that acidity, is energetically expensive, so
would only evolve if the level of pathogens in the human diet was high. The
authors surmise that humans were more of a scavenger than we thought.
However, there is a more likely conclusion if we take into account that humans
were a particular kind of carnivore. Unlike other carnivores, they took the meat
and fat of the big animals they hunted to a central place. Big animals, like
elephants and bison, and even smaller animals like zebra, provide enough calories
to last a 25 member HG group for days and weeks. During this time the pathogen
load would have increased to a higher level than even a regular scavenger is
bound to encounter under normal circumstances and hence the need for high
acidity.
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Reduced energy extraction capacity from plants
Most plant eaters extract a large part of their energy from the fermentation of fiber
by gut bacteria. For example, a gorilla extracts some 60% of its energy from fiber
[21]. The fruits that chimps are consuming are also very fibrous, and fiber can be
calculated, based on [22], to contribute approximately 50% of their energy.
Therefore, any adaptation that prevents humans from efficient exploitation of
fiber to energy points to a shift in the dietary emphasis away from plants towards
specialization in animal’s sourced food [See 23 considering criteria for
specialization]. A reduced mastication system already 1.7 million years ago (Mya)
in H. erectus indicates that his gut size was reduced [24]. Our gut is 40% smaller,
our colon, where fiber is processed to energy, is 77% smaller than that of a
chimpanzee. The size and our small intestine, where glucose fat and protein are
absorbed is 62% longer. Since the Chimpanzee was able to absorb a large amount
of sugar with a shorter small intestine, it is safe to assume that the 66% extension
represents an adaptation to consuming more fat and protein.
Endurance running
Bramble and Lieberman [25] list 22 specific adaptations to endurance running and
claim they represent an adaptation to ‘persistence hunting.’ There is some
disagreement as to the final purpose of the adaptation [26], but as it represents an
adaptation to better mobility, it indicates adaptations to operating in a larger home
range. Carnivores with a large proportion of flesh in their diets have particularly
large home-ranges [27].
Adaptation to a spear throwing
A similar indication of evolution towards carnivory is described by Roach et al.
[28]. Our shoulder is perfectly adapted to throwing, which must be used mainly in
hunting and protection from predators. The chimpanzee’s shoulder is adapted to
climbing trees, so we have another example of an adaptation, like the smaller gut,
that reduces our capability to obtain plants, fruits in this case.
High-fat reserves
Humans have much higher fat reserves than chimps [29]. Carrying a high amount
of fat cost energy and reduce the speed of chasing or fleeting. Most carnivores
and fleeting herbivores do not pack much fat. Recent HG were found to have
enough fat reserves to fast for three weeks for men and six weeks for women [30].
This ability may represent a unique adaptation to carnivory of large animals,
where bridging the variance of success is vital due to the larger animals’ relatively
lower abundance.
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Incomplete adaptation to metabolize starch
Humans have a varying number of AMY1 gene copies which synthesize salivary
amylase, representing different degrees of adaptation to consuming starch [31].
This variance in itself can be a testimony that the adaptation is relatively recent
and was not fixed yet.
Recent genetic adaptation to tuber consumption
Tubers are mentioned as a good candidate for Paleolithic plant-based diet [32].
Populations that presently depend on tubers are enriched in genes that are
associated with starch metabolism, folic acid synthesis, and glycosides
neutralization, but other populations are not [33]. These adaptations presumably
compensate for these tubers’ poor folic acid and relatively high content of
glycosides. The very limited geographic distribution of these genes may mean that
their presence in humans is quite recent so that tubers did not form an important
part of the human Paleolithic diet.
The earliest evidence for caries - 15,000 years ago
A high prevalence of cavities (carries), a sign of intensive consumption of
carbohydrates, first appears some 15.0 Kya in a site in Morocco, together with
evidence for exploitation of starchy foods [34]. On the other hand, the oldest H.
sapiens jaws present a perfect set of teeth 300 Ka [35], which is typical
throughout the Paleolithic. This may mean that high carbohydrates (plants)
consumption is a recent phenomenon.
Reconstruction based on human Physiology – conclusion
Looking into the information that is stored in our body and behavior it is clear that
we were primarily carnivores during most of our evolution and remain adapted to
carnivory despite over 10,000 years of agricultural subsistence. It transpires, then,
that we are adapted to consume high quantities of protein but how high? The
answer lies in reconstructing our attitudes toward fat [36, 37].
How much protein? The answer is in the fat
Protein processing for energy in humans is limited to 35-50% of calories although
no definitive limit is universally accepted [38, 39] . It means that if humans were
at the protein limit, the remaining 50-65% of the calories should have come either
from fat or carbohydrates, namely plants.
The archaeological record shows that many of humans’ particular hunting and
food exploitation behaviors can be interpreted as stemming from the need to
obtain fat. The preference of hunting larger animals and prime adults [36, 40, 41],
the preference to bring fatty parts to a central place and the extraction of bone fat
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[42], at great energetic costs, all point to a strategy of fat maximization. These
energetically expensive set of behaviors supports the conclusion that plants could
not provide a significant contribution to complement the protein at the limit of its
consumption.
This reconstruction also answers the question that is asked in the title of this
paper. If humans invested so much energy in obtaining fat, it means that they
were at the limit of their protein consumption.
This limit translates to approximately 3.8 gram per kg of body weight for energy
[43] plus 0.6-0.8 gram per kg bodyweight for protein replacement so in total 4.4 –
4.6 grams per kg bodyweight although, as noted, there is no universally accepted
limit.
In conclusion, a high consumption of protein a high consumption of fat and low
consumption of plant-sourced food seems to be the human evolutionary status
during the Paleolithic.
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