What did Ancient Humans Eat?

What did Ancient Humans Eat?
Eben van Tonder
15 July 2023


Biologically humans are adapted to eat meat and plant matter – both! Scientific data suggests that by the time the stone age appeared, ancient humans ate mainly meat. Here I present some of the data based on work done by Miki Ben-Dor and Ran Barkai from the Department of Archaeology, Tel Aviv University and Raphael Sirtoli from the Health Sciences, University of Minho, Braga, Portugal. (Ben-Dor, 2021) Importantly, it is NOT an analysis of what the best use of resources will be for the world in terms of food production where we find ourselves at the beginning of the 21st century. The study of Bonhommeau (2013) which I reference deals with that question. Nor is it a study of the cost of nutrition in various environments around the globe where it becomes important, especially in underdeveloped or developing economies. My sole interest is the history of meat consumption vs plant matter by humans from the perspective of the ice age or the Pleistocene, as it is referred to. This is the geological time period from c. 2.58 million to 11,700 years ago, which is the most recent period when glaciers were advancing.

From the perspective of humans and the tools we used, the ice age or Pleistocene is more or less concurrent with the Paleolithic or Old Stone Age period. In terms of the stone age, we then also find the Mesolithic or Middle Stone Age and the Neolithic or the New Stone Age. Each period is a reflection of the degree of sophistication used by humans to fashion and uses stone tools.

The Palaeolithic era spans from about 2.6 million years ago to around 10,000 BCE when humans were primarily hunter-gatherers, relying on stone tools and natural resources for survival. The Mesolithic or Middle Stone Age, stretch from 10,000 BCE to about 6,000 BCE. During this period, human societies started to show signs of domestication, but hunting and gathering remained essential for sustenance. Lastly, the Neolithic or New Stone Age lies between 6,000 BCE to around 2,000 BCE, although the timing varied across different regions. During this time, agriculture led to the development of permanent settlements, and humans started living in larger groups. The use of polished stone tools, pottery, and other technological advancements further characterizes this period.

The Human Trophic Level (HTL)

This evaluation uses the concept of Tropic Levels. In ecology, a trophic level refers to the position that an organism occupies in a food chain. It describes the feeding relationship between different organisms in an ecosystem. “The Human Trophic Level (HTL) is a mean of the trophic level of food items in the diet, weighted by quantity. Trophic levels are a measure of the energy intensity of diet composition and reflect the relative amounts of plants as opposed to animals eaten in a given country. A higher trophic level represents a greater level of consumption of energy-intensive animals.” (Bonhommeau, 2013)

Bonhommeau (2013) says that “trophic levels describe the position of species in a food web, from primary producers (such as plants or phytoplankton) to apex predators (range, 1–5). Small differences in trophic level can reflect large differences in diet. Although trophic levels are among the most basic information collected for animals in ecosystems, a human trophic level (HTL) has never been defined.” Their work sets a “global HTL of 2.21.” They found that “this value has increased with time, consistent with the global trend toward diets higher in meat.”

Ben-Dor (2020) concluded that the evolutionary lineage of hominins including modern humans (Homo sapiens) and their extinct close relatives in the food chain (the so-called trophic level) that most probably led to modern humans, evolved from a low base to a high, carnivorous position during the Pleistocene, beginning with Homo habilis, one of our close relatives who existed between 2.1 million years ago and 1.5 million years ago and peaking in Homo erectus, that existed approximately from 1.9 million years ago to around 143,000 years ago.

A reversal of that trend appears in the part of the Stone Age referred to as the Upper Paleolithic, specifically from approximately 40,000 to 10,000 years ago and culminated in the advent of agriculture. The reversal of the trend from a carnivorous position was strengthening in the Mesolithic/Epipaleolithic which is 10,000 to approximately 5,000 years ago and the Neolithic, around 10 000 to around 2000 before the present, culminating with the advent of agriculture. (Ben-Dor, 2020)

Ben-Dor (2020) showed that it is possible “to reach a credible reconstruction of the HTL without relying on a simple analogy with recent hunter-gatherers’ diets.” Instead, they depend on what they refer to as “the memory of an adaptation to a trophic level that is embedded in modern humans’ biology in the form of genetics, metabolism, and morphology” to elucidate past HTLs, an approach which is expected to gain momentum among scientists due to its productive nature and fruitful results. (Ben-Dor, 2020)

The Traditional Approach vs the Ben-Dor Research Group’s Approach: Methods and Results

It is easy to look at recent hunter-gatherer communities and infer that their habits go back millions of years ago. I have been guilty of the same many times over, but a close examination of this assumption shows that it can have little validity!

Ben-Dor (2021) points out that I am not alone in this error. They state that previous attempts to reconstruct the HTL were largely based on this assumption, namely that the diets of hunter-gatherer groups reflect customs and culture that existed for millennia. Upon close examination, it is recognised that there exist large technological and ecological differences between the Pleistocene and the Anthropocene or, generally speaking, between the period 2.6 million years ago to 11,700 years and those after this period.

The Pleistocene, as we said, is a geological epoch that began around 2.6 million years ago and ended approximately 11,700 years ago. The Anthropocene refers to an epoch that is defined by the significant impact of human activities on the Earth’s geology and ecosystems. The term “Anthropocene” is derived from “Anthropos,” meaning “human,” indicating that this epoch is characterized by the influence of human activities on a global scale. The Anthropocene concept suggests human actions, such as industrialization, deforestation, urbanization, pollution, and the burning of fossil fuels.

Exactly what time the Anthropocene should refer to is still in dispute. Some suggest the start of the Industrial Revolution with a date of c 1780. Others suggest a time of around 12 000 BP coinciding with the rise of agriculture or the Neolithic Revolution. Sope wants to push the beginning of the Anthropocene as far back as 14,000 – 15,000 years BP while others want to push it as far back that it would coincide with the Holocene which is the current geological epoch which, according to some, began 11,700 years ago.

However, these dates are not fixed, and it is easy to see that based on huge technological differences between the more recent time and the further one goes back into antiquity, the more recent dietary habits of human groups, even hunter-gatherers can not reflect that of much older groups. The general methodology used to arrive at the HTL from the Old Stone Age and into the early Middle Stone Age (which is concurrent with the ice age of the Pleistocene) relies “surprisingly little on systematic evolution-guided evidence.” (Ben-Dor, 2021)

In contrast to this widely practised system, the methodology used by Ben-Dor (2021) to reconstruct the HTL during the Pleistocene is the following:

– the research group reviewed “evidence for the impact of the HTL on the biological, ecological, and behavioural systems derived from various existing studies.”

– they “adapted a paleobiological and paleoecological approach, including evidence from human physiology and genetics, archaeology, paleontology, and zoology.”

The result of their work yielded 25 sources of evidence.


1.1 Bioenergetics

Compared with other primates, humans have a higher energy requirement for a given fat-free mass (Pontzer et al., 2016), and thus faced intense selective pressure to efficiently acquire adequate and consistent energy, especially to reliably energize the brain (Navarrete et al., 2011). Additionally, due to tool acquisition, prolonged child care, and education, humans need more time free from food acquisition than other animals (Foley & Elton, 1998).

Animal-sourced calories are generally acquired more efficiently; carnivores, therefore, spend less time feeding than similar-sized herbivores (Shipman & Walker, 1989). For example, baboons (Papio cynocephalus) devote almost all their daylight hours to feeding (Milton, 1987, p. 103) while adult Ache and Hadza men spend only a third of the day in food acquisition, preparation, and feeding (Hawkes et al., 1997; Hill et al., 1985).

Acquiring and consuming medium size animals, at a return rate in the range of tens of thousands of calories per hour, is an order of magnitude more time-efficient than plant-gathering (Kelly, 2013, table 3-3, 3-4). In other words, the price differences in “the supermarket of nature” were likely opposite to the price differences in the supermarkets of today. In nature, for humans, plant-sourced calories cost 10 times the price of meat if it is available. Given limited time and energetic budgets, such a difference in energetic returns leaves little room for flexibility (also referred to as plasticity) in the selection of the two dietary components. Nonetheless, a background consumption of plants and smaller prey is expected when women gather and do not participate in hunts (see Plants section for discussion). Also, differences in the relative availability of plants and animals affect the actual consumption.

In particular, large animals are the highest-ranking food according to ethnographic data (Broughton et al., 2011). According to classic optimal foraging theory, an animal would specialize in the highestranking type if the encounter rate is high enough (Futuyma & Moreno, 1988). Applied to humans, it means that they should have specialized in large prey if the encounter rates were high enough. Moreover, seasonal fluctuations in many plant species’ availability may hinder their reliability as food for a significant portion of the year. In contrast, animals are always available, although with fluctuating fat content.

Carnivory could have, therefore, been a more time-efficient and reliable caloric source. The relative abundance of large prey, and thus encounter rate, relative to smaller prey and plants, was probably higher during most of the Pleistocene, at least before the Late Quaternary extinction of megafauna (see the Ethnography, Paleontology and Zooarchaeology sections and Ben-Dor and Barkai (2020a) for references).

1.2 Diet quality

In relation to body size, brain size is strongly associated with dietary energetic density in primates and humans (Aiello & Wheeler, 1995; DeCasien et al., 2017; Leonard et al., 2007). Human brains are over three times larger than other primates’ brains, and as such, human dietary energetic density should be very high. The most energy-dense macronutrient is fat (9.4 kcals/g), compared with protein (4.7 kcals/g) and carbohydrates (3.7 kcals/g) (Hall et al., 2015). Moreover, plant proteins and carbohydrates typically contain anti-nutrients, which function in plant growth and defense (Herms and Mattson, 1992; Stahl et al., 1984). These anti-nutrients, such as lectins or phytate, appear in complex cellular plant matrix and fibers and limit full energetic utilization and nutrient absorption by humans (Hervik & Svihus, 2019; Schnorr et al., 2015). The most generous estimates from in vitro, human, and animal data suggest that well below 10% of total daily caloric needs can be met from fiber fermentation, and most likely below 4% (Hervik & Svihus, 2019; Høverstad, 1986; Topping & Clifton, 2001). Hence, the protein and fat mixture in animals would probably have provided higher energetic density, and therefore dietary quality. Brain size declined during the terminal Pleistocene and subsequent Holocene (Hawks, 2011; Henneberg, 1988), indicating a possible decline in diet quality (increase in the plant component) at the end of the Pleistocene.

1.3 Higher fat reserves

Humans have much higher fat reserves than chimpanzees, our closest relatives (Zihlman & Bolter, 2015). Carrying additional fat has energy costs and reduces human speed in chasing prey or escaping predators (Pond, 1978). Most carnivores and herbivores do not have a high body fat percentage as, unlike humans, they rely on speed for predation or evasion (Owen-Smith, 2002, p. 143). Present-day Hunter-Gatherers (the Hadza) were found to have sufficient fat reserves for men and women to fast for three and six weeks, respectively (Pontzer et al., 2015).

Humans seem very well adapted to lengthy fasting when fat provides their major portion of calories (Cahill Jr & Owen, 1968). Rapid entry to ketosis (when the liver synthesizes ketones from fat) allows ketone bodies to replace glucose as an energy source in most organs, including the brain. During fasting, ketosis allows muscle-sparing by substantially decreasing the need for gluconeogenesis (the synthesis of glucose from protein), and humans enter ketosis relatively quickly.

Dogs share similar digestive physiology and animal-rich dietary patterns with humans but do not enter ketosis quickly (Crandall, 1941). Indeed, dogs typically require a diet supplemented by medium-chain triglyceride to increase blood ketones to derive therapeutic benefit, but even then, they do not achieve deep physiological ketosis like humans (Packer et al., 2016). Cahill Jr (2006, p. 11) summarizes the evolutionary implications of humans’ outstanding adaptation to ketosis: “brain use of βOHB [a ketone body], by displacing glucose as its major fuel, has allowed man to survive lengthy periods of starvation. But more importantly, it has permitted the brain to become the most significant component in human evolution.” Rapid entry into ketosis has been found in the brown capuchin monkey (Friedemann, 1926),
suggesting that this adaptation to fasting may have already existed in early Homo.

Researchers who argue against a massive reliance on acquiring large animals during the Pleistocene mention their relative scarcity (Hawkes, 2016). However, besides the fact that they were more prevalent during the Pleistocene (Hempson et al., 2015), the ability to store large fat reserves and to more easily endure fasting may represent an adaptation, enabling humans to endure extended periods between acquiring the less frequently encountered large animals.

1.4 Genetic and metabolic adaptation to high-fat diet

Swain-Lenz et al. (2019) performed comparative analyses of the adipose chromatin landscape in humans, chimpanzees, and rhesus macaques, concluding that their findings reflect differences in the adapted diets of humans and chimpanzees. They (p. 2004) write: “Taken together, these results suggest that humans shut down regions of the genome to accommodate a high-fat diet while chimpanzees open regions of the genome to accommodate a high sugar diet.”

Speth (1989) hypothesized that humans eating an animal-based diet would display an obligatory requirement for significant fat amounts because they are limited in the amount of protein they can metabolize to energy. Dietary fat is also a macronutrient with priority storage within subcutaneous fat stores; this agrees with assumptions of adaptation to higher fat consumption.

The ability to finely tune fat-burning is a prominent feature of human metabolism (Akkaoui et al., 2009; Mattson et al., 2018). The lipase enzyme plays a dominant role in fat storage and metabolism. Comparing the pace of genetic changes between humans and other primates, Vining and Nunn (2016) found that lipase production underwent substantial evolution in humans.

Weyer and Pääbo (2016) found some indication of differences in both the regulation and activity of pancreatic lipase in modern humans compared with Neandertals and Denisovans. Given that Neandertals probably consumed a diet higher in meat and fat than anatomically modern humans, the latter was possibly adapting to lower fat consumption. However, these changes are also found in present-day humans, but there is no indication of how early they occurred in H. sapiens evolution. They could have resulted from a shift to a diet higher in plants in the period leading up to the adoption of agriculture, in which a marked increase in genetic changes is evident (Hawks et al., 2007). Additionally, storing larger fat reserves is a derived trait in humans, regardless of nutritional source (Pontzer, 2015). Thus, changes in fat metabolization capacity may, in part, be associated with metabolizing stored fat.

In humans, eating predominantly animal foods, especially fatty animal foods, promote nutritional ketosis. This pattern provides generous amounts of bioavailable essential micronutrients with crucial roles in encephalization, such as zinc, heme iron, vitamin B12, and long-chain omega-3 and 6 fatty acids (DHA and arachidonic acid, respectively) (Cunnane & Crawford, 2003). Infants’ brains meet all of their cholesterol needs in situ, with 30% to 70% of the required carbons being supplied by ketone bodies (Cunnane et al., 1999). Recently, nutritional ketosis has gained popularity as a possible therapeutic tool in many pathologies, such as diabetes, Alzheimer’s disease, and cancer (Ludwig, 2019).

1.5 Omega 3 oils metabolism

Another aspect of fat metabolism is the hypothesis that the early human brain’s enlargement was made possible through acquiring aquatic foods. Presumably, these foods were the only source of high amounts of docosahexaenoic acid (a long-chain omega-3 fatty acid; DHA) found in the expanding human brain (Crawford, 2010; Cunnane & Crawford, 2014; Kyriacou et al., 2016). In contrast, Cordain et al. (2002) argue that terrestrial animal organs contained sufficient DHA amounts for brain growth. Furthermore, Speth (2010, p. 135) proposed that humans biosynthesized sufficient DHA de novo from precursors. This last argument is compatible with the present existence of several billion people, including some Hunter-Gatherers, who have never eaten aquatic-sourced food, yet they and their offspring can grow and support much larger brains than early humans. A large part of this population does not consume high proportions of animal-derived food and practices multigenerational vegetarianism without cognitive decline (Crozier
et al., 2019). An increased need for DHA to sustain larger brains cannot even support claims for a terrestrial animal-based diet in early humans. Stable isotope analysis shows that at least some Neandertals did not consume much, if any, aquatic dietary resources (M. Richards & Trinkaus, 2009), though their brains were at least as large as that of modern humans.

Mathias et al. (2012) identified a genetic change that occurred in African humans about 85 thousand years ago (Kya) in the fatty acid desaturase (FADS) family of genes, showing a marginal increase in efficiency of converting plant-derived omega-3 fatty acids into DHA. This change may signify an increase in dietary plant components at that time in Africa. In Europe, however, a similar change took place only with the arrival of the Neolithic (Ye et al., 2017), suggesting that a plant-based diet was uncommon beforehand. Furthermore, tracer studies show modern adult humans can only convert <5% of the inactive plant-derived omega-3 polyunsaturated fatty acid alpha-linolenic acid (18:3Ω3, ALA) into the animal-derived active version docosahexaenoic acid (20:6Ω3, DHA) (Plourde & Cunnane, 2007).

Ye et al. (2017) found that positive genetic selection on FADS in Europe took the opposite direction in Hunter-Gatherer groups in the period leading up to the Neolithic, possibly signifying increased reliance on aquatic foods. The pre-Neolithic surge in aquatic foods exploitation is also supported by stable isotope analysis (see the section Isotopes and trace elements).

1.6 Late genetic adaptation to the consumption of underground storage organs.

A recent adaptation to a high-starch diet may be postulated from a study by Hancock et al. (2010, table 4), which showed that populations presently dependent on roots and tubers (underground storage organs [USOs]) are enriched in single nucleotide polymorphisms (SNPs) associated with starch and sucrose metabolism and folate synthesis, presumably compensating for their poor folic acid content. Another SNP in these populations may be involved in detoxifying plant glycosides, such as those in USOs (Graaf et al., 2001). Some researchers consider USOs ideal candidates for significant plant consumption by early humans (Dominy, 2012; B. L. Hardy, 2010; K. Hardy et al., 2016; Henry et al., 2014; R.W. Wrangham
et al., 1999). If genetic adaptations to USOs consumption were rather recent, it suggests that USOs did not previously comprise a large dietary component.

1.7 Stomach acidity

Beasley et al. (2015) emphasize the role of stomach acidity in protection against pathogens. They found that carnivore stomachs (average pH, 2.2), are more acidic than in omnivores (average pH, 2.9), but less
acidic than obligate scavengers (average pH, 1.3). Human studies on gastric pH have consistently found a fasted pH value <2 (Dressman et al., 1990; Russell et al., 1993). According to Beasley et al. (2015), human stomachs have a high acidity level (pH, 1.5), lying between obligate and facultative scavengers. Producing acidity, and retaining stomach walls to contain it, is energetically expensive. Therefore it would presumably only evolve if pathogen levels in human diets were sufficiently high. The authors surmise that humans were more of a scavenger than previously thought. However, we should consider that the carnivorous activity of humans involved transporting meat to a central location (Isaac, 1978) and consuming it over several days or even weeks. Large animals, such as elephants and bison, presumably
the preferred prey, and even smaller animals such as zebra, provide enough calories to sustain a 25-member Hunter Gatherer group from days to weeks (Ben-Dor et al., 2011; Ben-Dor & Barkai, 2020b; Guil-Guerrero et al., 2018). Moreover, drying, fermentation, and deliberate putrefaction of meat and fat are commonly practiced among populations that rely on hunting for a large portion of their diet (Speth, 2017), and the pathogen load may consequently increase to a level encountered by scavengers.

1.8 Insulin resistance

Another hypothesis claiming a human genetic predisposition to a carnivorous, low-carbohydrate diet is the “Carnivore Connection.” It postulates that humans, like carnivores, have a low physiological (nonpathological) insulin sensitivity. It allows prioritizing of glucose toward tissues like the central nervous system, erythrocytes, and testes that entirely or significantly depend on glucose, rather than muscles which can rely on fatty acids and ketosis instead (Brand-Miller et al., 2011); this sensitivity is similarly lower in carnivores (Schermerhorn, 2013). Brand-Miller et al. (2011) speculate that physiological insulin resistance allows humans on a low-carbohydrate diet to conserve blood glucose for the energy-hungry brain. The genetic manifestation of insulin resistance is complex and difficult to pinpoint to a limited number of genes (Moltke et al., 2014). However, Ségurel et al. (2013) found a significantly higher insulin resistance (low sensitivity) in a Central Asian population (Kirghiz) of historical herders, compared with a population of past farmers (Tajiks), despite both groups consuming similar diets. Their findings indicate a genetic predisposition to high physiological insulin resistance levels among groups consuming mainly animal-sourced foods. Additionally, a significant difference in the prevalence of this resistance exists between groups with long-term exposure to agriculture and those that do not, such as Australian
aborigines, who have higher resistance. If higher physiological insulin resistance is indeed ancestral, its past endurance suggests that high carbohydrate (starch, sugar) consumption was not prevalent.

1.9 Gut morphology

Most natural plant food items contain significant amounts of fiber (R. W. Wrangham et al., 1998), and most plant-eaters extract much of their energy from fiber fermentation by gut bacteria (McNeil, 1984),
which occurs in the colon in primates. For example, a gorilla extracts some 60% of its energy from fiber (Popovich et al., 1997). The fruits that chimps consume are also very fibrous (R. W. Wrangham et al., 1998). The human colon is 77% smaller, and the small intestine is 64% longer than in chimpanzees, relative to chimpanzee body size (Aiello & Wheeler, 1995; Calculated from Milton, 1987, table 3.2). Because of the smaller colon, humans can only meet less than 10% of total caloric needs by fermenting fiber, with the most rigorous measures suggesting less than 4% (Hervik & Svihus, 2019; Høverstad, 1986; Topping & Clifton, 2001). A 77% reduction in human colon size points to a marked decline in the ability to extract
the full energetic potential from many plant foods. The elongated small intestine is where sugars, proteins, and fats are absorbed. Sugars are absorbed faster in the small intestine than proteins and fats
(Caspary, 1992; Johansson, 1974). Thus, increased protein and fat consumption should have placed a higher selective pressure on increasing small intestine length. A long small intestine relative to other gut parts is also a dominant morphological pattern in carnivore guts (Shipman & Walker, 1989, and references therein).

This altered gut composition meets the specialization criteria proposed by Wood and Strait (2004) for adaptations that enable animals but hinder plant acquisition for food.

A marked reduction in chewing apparatus and a genetic change that reduced the jaw muscle bite force had already appeared 2–1.5 million years ago (Mya) (Lucas et al., 2006). A smaller mandibulardental
complex points to a smaller gut (Lucas et al., 2009); therefore, the carnivorous gut structure may have already been present in H. erectus.

1.10 Reduced mastication and the cooking hypothesis

Together with the whole masticatory system, teeth should closely reflect the physical, dietary form because masticatory action is repeated thousands of times each day and is thus under continuous
pressure to adjust to efficient dietary processing (Lucas et al., 2009).

One of Homo’s main derived features is the reduced relative size of the masticatory apparatus components (Aiello & Wheeler, 1995). This reduction is associated with a substantially decreased chewing
duration (approximately 5% of daily human activity, compared with 48% in chimpanzees), starting with H. erectus 1.9 Mya (Organ et al., 2011).

The masticatory system size in H. erectus, together with reduced feeding duration, is attributed to the increased dietary meat proportion and availability of stone tools (Aiello & Wheeler, 1995; Zink & Lieberman, 2016), high portion of dietary fat (Ben-Dor et al., 2011), or the introduction of cooking early in Homo evolution (R. Wrangham, 2017). We consider cooking plants as a possible but less likely explanation for the reduction in mastication since most researchers date the habitual and controlled use of fire to over a million years after the appearance of H. erectus (Barkai et al., 2017; Gowlett, 2016; Roebroeks & Villa, 2011; Shahack-Gross et al., 2014; Shimelmitz et al., 2014); but see R. Wrangham (2017). It seems that habitual use of fire appeared with the appearance of post-H. erectus species and so can signal increased plant consumption in these species. It should also be noted that although fire was undoubtedly used for cooking plants and meat, a fire has many non-cooking uses for humans (Mallol et al., 2007), including protection from predation, a significant danger in savanna landscapes (Shultz et al., 2012). Also, fire maintenance has bioenergetic costs (Henry, 2017), and in some environments, sufficient wood may not be available to maintain fire (Dibble et al., 2018). While the contribution of cooking to the consumption of plants is not contested, cooking also contributes to the consumption of meat. There is no archaeological indication of a net quantitative contribution of cooking to the HTL. We, however, assume that cooking signals a somewhat higher consumption of plants.

1.11 Postcranial morphology

Several derived postcranial morphologic phenotypes of humans are interpreted as adaptations to carnivory. Ecologically, the body size is related to trophic strategies. Researchers have attributed the increase in body size in Homo to carnivory (Churchill et al., 2012; Foley, 2001; T. Holliday, 2012). A recent body size analysis shows that H. erectus evolved larger body size than earlier hominins (S. C. Antón et al., 2014; Grabowski et al., 2015). Simultaneously, larger body size reduces the competitivity in arboreal locomotion and hence in fruit gathering. It is interesting to note that in Africa, humans’ body size reached a peak in the Middle Pleistocene, and H. sapiens may have been smaller than his predecessors (Churchill et al., 2012). Since carnivore size is correlated with prey size (Gittleman & Harvey, 1982), this development ties well with an apparent decline in prey size at the Middle Stone Age (MSA) in East Africa (Potts et al., 2018). A similar decrease in body size was identified in the Late Upper Paleolithic and Mesolithic (Formicola & Giannecchini, 1999; Frayer, 1981), also with a concomitant decline in prey size following the Late Quaternary Megafauna Extinction (Barnosky et al., 2004).

A series of adaptations to endurance running was already present in H. erectus, presumably to enable “persistence hunting” (Bramble & Lieberman, 2004; Hora et al., 2020; Pontzer, 2017). A recent genetic
experiment concerning exon deletion activity in the CMP-Neu5Ac hydroxylase (CMAH) gene in mice led Okerblom et al. (2018) to propose that humans, in whom the deletion was already fixed at 2 Mya, had already acquired higher endurance capabilities at that time. Whether this endurance was used for hunting, scavenging, or another unknown activity early in human evolution is debated (Lieberman et al., 2007; Pickering & Bunn, 2007; Steudel-Numbers & Wall-Scheffler, 2009). Comparing the Early Stone Age sites of Kanjera South and FLK-Zinj, Oliver et al. (2019) suggested that different ecological conditions required different hunting strategies, either cursorial (suitable for persistence hunting), or ambush, which is more appropriate for a woodland-intensive landscape. Some endurance running adaptations may also suggest adaptation to increased mobility in hot weather conditions, as expected from carnivores, given their relatively large home ranges (Gittleman & Harvey, 1982).

Another feature associated with hunting in the early stages of human evolution is an adaptation of the shoulder to a spear-throwing action, already present in H. erectus (Churchill & Rhodes, 2009; J. Kuhn, 2015; Roach et al., 2013; Roach & Richmond, 2015). Young et al. (2015) and Feuerriegel et al. (2017) argue that this adaptation came at the expense of a reduced ability to use arboreal niches, meeting the criteria proposed by Wood and Strait (2004) to support compelling morphological evidence of evolution toward carnivorous stenotopy.

1.12 Adipocyte morphology

Ruminants and carnivores, which absorb very little glucose directly from the gut, have four times as many adipocytes per adipose unit weight than non-ruminants, including primates, which rely on a larger proportion of carbohydrates in their diet (Pond & Mattacks, 1985). The authors hypothesize that this is related to the relative role of insulin in regulating blood glucose levels. Interestingly, omnivorous species of the order Carnivora (bears, badgers, foxes, voles) display more carnivorous patterns than their diet entails. Thus humans might also be expected to display organization closer to their omnivorous phylogenic ancestry. However, humans fall squarely within the carnivore adipocyte morphology pattern of smaller, more numerous cells. Pond and Mattacks (1985, p. 191) summarize their findings as follows: “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.”

1.13 Age at weaning

Evidence from Biology

I share with you my notes from the work of Voegtlin (1975), The Stone Age Diet. He begins by explaining digestion and nutrition for carnivores, then herbivores and then humans.

Carnivores – for example, a dog

His example of a carnivore is a dog. “The digestive tracts of all carnivores are remarkably similar in structure and function. The salivary glands do not have an important digestive function. Fat and meat are dissolved in the stomach made possible because this organ has the ability to manufacture and secrete
into its lumen a strong mineral acid called hydrochloric acid. Foodstuffs are held back in the stomach until solution has occurred.” (Voegtlin,1975)

“The carnivorous stomach, which has been filled with its normal ration of meat and fat, will be able to dissolve the entire meal and evacuate it into the small intestine within three hours.” (Voegtlin,1975) He states that “Very little actual digestion of food occurs in the carnivorous stomach,” but it has been shown that stomach acid and enzymes contribute to the initial breakdown of proteins and fats.

“The presence of the first few spurts of chyme (liquidised food from the stomach) in the duodenum, or first division of the small intestine, alerts chemical sensors or hormones. These stimulate the production
of digestive enzymes by the pancreas, which are in turn emptied into the intestine through the pancreatic duct. There they mingle with the chyme and immediately begin to break it down into its component parts. These chemical processes are continued as the chyme moves down the digestive tract, so that by the time it has reached the distal end of the small bowel, virtually all the material that can be digested has been absorbed from the digestive tube, leaving only a small indigestible residue to be emptied as a liquid suspension into the colon or large intestine for disposal as waste.” (Voegtlin,1975)

“Fat will not dissolve in water. Bile, which is manufactured by the liver, contains certain substances called bile salt, which act very much like modern laundry deterge to render fats soluble in the watery chyme, and thus render them susceptible to the action of fat-digesting enzymes. Fat in the carnivorous diet is present in large amounts on occasions and, since the need for bile is limited to times when there is considerable fat in the diet, the bile is not allowed to drain off, to be wasted during the interdigestive periods (between meals) of the carnivorous animals. Instead, it is diverted to the gallbladder where it is concentrated and stored until the presence of fat in the intestine again signals the need for its presence, whereupon a hormone, produced by the presence of fat in the intestine, causes the gallbladder to contract strongly and deliver great amounts of concentrated bile to the intestine.” (Voegtlin,1975)

Enzymes released by the pancreas aid in the breakdown of proteins, fats as well as carbohydrates. Voegtlin (1975) focuses on its role in breaking down fats, but it has been shown that it also facilitates the breakdown of carbohydrates.

“Digestion and absorption of the normal carnivorous diet – protein and fat with but little carbohydrate – is remarkably efficient. If “balance studies” are accomplished, which will merely measure the amount of a certain nutriment administered in the diet, and then determine how much of that same material appears in the animal’s excreta, it is found that the healthy animal never loses more than 4% of the ingested fat and only a trace of dietary protein.” (Voegtlin,1975)

The strong acid of the stomach guarantees that most microorganisms swallowed with the food or otherwise will be killed, or at least be attenuated and not allowed to multiply in that area. Those escaping the stomach are rarely able to withstand the digestive activity of the small intestine. However, toward the
lower end of the small intestine, where digestive activity has almost disappeared, a few surviving bacteria are to be found. But in the large intestine, myriad organisms thrive and serve some function in forming certain vitamins. The bacteria of the carnivorous colon are of the putrefactive type; they flourish on alkaline medium. The digestive tract we have been considering is practically sterile (devoid of organisms)
except in the large intestine.” (Voegtlin,1975)

He concludes that “the carnivorous digestive tract is simple, short, and of small capacity. A small variety of concentrated food is ingested at infrequent intervals. Food is digested only by enzymes which are manufactured by the animal itself. The meat-eating animals have no dependence upon microorganisms to assist in digesting the food. The food is almost completely digested and absorbed, leaving but little excretory bulk. Digestion is rapid, complete, and intermittent. The entire alimentary canal functions for a few hours, then enters upon an interdigestive period of rest. Significant digestive activity is confined to the small intestine. The carnivore is able to maintain life even after losing both stomach and colon, but cannot survive a loss of the small intestine.” (Voegtlin,1975)

Herbevores – for examplel, a sheep

“The most important change occurring in the rumen is the breakdown of cellulose to cellobiose, a process accomplished solely by the action of the microorganisms … Other carbohydrates are changed to volatile fatty acids, and still others are absorbed by bacteria and protozoa and reconstructed within their bodies into entirely different substances . . . While only 50% of the total cellulose ingested is used by the herbivorous animal, about 70% of that which is used is digested and absorbed by the rumen. Smaller amounts of cellulose may pass through the digestive tract to the cecum and colon, where bacteria again have an opportunity to digest it. No cellulose digestion occurs in other chambers of the stomach or in the small intestine. The rumen is never empty, even after prolonged of starvation. The residue serves to reinoculate fresh food with the bacteria and protozoa necessary to carry on this vital phase of the digestive process in the herbivore. . .  Digestion of proteins in the rumen is but poorly understood.
Since there are no enzymes secreted by the rumen, digestion of these protein substances is presumably also accomplished by the microorganisms. It has been recently pointed out that at least some ruminants. such as the camel, when in a state of protein deficiency, can secrete urea (a waste product containing nitrogen which is normally lost in the urine) into the rumen, where it can be used in making
new protein molecules. Much of the nitrogen-containing material found in the rumen is thought to be incorporated into the protoplasm of the bacteria and microprotozoa, the latter of which, it is to be remembered, are tiny animals.” (Voegtlin,1975)

“When these are subsequently digested in the intestine, they liberate their own body protein for use by the host animal. This, it is believed, is the ingenious mechanism that transforms plant protein into animal protein within the herbivorous digestive tract, making it possible for herbivores to survive without even traces of animal protein in their diet. . . . Nobel prize winner Artturi Virtanen has shown that test cows, ruminants structurally and functionally identical with the sheep, are able to grow normally, produce milk with normal protein content, and drop normal cows, even though their diet contained no protein of
any sort, either animal or vegetable. The cow accomplishes this biochemical miracle because the microbes of the rumen are able to manufacture animal proteins from nonprotein compounds of nitrogen, such as ammonia and urea, possibly from the nitrogen in the air itself. The prodigious numbers of these tiny animals, and the fact that they are never excreted in the faeces, make such a theory tenable. Since they are born, live, and die within the herbivore’s digestive tract theirs is indeed a monotonous life. About the only activity allowed them is that of procreation, which they accomplish with astounding rapidity.” (Voegtlin,1975)

“It is somewhat humiliating to learn that very probably most carnivorous nutrition, including that of us humans, depends for its very existence upon these lowly bacteria and protozoa – the vital link in the propagation and growth of our herbivorous food animals. Another interesting observation concerning microbial function in digestion by the herbivore is seen in the simple stomached, vegetarian mountain gorilla. In the free state this animal has many protozoa residing within its stomach, which doubtles plays a vital role in the digestion of plant substances and the synthesis of animal proteins. In captivity, these protozoa gradually disappear from the gorilla’s stomach. Then, being unable to synthesize his own
animal protein, the animal must be fed meat, milk, or other animal proteins if he is to remain healthy.
While the rumen probably does not by itself digest plant proteins, it is here that the cellulose envelope is stripped from the plant cell, exposing its nutriments (starch, vegetable proteins, and fats) to digestion by the true enzymes farther along the digestive tract. . . Contents from both the rumen and reticulum pass (the first two chambers), at the proper time, (passes) into the third chamber, the omasum. . . Their combined function is merely to prepare, by the action of microorganisms, ingested food for true or enzymatic digestion, by the fourth, or true stomach, which is called the abomasum.” (Voegtlin,1975)

“It differs from the first three chambers of the ruminant stomach already I described, for it possesses secreting glands within its walls which contribute hydrochloric acid, pepsin, and a weak, fat-splitting enzyme (lipase) to its contents. All of the digestive juices are present in much less concentration limn in the dog. They dissolve plant proteins and fats which have been freed of their cellulose investments. Of
more vital importance is their action in killing and dissolving the billions of bacteria and microportozoa arriving from the omasum. Seecls, cereal grains, bits of plant material, and cellulose which have escaped dissolution in the first three chambers, pass through the abomasum unchanged. They are emptied into the small intestine along with liquefied proteins, fats, starches, traces of sugar, and living and dead organisms.” (Voegtlin,1975)

“If we compare the gastric phase of digestion in the carnivore with that of the sheep, it may be seen that the dog swallows food directly into its glandular stomach, which is equivalent to the herbivorous abomasum, “simply because it does not have a rumen, reticulum, or omasum. The four-chambered stomach is unnecessary for flesh-eating animals, whose food is ready for enzymatic digestion immediately
upon being swallowed.” (Voegtlin,1975)

“The small intestine of the sheep is several times longer, proportionately, than is that of the dog. It has approximately the same structure and accessory glands of digestion, the pancreas. and the liver. As in the dog, the sheep’s pancreas secretes enzymes for the digestion of plant proteins, fat, and starch. A most important protein substance that is digested and absorbed by the small intestine is the moss of bacterin
and microprotozoa from the rumen, previously described. Since the herbivore is a continuous feeder whose digestive tract never rests, there is a continuous need for bile in the intestine. Therefore, most herbivores have no gallbladder for storage of bile or, if one is present, it has little or no ability to concentrate the bile or to expel it by contracting.” (Voegtlin,1975)

“The small intestine of the herbivore empties into the Large intestine or colon. While the carnivore has at this point merely a small blind pouch with no function at all, the cecum in the sheep is much longer and larger. It has an act. It has an active function to perform, that of further digesting seeds, cereal grains, bits of plant material and cellulose which reach it from the small intestine. Similar digestive activity takes place in the colon itself.” (Voegtlin,1975)

“The herbivorous animal ingests great quantities of food of low nutritional value, then proceeds to waste at least half of it, with a resulting low coefficient of digestion. Food is slowly digested at both ends as well as in the middle of the digestive tract. This activity is continuous because such a great quantity of material must be processed. Unlike the carnivore, herbivorous digestion is vitally dependent upon microbial activity. The bacterial organisms digest cellulose and “predigest” other food. Without this processing, the digestive enzymes of the intestine would be almost powerless to function. The protozoal organisms are thought to synthesize animal protein from plant food. The herbivore is a continuous feeder, and its digestive apparatus functions continuously around the clock. Much of the ingested food is not digested or utilized by the animal, causing the faeces to be voluminous and to contain much undigested material. The stomach, small intestine, cecum, and colon are all vital organs to the herbivore since it cannot
live without any one of them.” (Voegtlin,1975)

The Digestive System of Humans

“To describe the structure and function of a human digestive tract would be merely repetitious, for it is practically identical with that of the dog already delineated. The sole difference between the two is the presence in man of a rudimentary structure springing from a functionless cecum; this is called the appendix. This organ has long been considered to be a degenerated structure and is often cited as evidence that man was originally herbivorous and then became carnivorous. Supposedly, as this dietary change occurred, he gradually lost the use of his cecum, since he was no longer digesting vegetable material, and it gradually shrivelled into the vestige we know today. Others, however, suggest that man might not be losing his appendix at all, but is attempting to gain a functioning cecum. This would suggest that he was an original carnivore and that centuries of increasing plant food consumption caused this adaptive change in his digestive tract, in an effort to afford greater digestive capability for his civilized diet. Since this is a purely philosophical matter which will not be solved in less than another ten thousand years, it is pointless to pursue it further.” (Voegtlin,1975)

“To summarize the structure and function of man’s digestive tract, it is seen to be short, being only about five times the length of his body. Man, like the dog, has incisor teeth in both upper and lower jaw. The canine teeth are less well developed, but they are present. The jaw movements are up and down, which, with the ridged molars, suggests a tearing and crushing rather than a chewing function. When eating a
diet of meat, fat, and only a little carbohydrate, mastication is of little or no importance to the human.” (Voegtlin,1975)

“Man’s stomach is of a simple structure and small, having a capacity of two quarts or less. It secretes a strong acid which effectively dissolves all meat and fat before they leave the stomach. Plant substances are poorly dissolved. Man’s stomach functions intermittently, emptying a full meal in about three hours, then resting until he again eats. The human stomach is not a vital organ, as proved by hundreds of individuals who have lost all or most of it through surgery, yet manage to maintain normal nutrition. The human small intestine is also short. It is the only organ that digests food and absorbs the products of digestion to a significant degree. It is therefore a vital organ. On a diet of meat and fat with only modest amounts of processed carbohydrate, the small intestine of man is capable of digesting and absorbing practically all of the aliment, leaving but a small residue to be excreted by the colon. The accessory glands of digestion are well-developed. Enzymes manufactured by the pancreas, and to a minor degree the small intestine, constitute the only mechanism by which food is digested in the human body. The gallbladder is well-developed and functions strongly. It evacuates only when fat is present in the intestine.” (Voegtlin,1975)

“The human colon has no digestive function. Its chief activity is excretory, carrying indigested residue from the small intestine to the outside. By absorbing water from it, the colon forms the waste material into a small, compact mass. The cecum in mRn is functionless. The colon is not a vital organ and may be removed with no loss except convenience. The rectum is small, and on a proper diet should evacuate once each twenty-four to forty-eight hours. The stool should be firm and practically odourless.
Similar to the dog, digestion and absorption of foodstuffs do not occur at either end of man’s alimentary canal but only in the middle part, the small intestine.” (Voegtlin,1975)

“The human is not in the least dependent upon microorganisms as an aid to digestion. Man digests and absorbs his food normally even though the entire digestive tract has been sterilized with antibiotics. Except for the colon, there are but few bacteria and no protozoa within the human alimentary canal.” (Voegtlin,1975)

“Man is an intermittent feeder, although he usually eats more frequently than is good for him. Man never ruminates or chews his cud. Man cannot digest cellulose or unprocessed plant material. Man cannot survive with no animal protein.” (Voegtlin,1975)

“It appears certain that man is constructed as a carnivore. He functions as a carnivore and therefore, by inference, he should eat as a carnivore. His diet should be protein – mostly from animal sources – fat, and little or no carbohydrate. Even in small amounts, the latter substance, as well as vegetable proteins, should not be consumed unless they have been processed to allow their digestion and absorption.” (Voegtlin,1975)

Incorporate into text

  1. Digestive enzymes are produced by the pancreas and are emptied into the small intestine to break down food into component parts.

Evaluation: This statement accurately describes the role of pancreatic enzymes in the digestion of food in the small intestine. The enzymes released by the pancreas aid in the breakdown of carbohydrates, proteins, and fats.

  1. Bile, produced by the liver, contains bile salts that render fats soluble in the watery chyme, facilitating their digestion by fat-digesting enzymes.

Evaluation: This statement accurately describes the role of bile salts in emulsifying fats, allowing them to be more effectively digested by fat-digesting enzymes. Bile aids in the absorption of dietary fats.

  1. Digestion and absorption of the carnivorous diet (protein and fat) are efficient, with minimal loss of ingested fat and protein.

Evaluation: This statement is generally correct. The digestive system of carnivores is adapted to efficiently digest and absorb protein and fat. However, the specific percentage of loss may vary depending on factors such as the animal species and the composition of the diet.

  1. Bacteria in the carnivorous digestive tract, particularly in the large intestine, are of the putrefactive type and thrive in an alkaline environment.

Evaluation: This statement is partially accurate. Bacteria in the carnivorous digestive tract are generally more abundant in the large intestine and can be involved in putrefactive processes. However, the environment of the large intestine is more commonly characterized as slightly acidic, not alkaline.

  1. The human appendix is considered a degenerated structure, potentially related to dietary changes, and its significance is debated.

Evaluation: The statement reflects the historical perspective on the human appendix. However, current research suggests that the appendix may play a role in the immune system, harboring beneficial bacteria, and aiding in the recovery of the gut microbiota after illnesses. The exact function of the appendix is still an area of ongoing scientific investigation.

  1. The human digestive tract is similar to that of dogs, with short intestines, well-developed accessory glands (pancreas and liver), and the small intestine as the primary site of digestion and absorption.

Evaluation: This statement accurately describes the general structure and function of the human digestive tract. The small intestine is responsible for the majority of digestion and absorption of nutrients.

  1. Humans are not heavily dependent on microorganisms for digestion, and the presence of bacteria and protozoa in the human digestive tract is limited.

Evaluation: This statement is generally correct. While the human gut does contain bacteria, particularly in the colon, humans are not reliant on microorganisms for digestion to the same extent as herbivores. The human digestive system produces its own digestive enzymes and is capable of digesting and absorbing nutrients even with a low microbial presence.

  1. Humans are adapted as carnivores and should primarily consume protein (mostly from animal sources), fat, and minimal carbohydrates.

Evaluation: This statement reflects the author’s perspective, which advocates for a diet resembling that of carnivores. However, current scientific understanding supports a more balanced and varied diet for human health, including a mix of proteins (both animal and plant sources), fats, and carbohydrates. The optimal composition of an individual’s diet can vary based on factors such as age, health conditions, and personal preferences.

Overall, while some statements align with current knowledge, it is important to note that our understanding of the digestive system and dietary needs has evolved since the publication of the book in 1975. Research continues to contribute to our understanding of the complex processes involved in digestion and nutrition.


Bonhommeau et al (2013). Eating up the world’s food web and the human trophic level. PNAS, Vol 110 – 51.

Ben-Dor, M., Sirtoli, R., Barkai, R.. (2021) The evolution of the human trophic level during the Pleistocene, First published: 05 March 2021 https://doi.org/10.1002/ajpa.24247

Voegtlin, W. L.. (1975) The Stone Age Diet. Vantage Press.

Conny Waters (2022) Humans Were Apex Predators For Two Million Years, AncientPages.com