Humans are Carnivores

11 min readFeb 28, 2019

It is widely accepted that humans are omnivores. We don’t quite have all the traits of a classic carnivore, such as fang-like teeth, claws, or the ability to produce our own vitamin C. We are able to survive solely on plants for years and decades. But I would argue that does not mean we are omnivorous, or at least not optimally. Our brain and capability to makes tools and think strategically make up for the fact that we do not have fangs or claws. We are also able to consume vitamin C from animal parts, particularly organs.

Simply looking at all of our herbivoros traits compared to complete carnivores is not meaningful. Instead, we should compare our characteristics to the primates that we evolved from, in order to get a sense of our ‘evolutionary trajectory’. All of our ‘herbivorous’ traits have declined since we began evolving from primates 2.5 millions years ago, and without the intervention of modern agriculture, we would have continued to evolve into full carnivory. In this paper I will argue why we are highly carnivorous, and why our bodies are not designed for eating plant-based diets.

To understand what our body is designed to consume, we must study our ancestor’s dietary patterns to see how our body evolved into the modern human. Apes did, and continue to, eat a diet of mostly plants. There must have been some change in our diet and behaviour that lead to us evolving away from them. Feces fossils (coprolites) of our ancestors contain small bird/fish bones, feathers, egg shells, but no plant fibre or seeds [1]. This is surprising, considering seeds were designed to survive an animals digestive system. If they were consumed, we should be able to observe them.

The fatty acids found in animals (AA, DTA, DHA, EPA) make up 90% of our brain, and are not available in plants [1]. We have a poor ability to convert ALA to DHA/EPA [2], and since we didn’t have flaxseed oil or constant access to nuts back in the day (which still doesn’t work for everyone [2]), we could only develop our brain through animals. But just how powerful is our brain? Maybe we could have simply developed it by increasing our ability to gather and harvest plants, with animal fats being a minor source? The answer to this lies in Kleiber’s law.

Kleiber’s law is a formula for determining the metabolic demand of a species in relation to their mass. In regards to brain size and energy use, we can evaluate a meaningful rate with the encephalization quotient: 1 indicates the size or energy usage is expected based on the mass of the animal, < 1 indicates smaller than expected, > 1 means larger than expected. Our brain’s size quotient is 7, and our brain’s energy usage quotient is almost 30 [1]. Our brain is a powerhouse. Proportional to the rest of our body, it is an energy hog. Can we feed this brain with plants alone? It may be possible if we have a very large gut with a large absorptive surface and the ability to break down fiber into a significant number of SCFAs (short-chain fatty acids).

So do we have a large gut? Compared to our primate ancestors, our cecum and colon (large intestine) are significantly smaller, proportionally speaking [12]. If you look at the size of a gorilla’s abdomen compared to a humans, it is massive. It consists largely of cecum and colon, which is where plant matter is broken down. Our brain’s massive energy requirements are compensated by our weak digestive tract. Our small intestine, however, is longer. This is where foods that are smaller, denser, and easier to break down are absorbed. We sacrificed our ability to break down plant food in order to better absorb animal foods. We need a nutrient-dense diet that is high in essential fats. Our gut cannot produce them from plants sufficiently. Evolution allowed our brains to flourish at the expense of our ability to process plant foods because our ancestors ate a highly nutrient-dense, carnivorous diet.

If this is indeed true, then we should expect our bodies to have many carnivoros characteristics, or at least, characteristics that were conducive to carnivory given our intellect level and opposable thumbs. A recent paper[3] explains a multitude of these characteristics.

Our carnivoros traits

Humans have many small fat cells like all carnivores. ‘Pond and Mattacks 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.”’

Humans have the stomach acidity of a unique carnivore. ‘Humans had a high level of acidity of 1.5 that lies between that of obligate and facultative scavengers. Producing acidity, and retaining the stomach walls to contain that acidity, is energetically expensive, so would presumably only evolve if the level of pathogens in the human diet was high.’ Note that herbivoros primates have a stomach pH varying from around 4 to 6 [17]. Most omnivores are between 2 and 4 [17].

As previously noted, humans have a reduced energy extraction capacity from plants. ‘Our large intestine, where fiber is processed to energy, is 77% smaller by volume than that of a chimpanzee our size … The size and our small intestine, where -macronutrients are absorbed is 62% larger than that of a chimpanzee our size … the gut morphology adaptations both improved animal food exploitation and at the same time hindered the full exploitation of fibrous plant foods’

Humans substituted primate climbing capabilities for an adaptation to throwing. ‘Roach et al claim that the structure of our shoulder represents an adaptation to carnivory. They describe how our shoulder is perfectly adapted to throwing, which must be useful, in their opinion, mainly in hunting and protection from predators. They show that in contrast, the chimpanzee’s shoulder is adapted to climbing trees.’

Humans have much higher fat reserves than chimps, our closest relatives [3]. Carrying a high amount of fat costs energy and reduces the speed of chasing or fleeting, but gives us the ability to go periods without food while we hunt and scavenge. Apes are constantly eating and surrounded by plants, and don’t need this function. If we were constantly eating plants, we wouldn’t have adapted this way.

Human jaws and teeth shrank, forfeiting chewing capabilities. We know early humans crafted tools to help process meat, even before fire. It takes 39% to 46% less force to chew and swallow processed meat than processed root foods [11]. Evolution chose to forgo the ability to properly chew certain plant-based foods to allow for more room in the skull [11]. The ability to use tools also explains why we don’t have carnivorous fangs — we didn’t have to tear raw flesh from a carcass with our bare teeth.

From all of these adaptations, it is clear that humans have been moving further from herbivory/omnivory, and closer to carnivory. But there are counter-arguments that need to be acknowledged.

The counter arguments

Anthropologists like to bring up the point of tuber consumption. The starch from tubers has been discovered on Paleolithic humans (via teeth deposits), and has been used as an argument that we have adapted to eating plants and carbs in our diet. Miki Ben-Dor acknowledges this in his paper[3]: ‘Tubers, which are available year-round and are as energy dense as wild fruits, are mentioned as a good candidate for Paleolithic plant-based diet. 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. 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.’ Due to the lack of gene distribution among humans, our ancestors were likely not eating many tubers or carbs for a significant period of time.

Anthropologists also argue that our ancestors, or at least some of them, likely ate a large amount of plants because many modern hunter-gatherer tribes do. And not only that, but many modern tribes are quite unsuccessful at hunting meat, and are unable to obtain enough to sustain themselves alone. Thus, we must have been eating a significant amount of plant-based foods historically. However, this is likely not the case.

As Miki Ben-Dor mentions, modern hunter-gatherers are not indicative of past. Not only did the most recent ice age begin approximately 2.5 million years ago, around the same time we evolved from apes, but it ended around 11,000 years ago, around the time of the agricultural revolution. During the ice age, there were vastly greater amounts of large mammals available for the taking, which only began to rapidly decrease in the last 100,000–200,000 years [4]. Thus, majority of our paleolithic era evolution provided our ancestors an abundance of large, fatty animals for consumption and a decreased dependence on plants.

But what about the last 10,000 years (the Neolithic era)? That’s enough time for us to adapt to the vast rise of agriculture and plant products, right? There is a fair point to this argument:

‘Since the agricultural transition, humans have continued to undergo genetic change. Changes in the salivary amylase (AMY1), alcohol dehydrogenase (ADH) and lactase (LCT) genes in very recent human history illustrate the potential for diet to drive genetic change in this time period’. [5]

However, it appears that we are limited to those few changes:

‘Genome scans searching for signatures of positive selection in recent human evolutionary history have found very little evidence of other genetic changes in response to diet. New insights into the selection of AMY1, ADH, and LCT variants suggest that unusually strong selection pressures were applied on these genes during, and even before, the Neolithic era. These genetic changes are therefore exceptional examples, and do not represent a broader level of genetic adaptation to the Neolithic diet … Considering the short length of time since the agricultural revolution, there has therefore been limited opportunity for such mutations to arise in response to modern foods. Conversely, the length of the Paleolithic era (around 2.5 million years) provided far more opportunity for mutations of all kinds to arise and to undergo natural selection. Given the small number of genetic changes that have occurred in the last 10,000 years in response to diet, it can be concluded that a Paleolithic dietary template is most closely aligned with our genetic make-up.’ [5]

As we can see, modern hunter-gatherers are not indicative of the past. There is no evidence that our ancestors consumed a diet mostly consisting of plant-based foods. And our genes have not adapted to the modern diet that we have been consuming since the agricultural revolution.

Humans need meat

Our brain was the largest 90,000 years ago, when it slowly began to shrink. Approximately 20,000 years ago, a larger shrinkage began and has continued into today, causing a 10% decrease in size [1, 13]. There could be many confounding factors for this, but as one study suggests, the shrinkage strongly correlates with a lack of animal meat availability and an increased dependence on plant consumption [13]. Similarly, the agricultural revolution, which saw a vast rise in carbohydrate consumption and a vast decrease in animal consumption, has only damaged our health and led to vast nutritional deficiencies compared to hunter-gatherers, even in the well-fed farmers [10]. This has surely contributed to decreased brain size as well.

Ancient societies understood the value of fatty meat

Our appreciation for animal fat goes back into ancient times. It is even acknowledged many times in the bible. From the old testament, Cain, a vegetarian, provides an offering of the ‘fruit of the ground’. Abel, on the other hand, offered his sheep and ‘the fat thereof’. ‘But unto Cain and his offering, he had no respect.’ In the new testament, when the prodigal son returned home, his father ‘slew a fatted calf.’ Even studies of the most modern ‘traditional’ hunter-gatherers show that they search specifically for fatty meat, and may sometimes toss away meat if it is too lean.

Dr. Carl Lumholtz studied aboriginal tribes in the late 1800s — early 1900s, and noticed that they would never eat vegetables when animal sources were available. Children were always given the best, fattiest meat first, as it was necessary for development. Sir George Hubert Wilkins noted the same thing in his studies. Many modern aboriginals eat solely (or almost entirely) meat. Or at least did until the past century or so, which is when health problems started to crop up. These are tribes such as the Siberians, North Inuit, Marsh Arabs, Berbers, Samburo, and many more.

The modern era and our health epidemic

With the industrial revolution, we no longer had to rely on local, in season produce and meat, and we could simply eat whatever we wanted. Fruit became artificially sweeter (thanks to genetic engineering). We also began to have access to the world’s sweetest fruits at any point in time. It is also not surprising that coronary heart disease, which was essentially unheard of in the 1930s, skyrocketed during and after the 1930s and continues to rise today. In the 1950s, Ancel Keys implicated fat as the culprit, with extremely weak epidemiological evidence which eliminated data that didn’t support his conclusion. And somehow, that lead to regulations and dietary enforcements over fat intake. The hypothesis has still never been shown to be true outside of epidemiology and cherry-picked data, yet it still greatly influences our society.

It simply makes no sense that a traditional food, widely eaten by every human throughout history, is causing the modern epidemic of disease and cancer. And yet when we do say fats are good, we say avoid saturated animal fat and instead prefer polyunsaturated fats like ‘vegetable’ oils. This again makes no sense. We never consumed these things until recently. In the early 1900s they were used as lubricants in machines. We then realized how cheap it is to mass manufacture, and decided it would be profitable to sell them as ‘healthy’ dietary items. Since most nutritional science is funded by the food and pharma industry [14], it’s not surprising that these made their way into dietary guidelines. Scientists have recently begun to speak out against the nutritional guidelines for their lack of scientific rigor [15].

In the 1980s, the dietary guidelines were introduced recommending more carb-based foods, more plants, vegetable oils, less meats, and all of our health problems began rising substantially. Since then we have only added more and more unnatural foods, like processed carbs, more vegetable oils, and more sugary desserts (and non-desserts). And again, mortality rates increased, leading us into our modern health epidemic. But why is the mediterranean diet is so healthy? It must be all of the plants, grains, and olive oil, right? Guess what else they eat: High fat meat, lots of butter, and lots of fatty cheese and cream. What about those healthy low-fat plant-based dieters? Cutting out processed carbs, sugar, processed vegetables oils, and eating whole foods is better than the Standard American Diet. There is also no evidence that adding meat to a largely plant-based diet leads to any negative health effects [16]. Meat is not the problem.

So are we even omnivores? We have the survival capabilities to live off of plants for years and even decades, but that hardly means we should be eating them instead of meat. We didn’t just evolve to eat meat, we evolved because we ate meat. The industrial revolution has only harmed our bodies and health. It is clear that eating mainly meat is what allowed humans to flourish over 2.5 millions years. There is no nutrition in plant foods that cannot be found in animals, but the same cannot be said for plants. If we continue to eat plant-based diets, our brains will continue to shrink, our gut will expand, and eventually we will find ourselves amongst the monkeys in the jungles like 2.5 million years ago.

Disclaimer: I am not saying you should abandon all fruits and veggies from your diet. I am simply trying to propose a way of thinking about our evolution, and challenge the conventional wisdom that meat is dangerous and that humans shouldn’t be eating it.

References

Humans are Carnivores

Written by Kevin MacPhee