Biology

Evolutionary Adaptations to Meat-Eating in Humans

Abstract

Humans have eaten meat since the dawn of the Homo genus around 2.6 million years ago. This article reviews the myriad of genetic, physiological, morphological, and nutritional adaptations in humans to eating meat. Literature reviews will be conducted on research involving the effects of vitamin B12, fatty acids and amino acids, haem absorption, meat-adaptive genes, and parasite co-evolution. Conversely, arguments that humans are adapted to live a herbivorous lifestyle are explored. Evidence suggests that humans are omnivores, being well equipped to eat substantial portions of animal tissue. 

Introduction

For clarification, in this article, “meat” encompasses all animal tissue (mammals, reptiles, birds, insects, fish). 

Since the genus Homo originated millions of years ago, ancestors of humans have consumed large portions of animal matter; this is evidenced by both archaeological remains and isotopic analysis.[1][2][3] Before the dietary shift towards meat, the Homo sapiens diet was most likely similar to that of modern chimpanzees; mostly containing plant foods such as fruits, seeds, and leaves, with minor amounts of meat and insects.[4]

Today meat is still an important part of our diet; a recent report from the OECD with the FAO has estimated the amounts of the main types of meat eaten. The United States consumed 48.9 kg/capita of poultry and 25.9 kg/capita of beef and veal, with these amounts predicted to increase.[5] Despite the long evolutionary history of meat-eating, there is still a substantial portion of the public and some academics that believe humans are “naturally” herbivores and that our current meat-eating habit is facultative.

Understanding the differences between our current and ancestral diet provides information to be used in evolutionary medicine, a field that explores health and disease relating to evolutionary history. For example, some arguments show that the paleo or “stone age” diet most closely fits our genetic makeup, as the primary theory behind the diet is to eat what our bodies are “designed” to eat. This suggests a relatively large amount of energy should be from animal foods.[42]  Regardless of discussions as to whether this is the “optimal” diet, it has attracted a large following. Alternatively, many vegan or vegetarian groups and individuals claim that humans are naturally herbivores – so a vegetarian diet is healthiest.

Possible Meat-Eating Adaptations in Humans

Nutritional Adaptations
Vitamin B12

Vitamin B12, or cobalamin, plays an important role in human health. Deficiencies in B12 can lead to megaloblastic anemia and fatigue, as well as being linked to several neurological issues, such as depression, dementia, and in extreme cases, irreversible neurological damage.[7][8] Cobalamin can be obtained in the human diet via consumption of animal products[7], as there are very few plant sources containing sufficient amounts of cobalamin, and thus would most likely not have made up a significant portion of the human diet throughout evolution. 

Herbivores get most of their cobalamin from gut bacteria that synthesize the vitamin.[9] In humans, however, it is unlikely that these bacteria are able to act as a significant source of cobalamin, as they only account for about 2% of the total corrinoid content in faeces. It is also produced in the colon, a part of the large intestine, so it cannot be absorbed into the small intestine.[9][10]

This evidence suggests an adaptation to meat eating in order to obtain the required amount of vitamin B12; a conclusion that is supported by the finding that there are lower levels of B12 in vegetarian and vegan diets.[11-13]

Taurine Synthesis

Our species has a limited ability to synthesize taurine, which is a biologically important amino sulfonic acid derived from cysteine. It is essential for the development and function of the retina, skeletal muscle, and the cardiovascular and central nervous systems.[14][15] Vegetarian and vegan diets in humans result in lower concentrations of taurine, which is found naturally in meats and fish, but hardly ever in plants.[16][17]  As taurine has been obtained in the diet, the need to internally synthesize taurine may have been reduced due to a lower selective pressure.[18]

20 and 22 Carbon Fatty Acids

Like obligate carnivores[19], humans have an ineffective ability to convert 18-carbon backbones to longer-chain fatty acids due to a lack of desaturase and elongase enzyme activity respectively. These fatty acids are crucial for the function of the cell membrane and brain tissue[20], despite humans not having the enzymes necessary to synthesise them. This indicates that 22 and 20 carbon fatty acids must have been obtained via a different method, namely through diets. Although they can be found in plant foods, they are only present in trace quantities, so it is more likely that animal tissue was the main source of the fatty acids available throughout Hominid evolution.[21] This indicates that animal foods were increasingly incorporated into our ancestors’ diet.

Physiological and Morphological Adaptations
Dental Anatomy

Analysis of teeth from early Homo species have shown that they are adapted to mechanically break down tough foods. Meat is the main example of this; other resources include USOs. These are carbohydrate-rich underground plant storage organs that are often fairly brittle and with limited nutritional value compared to animal tissue, so it is likely they would not be a cornerstone resource.[22] Recent studies used dental topography to analyse ancestral specimens, showing that increased occlusal relief and steeper-sloped cusps than that of related species as seen in Figure 1. This would reduce the possibility of animal tissues stretching and absorbing energy whilst being mechanically digested, thereby allowing easier consumption.[22] 

Figure 1: Mandibular cheek teeth of early Homo erectus, showing high occlusal relief. Image copyrights belong to the National Museums of Kenya (KNM-WT15000)[22]

Reviews of ancestral dental anatomy observing changes in microwear over time have suggested that food required more preparation from the incisors and greater amounts of molar shearing. The available evidence suggests a shift in the diet of early Homo genus, especially Homo erectus, to include some foods with tougher textures such as meat.[1] However, this evidence is still not definitive, as there are limited numbers of fossil samples from our ancestors.

Gut Morphology

Observing and comparing the gut structure of carnivores, herbivores, and omnivores can provide predictions of dietary adaptations. Carnivores tend to have well-developed stomachs and long small intestines, while herbivores tend to have a chambered stomach with a well-developed caecum and colon. Humans, however, fit neither of these patterns. The human gut has a simple stomach, relatively elongated small intestine, and reduced caecum and colon, suggesting a relatively high dependency on meat.[23][24]

It should be noted that the gut is quite malleable and thus its proportions have adapted to the current diet. This does not change the fact that the human gut is designed for eating at least some meat, as its plasticity is quite limited in scope. Equally, suggesting that the human gut has adaptations to meat eating does not necessarily mean that humans have a dietary preference to faunivory (animal matter-eating). Analysis of whether the human gut specification fits a preferred faunivorous or frugivorous (fruit-eating) diet often yields conflicting results depending on which analytic technique is used.[25] However, the human gut is probably more firmly implanted into the frugivore range, similar to that of a group of organisms that eat mostly fruit, including a number of insects and some vertebrates.[26]

Haem Absorption

Humans have the ability to digest haem iron due to specific cell surface receptors and transporters from the extracellular to the intracellular environment[27]. Haem iron is only found in plants at very low levels, so the amount in plants is not nutritionally significant, and thus iron is sourced almost entirely from animal foods. This also explains why vegetarians are more prone to lower iron levels than meat-eaters.[28][29] The presence of these receptors and transporters indicate a physiological adaptation to animal foods in the diet; this point is further compounded by the fact that herbivores are unable to absorb these haem complexes and are therefore reliant on ionic iron, whereas carnivores and other omnivores are not.[30]

Other Possible Adaptations
Meat-Adapted Genes

Some scientists hypothesize that the increased consumption of animal-sourced food during human evolution selected for “meat-adaptive genes”, in order to increase resistance to harmful effects of fat, toxins, and pathogens, and to delay dysfunctions in the brain and heart, that are associated with consuming increasing. These genes may also enable a major increase in lifespan, which could partly explain the difference in life expectancy between humans and other great apes.[31] Meat-adaptive genes enabled the shift from herbivorous to omnivorous diets. However, this is currently just a hypothesis, and requires further research to be verified. 

Another hypothesis is that there might be a genetic basis for food preferences, affecting the choice of meat in the human diet. Indeed, studies have found that dietary patterns are partly heritable; for example, the heritability of red meat consumption in a UK sample was 39%, and certain genes have been found to correlate with increased meat consumption.[32][45] 

Co-evolution with Parasites

Many parasites and their hosts undergo co-evolution.[34] For example, Taeniidae are a family of parasites spread by the consumption of meat. T. saginata and T. solium are more commonly known as beef and pork tapeworm respectively, and use humans as their definitive host as opposed to other great apes.[35] This indicates a substantial period of co-evolution between parasites and humans due to increased meat consumption during the evolution of early Homo.[36]

The Herbivore Argument

As stated in the introduction, there are still many who believe humans are herbivores, despite overwhelming evidence opposing this. Many of these people are part of animal rights or vegan groups, as part of spreading their ideology. For example, the well-known animal rights organization PETA holds the position that: “According to biologists and anthropologists who study our anatomy and our evolutionary history, humans are herbivores who are not well suited to eating meat.”[37]

There are also a few academics who hold this position, such as Milton R. Mills, M.D. in his article The Comparative Anatomy of Eating,[38] and William Clifford Roberts, M.D who concludes that humans “are natural herbivores” in an article published in 1990.[39] The contents of this article will now be considered.

Before addressing the individual evidence, a key assumption made by Roberts is that humans must either be natural herbivores or carnivores; he allows no option that humans could be omnivores.  This binary view on diet means that the evidence presented by Roberts is merely a comparison between humans, carnivores, and herbivores. It should also be noted that Roberts provides very few sources for his claims. 

Firstly, Roberts argues that humans cannot be carnivores due to a lack of claws or sharp teeth, which are necessary to hunt and kill prey. However, there is no scientific reference provided that assures a universal requirement for these features in carnivores. As stated before, diet is binary, as there are more than two options to choose from. Humans have developed tools throughout evolution which have been used to butcher carcasses and hunt for millions of years.[40-42] In fact, large claws could be more of a hindrance than an advantage, as humans use their hands for more than just hunting. From the studies previously discussed, human teeth are in fact adapted to break down tougher foods.

Roberts also states that the human gut is more similar to that of a herbivore than a carnivore due to their respective intestine lengths. However, this is a very simplistic examination of the intestines, and from more detailed gut morphology analyses, the human gut is adapted to diets with relatively high proportions of animal tissue. Another distinction between carnivores and herbivores is in their methods of thermoregulation. Carnivores lap up water and pant to cool down, whereas herbivores sweat and sip water. Roberts claims that since humans also sip water and sweat, they display behaviours more similar to that of herbivores. This, however, is a point assuming that the cooling and drinking method is a universal rule for distinguishing between the two; in actuality, methods of cooling and drinking have little to do with the diet of an organism, and so lacks substantial evidence.

Roberts then says that carnivores can synthesise vitamin C, whereas both herbivores and humans cannot. The inability to synthesise vitamin C is due to mutations in the GLO gene, which codes for L-gulonolactone oxidase, an enzyme catalysing intermediate reactions in vitamin C synthesis.[43] Contrary to what Roberts states, these GLO gene losses and reactivations are unrelated to the diet of the species involved, thus the lack of vitamin C synthesis does not imply that an organism is a herbivore.[43] 

Figure 2: Phylogenetic tree showing whether the mammal species can or cannot synthesise Vitamin C are shown, those in black can and those in grey cannot. 
 As can be seen, there is little correlation between the expression for this enzyme and the dietary behaviour of the species.[43]

Finally, Roberts argues that carnivores are not sensitive to cholesterol-induced atherosclerosis although humans and herbivores are, further proving that humans share more dietary characteristics with herbivores than carnivores. To support this idea, one of the studies cited indicated that canines did not develop atherosclerosis even with average cholesterol levels 200 times that of a human. In contrast, adding roughly two grams of cholesterol to a rabbit’s diet over a period of two months resulted in significant changes in the lipid accumulated content of the arteries. To counter this argument, although carnivores are less likely to develop atherosclerosis, the probability of developing this disease is not zero. For example, diet-induced atherosclerosis can be caused in the domestic cat, and similar patterns are seen in omnivores such as pigs and mice.[44][45] Therefore, atherosclerosis formation does not necessarily correlate the diet of an organism.

Another often-cited piece of evidence that humans are herbivores is that meat consumption is linked to a host of health problems.[37] Indeed, meat consumption (especially red and processed meat) is often linked to increased risk of developing a number of cancers, diabetes, stroke, heart disease (albeit inconsistently), and obesity.[46-48] This, however, does not necessarily mean that humans are not evolved to eat meat. It is possible that the increased risk of diseases is a maladaptive effect; in the past it may have been quite beneficial to consume meat, but in modern times where meat is more readily obtained, our naturally-evolved inclination towards meat results in increased consumption, which leads to negative effects on health. Also many of the negative effects of meat could be due to cooking, processing and preservatives, rather than the meat itself.[49]

The idea that humans are herbivores is not without its merits. Our ancestors and our closest living relatives, chimpanzees, eat little to no meat. In light of the fact that meat seems to have negative health effects, it is not difficult to see why some argue that humans should be natural herbivores.

Conclusion

Long periods of meat consumption in members of the Homo genus have resulted in evolutionary adaptations in humans, to obtain key nutrients and better consume animal matter.

In light of all evidence presented, it would be difficult to say that humans have no adaptations to meat eating. In fact, the available evidence suggests that humans are quite well adapted to meat eating. As humans evolved and consumed greater amounts of meat, they developed genetic, physiological, morphological, and nutritional adaptations to eating animal tissue. Evidence cited by some arguing that humans are natural herbivores is lacking in comparison, although theoretically it is plausible. It must be noted, however, that some members of animal rights organizations and vegan and vegetarian groups may be controversially advocating for this in order to push their agendas to the general public. Further evidence should be collected before propagating a certain worldview or lifestyle in order to avoid moral quandaries.  

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About the Author

Jake Creasey, UK

Jake is a student currently studying for his A-levels in Geology, Biology, Maths and Chemistry.  He has a real passion for evolutionary theory, spending a large chunk of his time studying it, and wishes to go on to study it at university.

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