Evolutionary adaptations to meat-eating in humans.

Abstract

This article reviews the some of the myriad of different genetic, physiological, morphological and nutritional adaptations in humans to eat meat.  These adaptations suggest human ancestors had a high reliance on meat and challenge the idea held by a substantial portion of the general public that humans are “naturally” herbivores and that our current meat-eating habit is facultative. On the contrary, the evidence suggests that humans are omnivores, being well equipped to eat substantial portions of animal tissue and have eaten meat since the dawn of our genus “Homo” millions of years ago.

Introduction.

[For the following review the word “meat” encompasses all animal tissue, i.e. mammals, reptiles, birds, insects, fish, etc.]

For millions of years, since our genus “Homo” originated about 2.5 million yea
rs ago and likely before that time also our ancestors have dined on large portions of animal matter, this view is supported by fossil evidence and isotonic evidence.  [1] [2] [3] Before the dietary shift to include more meat in our diet, our diet was probably like that of modern chimpanzees, mostly herbivorous containing plant foods including fruits, seeds, nuts, leaves and flowers with minor amounts of meat and insects. [4]

Today meat is still an important part of our diet, for example, a recent report from the OECD with the FAO has estimated the consumption amounts of the main types of meat eaten, for example, the United States consumed 13.5 kg/capita of poultry, 6.4 kg/capita of beef and veal, 12.5 kg/capita of pork, 1.7 kg/capita of sheep and 20.2 kg/capita of fish and seafood between 2013-2015 (with a smaller portion of other types of meat), and that amount is predicted to increase [5].

Despite our long evolutionary history and our current desire to eat meat, there is still a substantial portion of the public which believe that humans are “naturally” herbivores and that our current meat-eating habit is facultative.  The following is a review of the evidence for many adaptations in human beings for eating animal tissue.

Possible adaptations in humans for meat eating:

Nutritional adaptations-

  • Vitamin B12

Vitamin B12 (cobalamin) is an important vitamin that plays a key role in human health, deficiencies of which are able to cause megaloblastic anaemia, fatigue, weakness, constipation, loss of appetite, and weight loss as the main problems. Additionally, it can cause difficulty maintaining balance, depression, confusion, dementia, soreness of the mouth or tongue and poor memory, as well as being able to cause irreversible neurological damage. [6] [7]

The natural source for cobalamin in human is via the eating of animal products [6], as there are very few plant sources containing significate amounts of cobalamin, which likely would not have made up a significant portion of the human diet as we evolved.

Herbivores get most their cobalamin from gut bacteria which synthesise the vitamin. [8] In humans, however, it is unlikely that gut bacteria are able to act as a significant source of cobalamin, due to the fact that cobalamin produced by gut microbes represents a minuscule amount (about 2%) of the total corrinoid content in faeces and that it is produced in the colon, which is below the small intestine where cobalamin can be absorbed. [8] [9]

This evidence suggests an adaptation to meat eating, in order to get the required amount of B-12.  This conclusion gains even more support when studying people on vegetarian and omnivorous diets, which finds that there are lower levels of B12 in vegetarian and vegan diets and that vegetarians and vegans are at high risk of B12 deficiency than people who eat some meat. [10] [11] [12]

  • Taurine synthesis

Another adaptation, our species has a limited ability to synthesize is the biologically important amino acid, taurine, which is essential for cardiovascular function, and development and function of skeletal muscle, the retina, and the central nervous system [13] [14]; it also has many other beneficial, non-essential, effects; As such taurine deficiency can have severe adverse effects. Vegetarian and vegan’s diets in human’s result in lowered concentrations of taurine [15], which is found naturally in meats and fish, but hardly ever in plants [16].  Like felines [17] the need to internally synthesise taurine may have been evolutionarily reduced in humans because it had been obtained in the diet, which had relaxed the selective pressure formerly requiring the need to synthesise this essential amino acid.

  • 20 and 22 carbon fatty acids-

Like obligate carnivores [18], humans have an ineffective ability to chain elongate and desaturate 18 carbon fatty acids to form their product 22 and 20 carbon fatty acids. Since these fatty acids are crucial for the function of the cell membrane and brain tissue among other things [19], then evolutionary reductions in the enzymes desaturase and elongase activity, which are the enzymes responsible for removing two hydrogen atoms from a fatty acid, creating a carbon/carbon double bond and catalyse the elongation of an aliphatic chain. This indicates 22 and 20 carbon fatty acids must have been obtained via the diet, 20 and 22 fatty acids are found mostly in animal food. Although they can be found in plant foods the amount in plant sources are only trace quantities, and it is more likely that animal tissue was the main source of 22 and 20 carbon fatty acids available to our Hominid ancestors. [20] This indicates that animal foods were increasingly incorporated into our ancestor’s diet instead of them being synthesised from 18 carbon plant fatty acid.

Physiological and morphological adaptations-

  • Dental anatomy

Analysis of teeth from early Homo has shown that early Homo’s teeth are adapted to eat tough food, meaning early Homo was more adapted to fracture tough, pliant foods.  Meat seems to be most likely be this key tough-food resource, as one of the other main sources of tough foods early Homo could have obtained, USOs (carbohydrate-rich underground storage organs of plants) are often fairly brittle compared to animal tissue and that they are of limited nutritional value, so probably would not be a cornerstone resource. [21] Some recent studies which used dental topography to analyse dental specimens of our ancestor’s show that increased occlusal relief and steeper sloped cusps yield sharper cutting surfaces that would give animal tissues less of an opportunity to stretch and absorb energy, thereby thwarting the major toughening mechanism [21]. So it is probable increased consumption of animal products may have played a role in the dental adaptations of our genus [Homo].  Similarly, reviews of dental evidence of our ancestors which analyse Dental Microwear, Dental Structure, Occlusal Morphology and Tooth size has shown that the available evidence suggests a shift in diet in early Homo and especially H. Erectus with broadening of diet to include at least some more tough foods. [such as meat] [1].

However dental evidence is quite limited as there is only small numbers of samples of fossils from our ancestors.

  •   Gut morphology:

By observing the differences between the gut of carnivores, herbivores and omnivores and comparing their guts to our gut we can predict what diets we are adapted to.

Carnivores and herbivores have morphological differences in their gut.  Carnivores tend to have well-developed stomachs and long small intestines, herbivores tend to have a chambered stomach with well-developed caecum and colon. Humans fit neither of these patterns.  The human gut has a simple stomach, relatively elongated small intestine and reduced caecum and colon.  Which suggests a relatively high dependency on meat. [22] [23]

It should be noted that the gut is quite malleable and it is able to adapt slightly to the current diet, altering the proportions of the gut.  This plasticity does not change the fact that gut is adapted to eating at least some meat as gut plasticity is quite limited in scope.

It should also be noted that 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 contradictory results depending on which technique is used, some methods give results that the gut is firmly in the faunivory range and others suggesting the frugivore range. [24]

But I would say,  that the human gut is probably adapted to a mostly frugivorous diet, similar to that of organisms which eat mostly fruit with the inclusion of a large portion of insects and small amounts of vertebrates. [34]

  •  Haem Absorption:

Humans have the ability to digest haem iron due to intestinal receptor, which are specifically for the absorption of heme iron. In Western societies, iron derived from heme sources make up about two-thirds of the average person’s total iron stores despite only constituting one-third of the iron that is ingested [25] Although heme iron is found in plants at very low levels, the amount in plants is not nutritionally significant, and heme iron is sourced almost entirely from animal foods (Which also explains why vegetarians are more prone to very low iron levels than people who eat red and processed meat). [26] [27] The presence of these receptors 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 reliant on the absorption of ionic iron and that carnivores and other omnivores, such as pigs [28], are able to absorb this molecule.

Other possible adaptations-

  •  Meat-adapted genes:

There are some genes in the human genome that are adaptations to eating meat. Some hypothesis that the increased consumption of animal-sourced food during human evolution, selected for “meat-adaptive genes” to increase resistance to harmful effects of fat, toxins, and pathogens, delaying dysfunctions and diseases of the brain and heart, etc. caused by this increasingly meat-rich diet. And enable a major increase in life span, which if true could partly explain the difference in life expectancy between humans and the other great apes. [29] Meat-adaptive genes enabled the shift from a herbivorous ape diet to the more omnivorous diet of hominids.  However, this is just a hypothesis at the moment, and requires further research to be verified, for example, an alternative hypothesis is that there might be a genetic basis for food preferences, affecting the choice of meat in the human diet.  Either way the human genome has adaptations (or inclinations) for meat eating. [30]

  • Co-evolution with parasites:

Many parasites and their hosts undergo co-evolution [31]. Cestodes of the family Taeniidae are parasites of carnivores spread by eating meat. Taenia saginata (commonly known as “beef tapeworm”) and T. solium (commonly known as “pork tapeworm”) use humans as their definitive host, whereas these parasites are not definitive for the other great apes [32].    This parasite which specifically lives of humans indicates a substantial period of co-evolution and meat consumption by humans and their ancestors. It is hypothesised that dietary shifts by humans from herbivory to increased carnivory during the evolution of early Homo caused these parasites to start infection humans. [33]

Conclusion:

Human ancestors have consumed relatively large portions of meat, this long period of dieting on meat has caused adaptations to take place in humans, to get key nutrients and better consume animal matter.

If you were to take each of the evidences of adaptations listed above individually it would be possible to reject each of the points, but when taken all together it would be difficult to say that humans have no adaptations to meat eating, in fact the available evidence suggests that humans are extremely well adapted to meat eating.

The evidence outlined above shows clearly that as humans evolved and consumed greater amounts of meats and have developed genetic, physiological, morphological and nutritional adaptations to eating animal tissue.

References

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[34] Ungar, P. (2007). Evolution of the Human Diet: The Known, the Unknown, and the Unknowable. 1st ed. Oxford: Oxford University Press, pp.316-318.

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