Evolutionary evidence consistently indicates that ancestral humans were habitual meat eaters. Adaptations related to hunter-gatherer lifestyles have shaped human anatomy, physiology, and metabolism. In general, animals thrive best on diets resembling the ones to which they are physiologically adapted - it is unlikely that Homo sapiens constitutes an exception to this principle. These adaptations include changes in cranial-dental and intestinal morphology and function, erect posture, reproductive characteristics, longer lifespan, brain expansion, and alterations in the gut microbiome. The building of a larger brain was supported by a reduction in gut size and the consumption of energy and nutrient-rich foods. Metabolic adaptations in humans include a dependency on nutrients found predominantly in animal source foods, such as vitamin B12, choline, taurine, and certain fatty acids like DHA and AA.
This article addresses the following three questions:
- Why is an evolutionary perspective relevant for nutritional theory?
- Which anatomical adaptations reflect the evolutionary importance of animal source foods?
- Which metabolic adaptations reflect the evolutionary importance of animal source foods?
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Why is an evolutionary perspective relevant?
Any debate about the healthiness of animal source foods should consider whether they are part of human evolutionary and species-appropriate diets. While there might not be a single optimal human diet, an evolutionary perspective indicates that humans evolved as habitual meat eaters rather than facultative ones. The vegetarian argument for a naturally herbivorous human diet is based on a phylogenetic relationship with apes. This assumption is extremely naïve. By focusing on the diet of common ancestors, it overlooks the evolutionary divergences that separate humans from non-human primates. Moreover, even chimpanzees eat substantial amounts of meat. Vegetarianism lacks an evolutionary precedent within the hominin lineage; without doubt, hominin ancestors were adapted to obtain nutrients from both plants and animals.
Further reading (summary of the literature):
It would be highly unlikely that Homo sapiens constitutes an exception to the fact that animals thrive on species-adapted diets [Leroy & Cofnas 2020]. Any discussion on the healthiness of ASFs should at least address whether or not they are part of our species-appropriate diets [while acknowledging that there may not be a single optimal human diet; Pontzer et al. 2018]. From an evolutionary angle [see elsewhere], Homo emerged with the anatomical and physiological equipment of a habitual rather than facultative meat eater [Henneberg et al. 1998; Leroy et al. 2023]. Those who argue that the human diet is naturally herbivorous based on a phylogenetic relationship with apes [Barnard & Leroy 2020], overlook the divergences that occurred during evolution [Leroy & Barnard 2020]. Undeniably, our hominin ancestors are adapted to procure nutrients from both plants and ASFs, whereas veganism is without evolutionary precedent [O'Keefe et al. 2022]. Even some of the non-human primates, chimpanzees in particular, are known to regularly consume meat [Kaplan et al. 2000; Nishida 2012; Watts 2020; Pontzer & Wood 2021]. Moreover, the ability of some humans to subsist on vegetarian diets for prolonged periods may not be universal, for instance because of interindividual genetic differences that are related to lipid metabolism and brain function [Yaseen et al. 2023].
Anatomical adaptations
During human evolution, a shift to diets based on meat and animal fat led to adaptations in cranial-dental and intestinal morphology and functions, posture, reproductive traits, lifespan, and brain size. The expensive tissue hypothesis argues that such new and richer diets facilitated the formation of an increased brain size, not only by providing essential building blocks and calories, but also by enabling a sizeable reduction of the energy-demanding gut. The small intestine increased in size, while the large intestine responsible for fermentation decreased, differing from apes. The gut microbiome also changed to accommodate higher meat and fat intake. Transitioning from fibrous plants to animal source foods and tool use caused reductions in teeth, jawbones, and chewing muscles, resulting in weaker bite force. Humans spent less time on feeding compared to other apes, reflecting changes in diet. The human stomach became highly acidic, possibly to protect against meat-borne pathogens like carnivores. Finally, the human lineage showed anatomical and genetic adaptations related to hunting, running, heat regulation, vision, breathing, throwing, and clubbing.
Although one has to be careful when interpreting the incomplete human fossil record [Wood & Smith 2022], the probable conclusion is that a substantial ASF intake during most of human evolution has contributed to cranial-dental and intestinal morphological change, human erect posture, reproductive characteristics, longer lifespan, and brain expansion [Isler & Van Shaik, 2014; Baltic & Boskovic 2015; Mann 2007, 2018]. In addition, the gut microbiome is thought to have changed substantially [Moeller et al. 2014; Amato et al. 2015; Beasley et al. 2015], likely towards an adaptation to higher meat and fat intake [Moeller et al. 2014; Domínguez-Rodrigo et al. 2021]. Anatomical and genetic changes in the human lineage [Okerblom et al. 2018] are suggestive of ecological adaptations that involve the pursuit and hunting of animals, related to endurance running [Bramble & Lieberman 2004; Lieberman et al. 2006; Lieberman & Bramble 2007; Ruxton & Wilkinson 2011; Glaub et al. 2017; Holowka & Lieberman 2018], heat loss [Lieberman 2015; Halsey & Brice 2020], vision [Kobayashi & Kohshima 2001], breathing [Bramble & Carrier 1983; Carrier 1984; Franciscus & Trinkaus 1988], throwing [Darlington 1975; Isaac 1987; Knüsel 1992; Watson 2001; Young 2003, 2008; Roach 2012; Roach et al. 2013; Roach & Lieberman 2014; Roach & Richmond 2015; Kuhn 2016; Lombardo & Deaner 2018a,b], and clubbing [Young 2003, 2008]. Some of these traits and changes can be traced back to Homo erectus [Ruxton & Wilkinson 2011; Roach & Richmond 2015; Pontzer 2017; Hora et al. 2020; Domínguez-Rodrigo et al. 2021], others perhaps even earlier [Isaac 1987; Ungar 2012]The shift from fibrous plants to the inclusion of substantial amounts of ASFs, together with the use of tools, paralleled a decrease in teeth size and jawbones, a reduction in chewing muscles, and weaker maximum bite force capabilities [Teaford & Ungar 2000; Zink & Lieberman 2016]. Homo molars gained steeper slopes and more relief, also suggestive to an adaptation to meat eating [Ungar 2004]. Starting with Homo erectus, humans developed smaller molars and began to spend a lot less time on feeding than would be predicted from body mass and phylogeny with other apes (only 5% instead of a predicted 48% of daily activity in Homo sapiens) [Organ et al. 2011]. Likely as a protection towards meat-borne pathogens, the human stomach evolved into one of the most acidic in the animal kingdom, similar to carnivores and scavengers [Beasley et al. 2015; Dunn et al. 2020]. Adaptation also led to a small intestine comprising 56% of total gut volume and a shrinkage of the large intestine (and therefore fermentative capacity) to a mere 20%; which is the inverse situation of what is found in apes [Milton 2003]. The surface area of the human digestive tract is surprisingly small [Helander & Fändriks 2014], both human intestinal area and length being closer to cats and dogs than to herbivores or even baboons [Henneberg et al. 1998].According to the 'expensive tissue hypothesis', an increase in brain size was made possible - under selective pressure for more cognitive capacity - by an overall reduction in the size of the energy-consuming gut [Aiello & Wheeler 1995], as well as by the supply of energy and nutrients via ASFs (e.g., iron, zinc, vitamin B12, choline, docosahexaenoic acid, fat, cholesterol) [Chamberlain 1996; Mann 1998; Gupta 2016]. The inclusion of food processing within that dietary shift, mostly the use of fire and food fermentation, may also have contributed to this evolution by 'externalizing' part of the digestion [Bryant et al. 2023]. Nicotinamide (vitamin B3) has been explicitly cited as a key brain-trophic element in ASFs [Williams & Dunbar 2013; Williams & Hill 2017a,b]. The exceptionally high energy needs of the brain may also be the reason why humans - infants in particular - have higher body fat than non-human primates [Kuzawa 1999; Cunnane & Crawford 2003]. While the brain of an adult primate consumes <10% of the total resting metabolic rate, this amounts to 20-25% in the case of anatomically modern humans [Leonard et al. 2003].
Metabolic adaptations
Human energy metabolism is adapted to diets dominated by lipids and proteins. A comparison of the age at weaning of different animal groups suggests that early weaning in humans was facilitated by switching from maternal milk to nutrient-dense meat, marrow, organs, and fat. A comparison of the age at weaning of different animal groups suggests that early weaning in humans was facilitated by switching from maternal milk to nutrient-dense meat, marrow, organs, and fat. The enduring consumption of such foods during the Pleistocene has fundamentally changed the way the human body deals with nutrients. Humans show a preference for absorbing haem iron over ionic forms, which is not observed in herbivores, have mostly lost the ability to absorb vitamin B12 produced by gut bacteria in the absence of coprophagy, have a higher dependency on choline, abundant in animal source foods, compared to other primates, and a reduced ability to produce taurine from amino acids and convert alpha-linolenic acid into long-chain fatty acids like eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The human brain not only requires a very high amount of energy but also relies on DHA and arachidonic acid, which is harder to achieve adequately with plant-only diets.
Further reading (summary of the literature):
Due to the enduring consumption of ASFs - and in the absence of coprophagy - humans have mostly lost their ability to absorb vitamin B12 produced by gut bacteria [Domínguez-Rodrigo et al. 2012], although a limited remaining contribution of the colon has been suggested [Kurpad et al. 2023]. The reliance on ASFs, and meat in particular, may also be an explanation for the preferential absorption of haem iron over ionic forms in humans but not in herbivores [Henneberg et al. 1998; Mann 2007]. Likewise, a higher dependency on choline, most abundant in ASFs, is seen in comparison to other primates [Domínguez-Rodrigo et al. 2021]. Moreover, a reduction is seen in the human potential to produce taurine from amino acid precursors [Chesney 1998; Mann 2007] and to convert alpha-linolenic acid into the biologically important long-chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [Stark et al. 2015; Hodson et al. 2018; Pignitter et al. 2018; Greupner et al. 2018]. Humans, therefore, may not be able to make sufficient amounts of DHA for normal infant brain development [Cunnane et al. 2007]. The human brain has not only a particularly high requirement for energy, but also for DHA and arachidonic acid (AA). Even in the unlikely case that a high enough caloric density would have been available from nuts and seeds, plant-only diets would have been unable to deliver enough preformed DHA and AA [Cordain et al. 2001].Adaptation to the eating of ASFs can also be inferred from a comparison of the age at weaning of herbivores, omnivores, and carnivores [Psouni et al. 2012]. For humans, an early age was enabled by a switch from maternal milk to nutrient-dense meat, marrow, organs, and fat [Kennedy 2005]. Overall, human energy metabolism is adjusted to diets dominated by lipids and proteins [Pond & Mattacks 1985; Finch et al. 2004; Swain-Lenz et al. 2019].
The degree to which the human microbiome - which is a very variable constellation to begin with - reflects evolutionary adaptations to animal source foods intake is to be considered as developing science. It is possible that Neanderthals [Speth 2017] or even H. erectus already consumed fermented meat and fish, potentially affecting the gut microbiome [Dunn et al. 2020].
