Are humans meant to eat meat?

The objection

“Our ancestors ate meat, and it grew our brains. Our stomachs are as acidic as a scavenger’s, we have canine teeth, and we need B12, which plants don’t provide. We’re built for meat.”

The answer

The objection gets our past right. Humans are omnivores, our ancestors ate meat, and in a 2015 review of 68 bird and mammal species the human stomach was about as acidic as those of the carrion-eating birds in the sample [1]. What a body can digest is a separate question from what a person needs to eat, and the dietitians’ bodies of the UK, Canada and, for adults, the US say a well-planned vegan diet can meet those needs [2][3][4], the UK body adding that vegans need a reliable source of B12 [2].

Did eating meat help our brains grow?

Meat may have helped, but the evidence is weaker than the objection assumes: the best-known version of the idea, Aiello and Wheeler’s (1995) argument that a diet richer in animal products let a smaller gut pay for a bigger brain, failed a 2011 test across 100 mammal species, and a 2022 analysis found no sustained rise in evidence of meat-eating after Homo erectus appeared [5][6][14]. Aiello and Wheeler compared the organs of a “standard” 65 kg man with those expected for a primate of his weight. His brain weighed 1,300 g against an expected 450 g, while his gut, at 1,100 g, was only about 60% of the expected size, so the extra brain and the missing gut roughly cancelled out (their table 4, below) [5].

Organ	Expected for a 65 kg primate	Observed in a 65 kg man
Brain	450 g	1,300 g
Gut (gastrointestinal tract)	1,881 g	1,100 g
Liver	1,563 g	1,400 g
Heart	320 g	300 g
Kidneys	238 g	300 g
All five organs	4,452 g	4,400 g

They argued that a gut that small works only on high-quality, easily digested food, and concluded that eating more and more animal products “was essential in the evolution of the large human brain” [5]. In their conclusion they counted nuts and underground tubers, as well as animal products, among those high-quality foods, and suggested that cooking, which makes food easier to digest, might have helped later increases in brain size [5].

The trade-off at the heart of that account has not held up well. Navarrete and colleagues (2011) tested it in 100 mammal species, 23 of them primates, and found that, once fat-free body mass was taken into account, brain size was not negatively correlated with the mass of the digestive tract or of any other expensive organ, and they concluded that the expensive-tissue hypothesis was refuted [6]. Their paper accepts that the human digestive tract is smaller than expected for a primate of our size, and proposes instead that steadier energy supplies and energy diverted from movement, growth and reproduction paid for the human brain [6].

How much meat our ancestors ate is argued over too. Reviewing 25 sources of evidence, Ben-Dor and colleagues (2021) concluded that the human lineage rose from a low base to “a high, carnivorous position” in the food chain during the Pleistocene, beginning with Homo habilis and peaking in Homo erectus, before a reversal that began in the Upper Palaeolithic and culminated with the arrival of farming [7]. Others read the record differently. A 2022 analysis of the east African fossil record from 2.6 to 1.2 million years ago found that, once it corrected for how well each period had been sampled, there was no sustained increase in evidence of meat-eating after Homo erectus appeared, and its authors said this undercuts accounts that tie that species’ human-like traits to eating more meat (Barr et al., 2022) [14]. From the dental plaque of Neanderthals and early modern humans, Fellows Yates and colleagues (2021) found that oral bacteria able to bind the starch-digesting enzyme amylase are a general feature of Homo, which they took to suggest that starch-rich foods became important early in Homo evolution, before the Neanderthal and modern human lineages split more than 600,000 years ago [8]. Ben-Dor’s group replied that the oral microbiome cannot predict how much starch people ate in the Pleistocene [17]. Genetics offers a hint in the same direction as Fellows Yates: earlier studies had linked the number of copies of the salivary amylase gene, AMY1, to starch-rich diets, and a 2024 study comparing present-day human genomes with ancient and archaic ones suggested that a common three-copy version of the gene dates back as far as 800,000 years (Yilmaz et al., 2024) [16]. Even on Ben-Dor’s reading the diet changed over time, so there is no one ancestral diet to copy [7].

Are humans omnivores or herbivores?

Humans are biological omnivores, and the claim that we are herbivores by nature is wrong. Beasley and colleagues (2015) gathered stomach pH measurements for 68 species and found the human stomach, at pH 1.5, closer to the stomachs of carrion-eaters than to those of most carnivores and omnivores. Their group averages, in the chart above, were 1.3 for obligate scavengers, all of them birds that live mainly on carrion, 1.8 for facultative scavengers, which eat it some of the time, 2.2 for generalist carnivores, 2.9 for omnivores, a group that includes humans, 4.1 for plant-eaters that ferment their food further along the gut, such as horses, and 6.1 for those, such as cattle, that ferment it in a chambered stomach [1]. The authors offer two explanations, either that scavenging mattered more in human evolution than is usually thought, or that strong acid was favoured as a defence against infection whatever the diet, and they found no stomach pH data for any other ape [1].

Our inability to make vitamin C, sometimes taken as the mark of a fruit-eater, says nothing about whether we are meant to eat plants or meat. Every anthropoid primate shares it, because the gene for the last enzyme in the pathway stopped working in their lineage about 61 million years ago, and so do guinea pigs, teleost fish and some bats and perching birds. A 2011 review found these losses unrelated to the diets of the species involved, beyond those diets all being rich in vitamin C [10].

Our bodies are a generalist’s. Homo erectus already had relatively smaller teeth and gut, smaller chewing muscles and a weaker bite than earlier hominins, and from chewing experiments in people, Zink and Lieberman (2016) calculated that if meat made up a third of the diet, hominins would have needed 13% fewer chews a year, and that slicing meat and pounding roots and tubers with stone tools would have improved their ability to chew meat into smaller particles by 41% [9]. With cooking apparently uncommon until 500,000 years ago, they concluded that stone tools and meat together first made those smaller teeth and chewing muscles possible [9]. The canine teeth in the objection have an answer of their own. A body that chews meat better with a blade, a gut smaller than expected for a primate of our size and a stomach acidic enough to kill food-borne microbes are a record of what our ancestors could eat.

Whether we must eat meat is a question for nutrition science, and for adults the main dietitians’ bodies agree that a planned vegan diet can meet our needs. The British Dietetic Association says plant-based diets “can support healthy living at every age and life stage” and can provide all the nutrients needed for good health, with the exception of vitamin B12, which has to come from fortified foods or a supplement [2]. Dietitians of Canada (2020) says “anyone can follow a vegan diet”, from children to older adults, and calls it healthy even for pregnant or breastfeeding mothers [3]. The US Academy of Nutrition and Dietetics, in a 2025 position covering only adults who are not pregnant or breastfeeding, says appropriately planned vegan diets “can be nutritionally adequate and can offer long-term health benefits” [4]. The German Nutrition Society is more cautious. In 2024 it said a vegan diet can be health-promoting for the general population provided B12 is supplemented, food is well chosen and planned, and other potentially critical nutrients are covered, if need be with further supplements, but that the data are too limited for it to recommend for or against one for children, teenagers, pregnant or breastfeeding women and older people, for whom it strongly recommends advice from qualified specialists [11]. For children, a 2025 position paper from ESPGHAN, the European society for paediatric gastroenterology, hepatology and nutrition, found the evidence inconclusive on whether a strictly vegan diet supports normal growth, though the studies it reviewed, covering about 1,500 vegan children, found no significant differences from omnivorous children in height or body mass index z-scores; it recommends regular checks of diet, growth and nutritional status [15]. The answer on whether you need meat to be healthy sets out what the long-running studies of people who eat no meat have found.

Does needing B12 mean we’re built for meat?

Needing B12 shows that we need a reliable source of a vitamin made only by certain bacteria and archaea, never by plants or animals, and while most people get it from animal foods, fortified foods and supplements are reliable sources too [2][12]. The vitamin builds up in animal tissues from those microbes; cattle and sheep, for instance, absorb what the bacteria in their stomachs make, and a 2018 review concluded that humans get their B12 mainly from ruminants and from fish and shellfish (Watanabe and Bito, 2018) [12]. Plants rarely contain much of it, and what they do contain comes mainly from bacteria in the soil or on their surfaces [12]. The B12 in supplements and fortified foods comes from bacteria too: only a few bacteria and archaea can make it, so commercial production relies on microbial fermentation (Fang and colleagues, 2017) [13].

For anyone who eats no animal products, the BDA says the only reliable sources of B12 are fortified foods and supplements, and warns that too little can cause fatigue, anaemia and nerve damage [2]. It passes on the advice of the Vegan Society, a vegan charity: eat fortified foods at least twice a day, aiming for 3 micrograms of B12 a day, or take a supplement of at least 10 micrograms daily or 2,000 micrograms weekly [2]. For a vegan child, or during pregnancy or breastfeeding, get the dose from a GP or dietitian: the German Nutrition Society strongly recommends advice from qualified specialists for these groups [11], and ESPGHAN says supplements of specific micronutrients, B12 among them, are essential for children on a strict vegan diet [15].