The Diet We’re Meant to Eat, Part 2: Physiological & Biological Evidence

The subject of whether humans are naturally suited for plant-only diets or meat-only diets (or something in between) has been the source of ongoing controversy and some very strong opinions. It’s a topic in which I feel there’s much more misinformation out there than well-explained accurate science, so I’m tackling the question from all the angles.

In the first installment of this series, I explained why evidence from human history and modern hunter-gatherer populations squarely points to the idea that we’re very much omnivores. For literally millions of years, both humans and the ancestors we descended from consumed a mixture of plant and animal foods—never just one or the other.

But, there’s still more to the story! Often, arguments for the human species being truly herbivorous or truly carnivorous are based on analyses of other primates’ diets, comparative anatomy, and other features that might help clarify our taxonomy (e.g., teeth and length of digestive tract). After all, just because we choose to eat both plants and animals doesn’t mean that’s what we’re biologically suited for. We might choose to eat Krispy Kreme donuts when they’re sitting on the counter, too, but that doesn’t make them good for us!

So, in this post, we’ll be looking at the anatomical and physiological arguments for what humans are best suited to eat, and seeing what we can (or can’t!) conclude from them.

Nonhuman Primates: Are Our Closest Relatives Really Herbivores?

A common justification for humans being herbivores comes from comparative anatomy with other primates—especially chimpanzees, one of our closest living relatives. The argument typically goes that 1) the higher primates are essentially herbivores (with a trivial amounts of insect on menu, but not enough to be nutritionally important) and 2) human anatomy is very similar to primate anatomy, so 3) the diet they eat resembles the one we’re suited for. Okay, that sounds logical on the surface, but does it really hold up under scrutiny? Let’s look deeper at the evidence to find out!

First of all: it’s totally true that chimpanzees love eating fruit, leaves, and other vegetation, and these foods comprise the bulk of their diet. But, chimpanzees are actually far from pure herbivores. In every single site where chimps have been studied long-term, they’ve been observed to hunt and share meat—especially red colobus monkeys, which are their preferred prey. Their hunting strategy involves going out in groups, patrolling for smaller vertebrates, and deliberately chasing their future dinner over long distances (although chimps will also hunt spontaneously when a tasty opportunity arises). And, it turns out chimps are pretty impressive hunters: they have over a 50% success rate for their attempted kills!

Not only that, but certain groups of chimpanzees actually use tools to hunt—a behavior we used to think was exclusive to humans. Both male and female chimpanzees in Fongoli, Senegal will whittle branches into sharp spears, which they use to capture and impale bush babies, mongooses, vervet monkeys, patas monkeys, bushbucks, and baboons. During a recent 10-year study, researchers observed a total of 308 instances of tool-assisted hunting (and that’s not counting the numerous other hunting instances that didn’t feature tools). Both the frequency of hunts and the deliberation that goes into planning them (strategy-wise and tool-wise) implies that for chimpanzees, meat consumption isn’t trivial, but an important part of their dietary ecology.

Evidence of meat-eating even shows up in chimpanzees’ body tissues—which helps counter the argument that their meat consumption is an infrequent luxury rather than something nutritionally important (a claim sometimes put forth by proponents of the idea that chimpanzees are nearly vegan). In a 2013 study in Taï National Park, Ivory Coast, nitrogen isotopes in male chimpanzees’ hair keratin (which reflects short-term protein intake) as well as their bone collagen (which reflects long-term protein intake) revealed levels that could only be reached with a regular intake of meat. Contrary to the long-standing belief that chimps get most of their protein from nuts and fruit, the researchers concluded that for the residents of this territory, “meat is a frequently sought-after protein resource.”

Insects: Creepy, Crawly, and Nutritious!

Along with chimpanzees’ meat intake, large-bodied great apes are well known connoisseurs of another animal food: insects! Chimpanzees frequently consume ants, termites, caterpillars, bees, wasps, and beetle grubs; bonobos (who are about as close to us genetically as chimpanzees are) feast on larva, termites, ants, earthworms, millipedes, and bees; orangutans love termites, ants, bees, crickets, caterpillars, and gall wasps; and mountain gorillas dine on ants, while western gorillas consume a wider variety of insects. Whew!

Despite insects’ low calorie yield relative to other foods (like ripe fruit), many primate populations spend a disproportionate amount of time searching for insects to eat—and often go to great lengths to find them. In Senegal, male chimpanzees allot almost a quarter of their foraging and feeding time to finding termites, despite the fact that insects only form about 1% (by weight) of their total diet (that figure is similar for most chimpanzee and gorilla populations). Some primates even use tools to speed up the insect-catching process: wild Sumatran orangutans make tools out of twigs, sticks, and branches to extract and consume insects, while wild chimpanzees often use branch or twig “wands” to catch ants (along with digging sticks to excavate ant nests).

Plus, we have evidence that insects aren’t only a “fallback food” when primates’ preferred items are scarce. For instance, even when ripe fruit is dripping from the trees, researchers have observed chimpanzees of the Nimba Mountains of Guinea still actively preying on ants—implying that they innately value insects regardless of whether other foods are abundant.

Some proponents of the “chimpanzees are herbivores” (and “humans are herbivores by genetic proximity!”) arguments make the mistake of assuming that just because insects form a relatively small volume of the diet, they don’t offer anything nutritionally significant. Actually, that couldn’t be further from the truth! Nutritional analyses show that even at only 1% of food intake by weight, insects provide significant levels of protein, vitamin B12, iron, manganese, sodium, copper, and other nutrients that are low or absent in plant foods. Unlike vegetation (which contains limited amounts of certain amino acids), the composition of insect protein consists of 46 to 96% essential amino acids, making them a valuable source of high-quality protein. Insects also supply amazing amounts of fatty acids, including essential fatty acids:

And, the great apes seem to have a knack for honing in on the absolute nutrient-richest insects they can find. In Cameroon, chimpanzees have been observed to selectively choose insects higher in fat and protein (and pass over the insects that are lower in these nutrients), and in Tanzania, chimpanzees routinely pick only the insects that are richest in iron, manganese, and other minerals. Likewise, when some primates suffer the aftermath of eating too much laxative fruit, they appear to eat termites for their antidiarrheal properties (nature’s version of Pepto-Bismol!).

So, can we really call chimpanzees “herbivores” when they deliberately go out to hunt meat, fashion tools specifically for catching animal foods, and consume large amounts of insects that supply nutrition sorely lacking in plants? I think the answer is a resounding no! And if genetic proximity is a valid argument, I think this points to the importance of animal protein (and dare I say insects???) in human diets as well!

Clues From Gut Morphology

Let’s answer that question a different way. We’ve established that our closest living relatives are technically omnivores, but let’s look at how much their diet informs the optimal one for us. We may share some impressive genetics, but how much anatomically do we have in common with other primates—and what does that tell us about the human diet?

In a general sense, the “design” of our digestive tract is similar to our great ape relatives: we all have the same gut anatomy that features a simple acid stomach, a small intestine, a small cecum, an appendix, and a colon. But, that’s where the similarities end. In humans, the greatest total gut volume (over 56%) is in the small intestine, whereas in other primates, the greatest total gut volume (over 45%) is in the colon. Basically, our colons are proportionately much smaller than we see in other primates, who have giant hindguts perfect for breaking down woody seeds, seriously tough plant fiber, and other bulky material. The small-intestine-dominant guts of humans, on the other hand, suggest we’re adapted to higher-density foods (like meat and cooked starches) that are easier to digest than hardcore plant roughage.

Although we can’t say for sure when our dramatic change in gut morphology occurred, the changes do suggest that we’ve adapted to a higher-quality omnivorous diet than our early ancestors and extant relatives. Both dense fat and protein sources (like meat) and dense unrefined carbohydrate sources (like starchy roots and tubers) likely allowed our intestinal anatomy to change into its current form over the few hundred thousand years (or more!) since our ancestors started routinely using fire for cooking (see more here). Of course, other components of our early ancestral diets can still benefit us, too: the rich micronutrient and phytochemical content of plant foods still demonstrate profound health benefits for us, and insects continue to be a part of indigenous human diets across the globe! (Exo bars, anyone?)

Amylase and Lactase: More Evidence for Omnivory (and Dietary Divergence from Other Primates)

Alpha-amylase is an enzyme in saliva that breaks down starch into sugar (or more technically, hydrolyzes starch into maltose, maltotriose, and larger oligosaccharides). This enzyme is coded by the AMY1 gene, which can range from anywhere between 2 to 15 diploid copies in humans. As a general rule, the more AMY1 copies you have, the more salivary amylase you’ll produce (although lots of other factors can influence your levels at any given moment!). And as another general rule, populations with a long history of eating starch-rich diets (like the Japanese and Hadza) have higher average AMY1 copy numbers than populations consuming low-starch diets (like pastoralist tribes and arctic hunter-gatherers).

Chimpanzees and other nonhuman primates, on the other hand, have very few AMY1 copies and almost zero variation within their species. Chimpanzees universally possess only 2 diploid AMY1 copies, and bonobos have 4 (although their coding sequence is disrupted and those copies are probably only minimally functional, if not completely useless!). In other words, humans are the only primates genetically equipped to handle larger starch loads.

So, what does this tell us? Basically, at some point in our history (scientists are still trying to figure out exactly when!), humans began facing selective pressure to digest starch-rich foods—possibly because of more plant USOs (underground storage organs, discussed more here) making their way into our diet, or because cooking freed up starch sources that were previously unpalatable. As a result, our ancestors with less-than-efficient starch digestion were at a survival disadvantage, while those who could produce more salivary amylase had a better chance of staying in the gene pool. Over time, this led to the pattern we see today: much higher AMY1 copy numbers in humans relative to our great ape cousins, and particularly high numbers in populations that have traditionally relied on starch as a staple.

So, how does this tie into the herbivore-carnivore-omnivore discussion? Well, it puts a giant nail in the coffin of the idea that humans are carnivores (with a nearly entirely carnivorous past). We have clear genetic evidence for a historical role of plant starch, significant enough to leave an imprint on our genome!

But, our species’ genetic quirks don’t make the argument for herbivory, either. Humans are also unique when it comes to lactase, the enzyme that breaks down the lactose in dairy.

Whereas most mammals stop producing lactase after they’re weaned, humans can exhibit the lactase persistence phenotype (LP)—the ability to digest lactose into adulthood. LP is an inherited dominant trait that’s widespread in populations with long histories of pastoralism and milk consumption (especially in Europe, the Middle East, and Africa). Basically, this phenotype lets many people obtain energy and nutrition from dairy products, when normally, the result would be gastrointestinal distress (as anyone with lactose intolerance understands!). If we were truly herbivores, we wouldn’t have developed a genetic adaptation to a very specific animal food!

But What About our Teeth, Jaws, Nails, and Eating Patterns?

Some arguments for humans being herbivores or carnivores revolve around the shape of our teeth, jaws, and nails; our eating patterns; and other features that tend to be consistent among herbivorous or carnivorous animals. For example, an article on Vegsource.com concludes that humans are herbivores because of our blunt nails, well-developed facial muscles, jaw position and motion, small mouth opening relative to our head size, broad and flattened incisors and molars, and blunt canines (just to name a few!)—all of which are claimed to be features seen only in herbivores. Meanwhile, an article from Second-Opinions concludes that humans are carnivores because we have incisors on both jaws, intermittent feeding habits, vertical jaw movements, a lack of rumination behavior, a well-developed gallbladder, and a lack of stomach bacteria (among other things!)—all of which are claimed to be features seen only in carnivores. What to believe?!

Actually, trying to understand the human diet based on general features of herbivores or carnivores is misleading from the get-go. For one, unlike virtually any other species on the planet, we co-evolved with increasingly complex tools and processing methods that allowed our diets to expand without requiring us to have certain physical attributes. While many meat-eating species have powerful jaws, needle-sharp teeth, and claws that help them kill and chew prey, humans developed tools (spears, knives, blades, etc.) that serve the same function. In other words, we do part of our “digesting” outside of our bodies!

And, while many plant-eating species ruminate to obtain more nutrition from their food, have flat teeth for crushing vegetable matter, and have jaws that swing side to side, humans found ways to process plant foods (through grinding, cooking, chopping, etc.) that bypass the need for those types of herbivorous attributes. Pretty clever, huh?

The end result is that general anatomical comparisons between humans and herbivores or carnivores are pretty much meaningless, because they don’t take into consideration the unique position humans are in—having co-evolved with big brains and tools that help us pre-process our food, lessen the burden on our digestive systems, and remove the need for specific physical traits used for acquiring and consuming plants or animals.

What Can We Conclude?

Once again, all signs point to “omnivore” when it comes to human anatomy and physiology! And, this isn’t even the end of story! Stay tuned for Part 3!

Citations

Bogart SL & Pruetz JD. “Insectivory of savanna chimpanzees (Pan troglodytes verus) at Fongoli, Senegal.” Am J Phys Anthropol. 2011 May;145(1):11-20.

Deblauwe I. “Temporal Variation in Insect-eating by Chimpanzees and Gorillas in Southeast Cameroon: Extension of Niche Differentiation.” International Journal of Primatology. 2009 Apr;30(2):229-252.

Fahy GE, et al. “Stable isotope evidence of meat eating and hunting specialization in adult male chimpanzees.” Proc Natl Acad Sci U S A. 2013 Apr 9;110(15):5829-33.

Van Huis A, et al. “Nutritional Value of Insects for Human Consumption.” Food and Agriculture Organization. FAO Forestry Paper 171. 2013.

Hamad I, et al. “Detection of Termites and Other Insects Consumed by African Great Apes using Molecular Fecal Analysis.” Sci Rep. 2014; 4: 4478.

Koops K, et al. “Chimpanzees Prey on Army Ants at Seringbara, Nimba Mountains, Guinea: Predation Patterns and Tool Use Characteristics.” American Journal of Primatology. 2015; 77:319–329.

Luca F, et al. “Evolutionary Adaptations to Dietary Changes.” Annu Rev Nutr. 2010 Aug 21; 30: 291–314.

Mandel AL, et al. “Individual Differences in AMY1 Gene Copy Number, Salivary α-Amylase Levels, and the Perception of Oral Starch.” PLoS ONE;5(10):e13352.

Mitani JC & Watts DP. “Why do chimpanzees hunt and share meat?” Animal Behavior. 2001;61:915-924.

O’Mally RC & Power ML. “Nutritional composition of actual and potential insect prey for the Kasekela chimpanzees of Gombe National Park, Tanzania.” Am J Phys Anthropol. 2012 Dec;149(4):493-503.

Perry GH, et al. “Diet and the evolution of human amylase gene copy number variation.” Nat Genet. 2007 Oct; 39(10): 1256–1260.

Pruetz JD, et al. “New evidence on the tool-assisted hunting exhibited by chimpanzees (Pan troglodytes verus) in a savannah habitat at Fongoli, Sénégal.” R. Soc. Open Sci;2:140507.

Rothman JM, et al. “Nutritional contributions of insects to primate diets: Implications for primate evolution.” Journal of Human Evolution. 2014;71:59-69.

Sugiyama Y. “Tool-use for catching ants by chimpanzees at Bossou and Monts Nimba, West Africa.” Primates. 1995 Apr;36(2):193-205.