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Human Ancestors Were Nearly All Vegetarians

Paleolithic diets have become all the rage, but are they getting our ancestral diet all wrong?

Scientific American

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Right now, one half of all Americans are on a diet. The other half just gave up on their diets and are on a binge. Collectively, we are overweight, sick and struggling. Our modern choices about what and how much to eat have gone terribly wrong. The time has come to return to a more sensible way of eating and living, but which way? One group of self-help books suggests we give up carbohydrates, another that we give up fats, another still that we lay off the protein. Or maybe we should just eat the way our ancestors did. A newer class of very popular self-help books recommends a return to the diets of our ancestors. Paleolithic diets, caveman diets, primal diets and the like, urge us to remember the good ole days. Taken too literally, such diets are ridiculous. After all, like all wild species, sometimes our ancestors starved to death and the starving to death diet, well, it ends badly. The past was no panacea; each generation we made due with the bodies and foods available, imperfect bodies and imperfect foods. But let's pretend, for the sake of argument, that it would be a good idea to eat like our ancestors ate. Just what did they eat?

Here is where the trouble starts. Collectively, anthropologists have spent many a career attempting to hone in on the diets of our most recent ancestors. Typically, they focus on our stone age (AKA Paleolithic) human ancestors or our earlier pre-human, hominid ancestors. Even if we just consider our stone age ancestors—those folks whose stories span the time between the first stone tool and the first agriculture—the sides of the debate are polarized. If you listen to one camp, our ancestors got most of their nutrition from gathered fruits and nuts; successful kills of big mammals may have been more of a treat than an everyday reality. A 2012 paper suggests that even Neanderthals—our north country cousins and mates—may have eaten much more plant material than previously suspected. Meanwhile, more macho camps of academics paint a picture of our ancestors as big, bad, hunters, who supplemented meaty diets with the occasional berry “chaser.” Others suggest we spent much of our recent past scavenging what the lions left behind, running in to snag a half-rotten wildebeest leg when the fates allowed. In other words, although “Paleolithic” diets in diet books tend to be very meaty, reasonable minds disagree as to whether ancient, Paleolithic diets actually were. Fortunately, research suggests answers (yes, plural) to the question of what our ancestors ate.

The resolutions come, in part, from considering the question of our diets in a broader evolutionary context. When we talk about "paleo" diets, we arbitrarily tend to start with one set of ancestors, our most recent ones. I want to eat like Homo erectus or a Neanderthal or a stone age human, my neighbors testify. But why do we choose these particular ancestors as starting points? They do seem tough and admirable in a really strong five o’ clock shadow sort of way. But if we want to return to the diet our guts and bodies "evolved to deal with" (a concept that wrongly assumes our bodies are fine tuned by engineers rather than cobbled together by natural selection), perhaps we should also be looking our earlier ancestors. In addition to understanding early humans and other hominids, we need to understand the diet of our ancestors during the times when the main features of our guts, and their magical abilities to turn food into life, evolved. The closest (albeit imperfect) proxies for our ancestral guts are to be found coiled inside the living bodies of monkeys and apes.

I should start by explaining what the “gut” is and does; I use the term too loosely. What I really mean is the alimentary canal and all of its gurgling bells and whistles. This canal is the most important and least lovely waterway on Earth. It takes you from the mouth through the body all the way down to the anus. But while most canals take the shortest course between two points, the one inside you takes the longest. The longer the canal, the more area over which digestion can occur. Food enters the canal through the mouth, where it is chewed and slimed with saliva. It then hits the stomach, where proteins are digested (and, I think, bacteria are filtered). Next, it is on down to the small intestine where simple sugars are absorbed. If you have just eaten a twinkie, the process essentially ends there. Everything worth consuming has been absorbed. But if you have eaten broccoli, an artichoke or a fig, things are just beginning. It is in the large intestine, where harder to break down carbohydrates (such as cellulose, the most common plant compound on Earth) are torn asunder. This system evolved so as to provide us with as many calories as possible (long to our benefit) and, also, as many of the necessary but hard to produce nutrients. The alimentary canal is, evolutionarily speaking, a masterwork. It makes energy from the food we are lucky enough to find.1

Although all guts are sublime, just how they do what they do varies among species, much as do the leaves on trees or beaks on birds. When considering evolution’s great innovations, Darwin dallied among the beaks, but he might just as well have focused on the gut or even simply colons2. A beak can pick something up, maybe crush it. Big deal. A colon can jump start the process of turning a bit of rotten fruit or leaf into usable energy and ultimately life. Science can replicate a beak; it is still working on making a good replica of a colon, much less replicating the great variety of colons and guts more generally found in nature. Carnivores such as lions have smooth stomachs big enough to hold a good sized hunk of a small antelope. In them, the muscles of prey are returned to the bits of protein out of which they are made. The stomachs of some herbivores on the other hand are dense with hair-like villi and, moving among them, the bacteria that aid in the breakdown of plant cell walls and their cellulose. The stomach of a cow is a kind of giant fermenter in which bacteria produce huge quantities of specific fatty acids the cow can easily use or store (You eat some of those fatty acids when you eat a cow). In other species, the stomach scarcely exists and fermentation takes place in a greatly enlarged large intestine.

Yet, for all of the vulgar and magnificent elaborations on the theme of tubes to be found inside animals, the guts of humans are boring (although see footnote 5). Our guts are remarkably similar to those of chimpanzees and orangutans--gorillas are a bit special--which are, in turn, not so very different from those of most monkeys. If you were to sketch and then consider the guts of different monkeys, apes and humans you would stop before you were finished, unable to remember which ones you had drawn and which ones you had not. There is variation. In the leaf-eating black and white colobus monkeys (among which my wife and I once lived in Boabeng-Fiema, Ghana) the stomach is modified into a giant fermentation flask, as if the colobus were kin to a cow. In leaf-eating howler monkeys the large intestine has become enlarged to take on a similarly disproportionate role, albeit later on in digestion. But in most species things are not so complex. An unelaborated stomach breaks down protein, a simple small intestine absorbs sugars and a large (but not huge) large intestine ferments whatever plant material is left over. Our guts do not seem to be specialized hominid guts; they are, instead, relatively generalized monkey/ape guts. Our guts are distinguished primarily (aside from our slightly enlarged appendix) by what they are missing rather than what they uniquely possess. Our large intestines are shorter than those of living apes relative to the overall size of our gut (more like 25 percent of the whole, compared to 46 percent of the whole in chimps). This shortness appears to make us less able to obtain nutrients from the cellulose in plant material than are other primates though the data are far from clear-cut. The variation in the size and details of our large intestines relative to those of apes or gorillas have not been very well considered. In a 1925 study the size of colons was found to vary from one country to the next with the average Russian apparently having a colon five feet longer than the average Turk. Presumably the differences among regions in colon length are genetically based. It also seems likely that the true human colonic diversity has not yet been characterized (the above study considered only Europe). Because of the differences in our colons (and ultimately the number of bacteria in them) we must also vary in how effectively we turn cellulose and other hard to break down plant material into fatty acids. One measure of the inefficiency of our colons is our farting, which we all know varies person to person. Each stinking fart is filled with a measure of our variety.3 Aside then from the modest size of our colon, our guts are strikingly, elegantly, obviously, ordinary.

So what do other living primates eat, the ones with guts mostly like ours, eat? The diets of nearly all monkeys and apes (except the leaf-eaters) are composed of fruits, nuts, leaves, insects, and sometimes the odd snack of a bird or a lizard (see more about chimpanzees). Most primates have the capacity for eating sugary fruit, the capacity for eating leaves and the capacity for eating meat. But meat is a rare treat, if eaten at all. Sure, chimpanzees sometimes kill and devour a baby monkey, but the proportion of the diet of the average chimpanzee composed of meat is small. And chimps eat more mammal meat than any of the other apes or any of the monkeys. The majority of the food consumed by primates today—and every indication is for the last thirty million years—is vegetable, not animal. Plants are what our apey and even earlier ancestors ate; they were our paleo diet for most of the last thirty million years during which our bodies, and our guts in particular, were evolving. In other words, there is very little evidence that our guts are terribly special and the job of a generalist primate gut is primarily to eat pieces of plants. We have special immune systems, special brains, even special hands, but our guts are ordinary and for tens of millions of years those ordinary guts have tended to be filled with fruit, leaves, and the occasional delicacy of a raw hummingbird.4

“But wait dude,” you might say, you have not gone far enough back in time. After all, most of the details of our guts, the size and shape of its different parts, are even older. Even prosimians, lemurs and their other adorable kin have guts similar to our guts. Maybe they were carnivores and we can still be “paleo” and eat a ton of meat? Maybe in thinking about our guts, we should look to the prosimians. Sure enough, most prosimians are (and likely were) carnivores. They eat and ate meat, BUT most of that meat comes from insects. And so if you are serious about eating a really old school paleo diet, if you mean to eat what our bodies evolved to eat in the “old” days, you really need to be eating more insects. Then again, our guts aren’t so different from those of rats. Maybe the rats….4

Which paleo diet should we eat? The one from twelve thousand years ago? A hundred thousand years ago? Forty million years ago? If you want to return to your ancestral diet, the one our ancestors ate when most of the features of our guts were evolving, you might reasonably eat what our ancestors spent the most time eating during the largest periods of the evolution of our guts, fruits, nuts, and vegetables—especially fungus-covered tropical leaves.

Of course, there might be differences between our digestive system and those of other species that have gone relatively undetected. Maybe someone will discover rapid evolution in the genes associated with our digestion over the last million years, the sort of evolution that might signal that we had evolved specialized (but so far hidden) features to deal with diets heavier in meat, an adaptationist just so story that makes a big steak seem not like an indulgence but instead our evolutionary birthright. If you want a justification for eating a meaty "paleodiet," in other words, the search should be for evidence that some aspect of our bodies evolved in such a way as to be better able to deal with extra meat or other elements of our stone age diets that differed from the primate norm. It could be there, as of yet undetected.

If you want my bet, the majority of the recent (last few million years) changes in our guts and digestion will prove to have had more to do with processing food and, later, agriculture rather than with meat-eating per se. As hominids and/or humans switched to eating more meat, their bodies might have evolved so to be able to better digest meat. I could be convinced. But, we know our human digestive systems DID evolve to deal with agriculture and the processing (fermenting and cooking) of food. With agriculture, some human populations evolved extra copies of amylase genes, arguably so as to better be able to deal with starchy foods. The case of agriculture is the most clear. With agriculture, several human populations independently evolved gene variants that coded for the persistence of lactase (which breaks down lactose) so as to be able to deal with milk, not just as babies but also as adults. Drinking milk of another species as an adult is weird, but some human populations have evolved the ability. With agriculture, the species in our guts seem to have evolved too. Some populations of humans in Japan have a kind of bacteria in their guts which appears to have stolen genes for breaking down seaweed, a foodstuff that became popular along with the post-agricultural Japanese diet. With agriculture, human bodies changed so as as to cope with new foods. Our bodies bear the marks of many histories. As a result, if you want to eat what your body “evolved to eat” you need to eat something different depending on who your recent ancestors were. We already do this to some extent. If your ancestors were dairy farmers, you can drink milk as an adult without trouble, you've "got lactase." But if they were not, you tend to get diarrhea when you drink milk and so you probably avoid the stuff (lest your friends avoid you). But the truth is, for most of the last twenty million years of the evolution of our bodies, through most of the big changes, we were eating fruit, nuts, leaves and the occasional bit of insect, frog, bird or mouse. While some of us might do well with milk, some might do better than others with starch and some might do better or worse with alcohol, we all have the basic machinery to get fruity or nutty without trouble. And anyway, just because some of us do better with milk or starch or meat than others doesn't mean such foods are good for us, it just means that those individuals who couldn't deal with these foods were more likely to die or less likely to mate.

What might be different, either between you and me or between you and me and our ancestors is the sort of gut bacteria we have to help us digest our food (which might also relate to the size and particulars of our colons). The new era in study of gut bacteria (and their role in digestion)—the era of the microbiome—may reveal that our stone age ancestors, by eating a little more meat, cultivated bacteria that help break down meat, which they then passed on to us (during birth which is messy and has long been), their maybe meat-eating descendents. Research by Joanna Lambert at the University of Texas, San Antonio and Vivek Fellner at North Carolina State University (my home institution) have revealed that the gut microbes of chimpanzees and gorillas do seem to work a little differently than those of monkeys (or at least the monkeys they studied). Bacteria from the guts of gorillas and chimps seem to produce more methane as waste than do those from monkey guts. Maybe this is just the tip of the fecal berg and the guts of different primates are fine-tuned to their diet in very sophisticated ways, including the fine-tuning of our own guts for eating more meat! Possibly, the next years will be exciting, both in terms of understanding the unique attributes of our microbes and the unique elements of our immune systems and the ways in which they regulate the composition of those microbes. These changes in bacteria might be mediated by changes in our immune systems themselves and how they relate to the microbes processing our planty food. Interestingly, if our gut bacteria responded rapidly to shifts in diets toward more meat during the stone age, they might be expected to have shifted again when we began to farm, at least for those of us with ancestors who began to farm early. When our gut bacteria met up with our agricultural diets, beginning twelve thousand years ago or so, they would have begun to compete with new microbial species that kicked ass at living off wheat, barley, corn, rice or any of the other grasses that have come to dominate the world, sometimes at our expense. This may even mean that which diet is best for you depends not only on who your ancestors were, but also who the ancestors of your bacteria were.

So, what should we eat? The past does not reveal a simple answer, ever. Our bodies did not evolve to be in harmony with a past diet. The evolved to take advantage of what was available. If the best diet we can, with billions of dollars invested in nutritional studies, stumble upon is the one that our ancestors of one or another stage happened to die less when consuming, we are in trouble. Should we take our evolutionary past into account when figuring out the optimal diet. Yes, definitely. But there are two big caveats. First, our evolutionary history is not singular. Our bodies are filled with layers of evolutionary histories; both recent and ancient adaptations, histories that influence how and who we are in every way, including what happens to the food we eat. The recent adaptations of our bodies differ from one person to the next, whether because of unique versions of genes or unique microbes, but our bodies are all fully-equipped to deal with meat (which is relatively easy) and natural sugars (also easy, if not always beneficial), and harder to digest plant material, what often gets called fiber.5 Our ancient evolutionary history influences how we deal with these foods, as does our stone age past, as do the changes that occurred to some but not all peoples as agriculture arose. With time, we will understand more about how these histories influence how our bodies deal with the food we eat. But the bigger caveat is that what our histories and ancestral diets offer is not an answer as to what we should eat. It is, more simply, context. Our ancestors were not at one with nature. Nature tried to kill them and starve them out; they survived anyway, sometimes with more meat, sometimes with less, thanks in part to the ancient flexibility of our guts.

As for me, I’ll choose to eat the fruits and nuts like my early ancestors, not because they are the perfect paleodiet but instead because I like these foods and modern studies suggest that consuming them offers benefits. I'll supplement them with some of the great beans of agriculture, too much coffee, maybe a glass of wine and some chocolate. These supplements are not paleo by any definition, but I like them. What should you eat? The truth is that many different diets consumed by our ancestors—al insect diet, mastodon diets or whatever you please—would be, although some perfect panacea, better than the average modern diet, one so bad that any point in the past can come to seem like the good ole days, unless you go too far back to a point when our ancestors lived more like rats and probably ate everything, including their own feces. Sometimes what happens in paleo should really stay in paleo.6

For another take on the troubles with looking to history for idealized answers to our modern problems see Marlene Zuk's great 2009 article in the New York Times.


  1. Well, into you and into excrement.
  2. It would have suited him. After all, he took great pains to document his own bowel movements.
  3. The most widely cited comparison of the guts of chimps, humans, gorillas and orangutans has sample sizes of one individual for both chimps and orangutans, so just how much larger the large intestines of chimps or orangutans are relative to ours is not yet known. Our relatively short large intestines might be an adaptation to our special diet, but might also be the consequence of a tradeoff between investing in big brains and big intestines. Or some mix thereof. Along these lines, it has been suggested that our shift to eating more meat historically might have allowed investment in bigger brains which might, in turn, have required us to eat more meat so as to feed the bigger brain and simultaneously made our large intestines and their fermentation less necessary. This idea is interesting and many-layered and comes with a number of untested but testable predictions. It would be fun to explore the genes associated with the changes in the size of our large intestine and when and whether they underwent strong selection.
  4. For a review of the ecology and evolution of primate guts, see the excellent work by my friend and colleague, Joanna Lambert. For example… Lambert JE. Primate nutritional ecology: feeding biology and diet at ecological and evolutionary scales. In Campbell C, Fuentes A, MacKinnon KC, Panger M, and Bearder S (eds): Primates in Perspective, 2nd Edition, Oxford University Press or Lambert, JE (1998) Primate digestion: interactions among anatomy, physiology, and feeding ecology. Evolutionary Anthropology. 7(1): 8-20.
  5. Sometimes it takes a friend to say things just right. In defense of human guts, my friend Gregor Yanega at Pacific University offered, "Our guts are special because they are less specialized. They can accomodate so many changes in the foods that surround us, can accomodate unusual abundance and a certain amount of scarcity: we can even eat some of the world's more difficult foodstuffs: grains, leaves, and plants. Berries, nuts, meats, sugars, those are easy. Eating them together is pretty rare."
  6. I know, what I have shown is not that our ancestors were vegetarians but instead that they tended to mostly eat vegetable matter. Here though I am using the definition of vegetarian that most humans use where someone is a vegetarian if they decline meat in public but occasionally, when no one is looking, sneak a beef jerky. The modern vegetarian's illicit beef jerky is the ancestral vegetarian's crunchy frog.

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This post originally appeared on Scientific American and was published July 23, 2012. This article is republished here with permission.

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