Ancient Humans Mostly Vegetarian, 'Paleolithic Diet' Critic Says

'Paleolithic Diets' Wrong About Ancient Eating Patterns, Critic Says

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

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? An entire class of self-help books recommends a return to the diets of our ancestors. Paleolithic diets, caveman diets, primal diets and the like, urge us to eat like the ancients. Taken too literally, such diets are ridiculous. After all, sometimes our ancestors starved to death and the starving to death diet, well, it ends badly. Yet, the idea that we might take our ancestral diet into consideration when evaluating the foods on which our organs, cells and existence thrive, makes sense. But what did our ancestors 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 paper out just this month suggests even Neanderthals–our north country cousins and mates– may have eaten much more plant material than previously suspected. Still, the more macho camps 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. Although “Paleolithic” diets in diet books tend to be very meaty, reasonable minds disagree as to whether ancient, Paleolithic diets actually were. Fortunately, new research suggests a clear answer to the question of what our ancestors ate.

The resolution does not come from new discoveries by archaeologists. It comes from 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, we should not be looking at our most recent ancestors. Instead, we need to understand the diet of our ancestors during the time when the main features of our guts, and their magical abilities to turn food into life, evolved. We need, in other words, to look at apes, monkeys and other non-human primates.

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 one 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 much of the digestion of proteins occurs. 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 or an artichoke 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 before whatever has your body and your microbes have been unable to digest (or have produced) is excreted. 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.

Not only are guts significant, they vary 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 colons. A beak can pick something up, maybe crush it. Big deal. A colon can turn a bit of rotten fruit or leaf into energy and hence 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 cougars or mountain lions have smooth, stomachs big enough to hold the rump of an antelope. In them, the muscles of prey are return 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. Our guts are remarkably similar to those of gorillas, chimpanzees and orangutans which are remarkably similar to those of other apes, 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 some variation. In the leaf-eating black and white colobus monkeys (among which my wife and I once lived) 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 similar role. Lampreys do without a stomach at all. The alimentary canal varies tremendously, 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, monkey/ape guts. The only real distinction between our guts and those of other primates, aside from a slightly enlarged appendix, is that we have slightly less large intestine relative to small intestine relative to other species, which might make our guts a little less efficient, though it might not (And even then such inefficiency wouldn’t predispose our bodies to any particular diet or culinary norm, it would just make us more likely to fart). Aside from the potential for farting, our guts are strikingly, elegantly, obviously, ordinary.

Image 1a (left). Chimpanzee eating an ordinary food, a fig.

So what do apes eat or what do most monkeys 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. They have the capacity for eating sugary fruit, the capacity for eating leaves and the capacity for eating meat, although that capacity tends to rarely be invoked. Sure, chimpanzees sometimes kill and devour a monkey, but the proportion of the diet of the average chimpanzee composed of meat is small, less than 3% by mass. Even with that modest preference for flesh, chimps are extreme. They eat more 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 to eat 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.

1b (right). Chimpanzee eating a rare delicacy, a colobus monkey. Photo by Joanna Lambert.

“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 your 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… , well, I’ll come back to them .

A paleo diet is an arbitrary thing. Which paleo diet should we eat? The one from twelve thousand years ago? A hundred thousand years ago? Forty million years ago? I would argue that, IF we want to return to our ancestral diets, we might reasonably eat what our ancestors spent the most time eating during the largest periods of the evolution of our guts. If that is the case, we need to be eating fruits, nuts, and vegetables—especially, fungus-covered, tropical, leaves.

Image 2. A blue monkey eating a fig. Photo by Joanna Lambert.

What would convince me otherwise? I would be swayed if someone were to discover rapid evolution in the genes associated with our digestion over the last hundred thousand years, the sort of evolution that might signal that we had specialized on recent diets heavier in meat. I want evidence, in other words, that some aspect of our bodies evolved in such a way as to be better able to deal with meat. But that wouldn’t be enough. Our bodies would have to have simultaneously have evolved to be less able to deal with more ordinary primate diets—again, the nuts, fruits and leaves. It could be there, as of yet undetected. But 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 agriculture than with meat-eating.

As hominids and/or humans switched to eating more meat, their bodies might have evolved so to be able to better digest meat. Maybe, though show me the evidence. But, our human digestive systems DID evolve, we know, to deal with agriculture. With agriculture, some human populations evolved extra copies of amylase genes, arguably so as to better be able to deal with starchy foods. 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. 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.

All this means that 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 the occasional bit of insect, frog, bird or mouse and so 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.

There are, however, trillions of microscope caveats. 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. 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. Recent 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. In other words, gorillas and chimps (and in all likelihood humans) are extra farty. 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, but if our gut bacteria responded rapidly to shifts in diets toward more meat during the stone, age (if they occurred), they might be expected to have shifted again when we began to farm. 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. In the future, one imagines a day in which we could inoculate ourselves with bacteria appropriate to our diets, but for now we are far too ignorant to distinguish good from bad.

Image 3. A Black and White Colobus Monkey eating leaves. Photo by Joanna Lambert.

So, what should we eat? The past itself does not reveal the answer, though it does suggest our bodies are fully-equipped to deal with meat (which is relatively easy) and simple sugars (also easy), but also harder to digest plant material, what often gets called fiber. We have colons filled with bacteria just waiting for the stuff. So, go ahead and eat some grass. Go ahead too and pick your favorite time in the past, and eat like they did back then. I’ll choose to eat fruits and nuts (supplemented with some of the great beans produced by the agricultural revolution). I’ll also have some coffee, maybe a glass of wine and some chocolate. They are not paleo, but I like them. Go figure. Maybe you are “new school” and will choose to eat only the meat of giant sloths and mastodons, in doing so, ignoring your bacteria. Or maybe you are really paleo and you are going to focus on insects, which might favor other bacteria (able to break down insect chitin). Any of these possibilities are 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.


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- 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 both. Or something else entirely.

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.

5-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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