Dietary Evolution
We are used to designating animals as either herbivores, omnivores, or carnivores. For example, humans are omnivores, but the level individuals consume plants and animals varies widely. For instance, if a person eats a plant-based diet but occasionally eats meat, they are technically an omnivore, even though they eat 99.9% vegetables and fruit. It may surprise you to hear that wolves and dogs are considered omnivores; however, wolves seldom eat anything but meat in the wild.
Because of the confusion, scientists use the term trophic level to describe diets instead. Trophic levels represent the positions of species in a food web, from primary producers to apex predators. For example, plants are trophic level 1. To figure out trophic levels of animals, we take the trophic levels of food items in a species' diet, weighted by quantity, and add one. For example, cows feed on plants (trophic level 1); thus, their trophic level is 2. Polar bears or killer whales are extremely carnivorous, and have trophic levels that range up to 5.5.
Human Trophic Level (HTL)
Bonhommeau and fellow researchers have determined modern humans living today have a trophic level (Human Trophic Level, HTL) of 2.21. This designation puts us in the realm of pigs and anchovy fish. Slight differences in a species trophic level can reflect significant differences in diet. For instance, the people of Burundi had an HTL of 2.04, representing a 96.7% plant-based diet, compared to Iceland, where they eat 50% meat and 50% vegetables and have an HTL of 2.57. (1) People can be practically vegans or eat copious quantities of meat and still be considered omnivores.
Now and Then
Today we have the luxuries of science, transportation, and food production to provide us with a wide variety of foods to eat. However, this was not always the case. Climate, location, and technology all played a hand in limiting human diets. For instance, Inuits in the not-so-green arctic had very few plants in their diet, whereas natives of equatorial South America ate many different plant-based foods.
Stenotopy and Eurytopy
Researchers looked at 139 different mammals finding that 80% of them are omnivores. Still, 75% consume more than 70% of their food from either plants or animals, leaving only 20% of them as omnivores-generalists. (2) Scientists refer to dietary generalization as eurytopy. Most animals survive by taking specific advantage of their environment, exploiting availability via physical and strategic superiority, making specialization, also called stenotopy, the norm.
The Pleistocene
The Pleistocene was when the human genus first evolved.(3) It spanned from 2.6 million years ago until a bit over eleven thousand years ago. Repeated glaciations during the period caused habitat changes that resulted in less vegetation and gigantic mammals like the wooly mammoth. Anthropologists believe this climactic change helped humans evolve from plant-eating apes into3 omnivorous humans who acquired much of their food from animal-based sources. Since we first appeared and subsequently evolved into modern humans during the Pleistocene, our choices for plant-based foods were limited. In that same period, our propensity to hunt and consume large herbivores, however, was maximized. (4)
Genetic Mismatches Can Result in Adaptation
Genes and behaviors must match; if they don't, the mismatch can result in poor outcomes. Wolves and dogs are a current example. Most people don't realize that they are one species, believe it or not! When wolves were domesticated 23,000 years ago, their lifestyles changed. They teamed with humans in the hunt. Their proximity resulted in genetic changes: Their bodies shrank, barking arose, and they developed more enzymes to digest carbohydrates. The last difference is of particular importance for my discussion today. Since humans were still hunter-gatherers in the Pleistocene, eating many more plant foods than wolves (no grains yet), and the wolves had newfound access to them, they adapted to take advantage of their novel energy source, producing more digestive enzymes for carbohydrates. I wrote more about this subject three years ago, and you can read more about it here.
Genetic Mismatches and Disease
A human example of a genetic mismatch is sleep. Most scientists recognize that humans need about eight hours of sleep regularly. Science also shows that failure to do so results in disease and premature death. Simply put, our evolutionary legacy requires us to sleep eight hours, as expressed through our genes. That being said, many, if not most, animals are in constant states of genetic mismatches. When the mismatch is sudden and extreme, disease results. Modern dogs are a perfect example. They can only handle about a 7% carbohydrate diet; we feed them over 50%. They now have skyrocketing rates of obesity, diabetes, cancer, and degenerative arthritis. Sound familiar???
Form Follows Function and Vice Versa
If dogs were newly discovered, examining their behavior, morphology, biochemistry, etc., most scientists would conclude that they are carnivores without knowing what they ate. They would observe that they have a similar digestive system to other carnivores, can run for very long distances, live in packs with social orders like other carnivores, and gnawed bones abound where they live. In short, their form would inform us of their function. When encountering extinct animals, scientists must use clues to reconstruct how they lived and ate.
Domesticated Humans
Domesticated wolves (dogs) adapted to eating more plants than their wild relatives, so too did domesticated humans. Indeed, in the last ten or twenty thousand years, modern humans have domesticated themselves. The first thoroughly modern humans were larger and ate much more meat than we do today. Unlike dogs that can look significantly different from wolves, modern humans are much more similar to their ancestors in appearance. Yet, we differ physiologically.
For one thing, some of us can digest milk as adults. Our ancestors could not. We can farm and raise animals; our ancestors did not. We eat much more seafood and fish than before. We eat much smaller animals, and we eat many more vegetables than our ancestors in the Pleistocene.
Miki Ben-Dor
Dr. Miki Ben-Dor is trying to reconstruct the diet and behaviors of humans living in the Pleistocene. He holds that humans preferred to eat large animals because they were so plentiful, simple to hunt, and supplied all of their nutritional needs. (5) He questions whether our current trophic level matches our evolutionary design. He lays out his arguments in this paper he cowrote with Raphael Sirtoli and Ran Barkai. I suggest you read it; I think I enjoyed this paper more than any I have read in many years. Visit him at his website Paleostyle.com. Ben-Dor states, "To date, no complete evolutionary explanation has been given for the formation of the human species, and certainly not a complete and unifying explanation at the same time." Miki has laid out the evidence, admits that it is incomplete, and suggests lines of inquiry for future research. What follows are some quotes from his paper, followed by a limited summary of his findings. Feel free to skip to my conclusion and final warning if it gets too geeky.
Quotes
"Contemporary hunter-gatherers may be somewhat appropriate as an analogy to the end of the Stone Age when most large animals were already extinct; The dog is domesticated, and the bow was developed as a hunting tool. They are certainly not suitable as analogs for earlier periods, constituting about 99% of the Stone Age."
"20th-century hunter-gatherers, on which research has so far been based, are not a suitable source for reconstructing the relative amount of plants compared to animals in the diet of ancient (Pleistocene) humans."
"There is no reconstruction in the literature that does not rely on 20th-century hunter-gatherers and accordingly presents a flexible trophic level based on local ecological conditions."
Takeaways
The protein and fat mixture in animals would probably have provided higher energetic density, and therefore dietary quality enabled us to have such large brains.
Ketosis only occurs with high fat/low carbohydrate diets or starvation. Human ketosis has allowed man to survive lengthy periods of starvation. But more importantly, it has permitted the brain to become the most significant component in human evolution.
Carnivore stomachs are more acidic than omnivores but less acidic than obligate scavengers. Humans have extreme stomach acidity as high as scavengers, reflecting our predilection of consuming enormous animals that started to rot before finishing consumption.
Humans have insulin resistance like other carnivores, suggesting traditionally low carbohydrate diets in antiquity.
In general, large guts indicate herbivory, and small guts indicate carnivory. Humans have small guts.
Small jaws compliment small digestive systems. Humans have small jaws.
The colon ferments plant matter into energy. All carnivores have small colons. Ours are 77% smaller than chimps, indicating we don't need the volume because we don't eat nearly as much plant matter.
Protein and fat take much longer to digest and absorb than carbohydrates. Carnivores all have long small intestines to adequately digest fats and proteins. Our small intestine is 64% longer than in chimpanzees, relative to chimpanzee body size, indicating a diet much higher in animal protein and fat.
Gorillas and chimps have a large cecum designed to contain bacteria that ferment fiber into fats. We have no cecum; it has shrunken into the appendix.
We adapted shoulders for throwing, a beneficial change needed for superior hunting.
Herbivores have fewer fat cells, but they are larger. Carnivores have the reverse, as do humans.
Hunting skills fully develop at around 40 years, and the importance of hunting experience presumably caused longevity extension in humans who live much longer than any primates.
Hunting provides a tenfold higher energetic return per hour compared to gathering plants.
For eight of the ten vitamins, terrestrial mammal food is denser, and in most cases, several times denser than plants. Eating animals is therefore advantageous.
The term bioavailability describes the ease at which the gut absorbs nutrients. Animal foods are higher in bioavailability for humans than plant-based foods.
Since we are flesh and bone, it stands to reason that consuming them supplies their building blocks.
High carbohydrate diets result in cavity formation in teeth. We find low rates of human cavities during most of the Pleistocene, supporting low carbohydrate consumption and a high HTL during the Pleistocene.
Food sharing, food storage, cannibalism, surplus killing, interspecies intolerance, and alloparenting are carnivores' behaviors. Only one behavior, group defense, resembles other primates.
Most plants contain toxins designed to prevent consumption. Therefore, animals that evolve detoxification enzymes must stay in the limited area the plants grow. Hence, herbivores have smaller geographic distribution than animals.
Animal flesh is generally not toxic, so we need not be concerned about toxicity as long as we can digest it.
Humans and wolves are the most and second-most widely spread species, respectively. Their reliance on carnivory helped disperse them throughout the world in pursuit of multiple species of prey.
Hypercarnivores are carnivores that derive at least 70% of their food from animals. Human hunting of large mammals like mammoths suggests we were hypercarnivores in the Pleistocene.
Conclusion
We are not carnivores.
Our pre-human primate ancestors were primarily herbivores.
We broke off from other primates by adapting to eating meat acquired through scavenging and hunting.
The Pleistocene environment we evolved in providing an opportunity to get most of our nutrition from enormous animals like mammoths, but we were still omnivores.
We were omnivores with a high trophic level, meaning we ate primarily large animals.
We were omnivores specializing in hunting large fatty animals but ate large predators as well.
Big game became scarce at the end of the Pleistocene, causing us to hunt smaller animals.
Hunting smaller animals is more complicated. So we domesticated the dog to help pursue and corner them.
When the Pleistocene ended, the world got warmer, and we were able to find more edible plants.
We invented farming and increased our plant consumption.
Our trophic level has fallen since the Pleistocene.
Some of us are almost entirely herbivorous.
Some of us are almost entirely carnivorous.
We are all, in reality, omnivores.