We are what we eat, goes the old saying, and new research has revealed one of the deepest and oldest secrets of who we are as a species. Humans are natural born killers: super-predators designed by evolution to subsist mainly on the meat and fat of large animals, and genetically hardwired to hunt our prey into extinction, says a new study on the eating habits of prehistoric hominins going back 2 million years.
This meta-analysis collated information from some 400 previous studies, conducted over decades by unconnected scientists, and providing biological, genetic, archaeological and molecular data on the diet of our Stone Age ancestors.
The overwhelming evidence gleaned from this research belies the common belief that humans are adaptable omnivores who won at the evolutionary game because of their flexibility and smarts. Instead, it supports a new paradigm of our evolution, which is being proposed by the researchers behind this new study. And that idea is that we are specialized hypercarnivores who diversified our diet only at the tail-end of our evolutionary story, and only because we were forced to do so after killing off our main food source.
While it looks at the eating habits of hominin hunter-gatherers over hundreds of thousands of years in the past, this analysis can also help explain a number of problems that humans face today: from why so many of us have trouble digesting certain foods to why it’s so hard for us to preserve our environment.
The new study was published in March in the yearbook of the American Association of Physical Anthropologists by Dr. Miki Ben-Dor and Prof. Ran Barkai, two archaeologists from Tel Aviv University, and Raphael Sirtoli, a health sciences researcher at the University of Minho in Portugal.
The team investigated “The evolution of the human trophic level during the Pleistocene,” which is a technical way of saying that it tried to glean the position that Stone Age hominins occupied in the food chain – essentially, a prehistoric who-ate-who.
Homo erectus, the hypercarnivore
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Our very distant ancestors, such as the australopithecines and Homo habilis, had already started to move away from the typical plant-based diet of primates and incorporate more meat in their eating habits, the researchers say. But some two million years ago a new hominin emerged in Africa that would take the world by storm, and is believed to have eventually evolved into modern Homo sapiens.
That postulated direct ancestor of ours was Homo erectus. He was the first member of our species to climb to the top of the food chain and become what zoologists call a hypercarnivore – a mammal that obtains more than 70 percent of its food from other animals.
Since erectus, humans have remained at the top of the food chain and evolved into specialized hunters of megafauna, large herbivores of the Pleistocene, such as the mammoth and the aurochs, that are now extinct.
Some of the evidence for this comes from comparing our own physiology to that of other primates, particularly the chimpanzee, our closest ape relative. Other evidence stems from genetic studies and analysis of archaeological remains of Stone Age tools and hominin bones.
From all these sources, the researchers have unearthed at least 25 pieces of evidence pointing to our carnivorous past.
“There is no argument that prehistoric humans ate meat, and that this separated us from our predecessors,” says Ben-Dor, who is the lead author on the study. “But most researchers hold the view that humans could do anything, they could eat whatever they wanted: plants, animals, whatever they liked and whatever was available.”
Mistakes in the paradigm
From this assumption comes the idea is that Homo sapiens evolved and spread across the world because it was extremely flexible, Ben-Dor tells Haaretz. But that paradigm is flawed, because it is largely based on studies of the behavior of modern-day hunter-gatherers. These groups have access to technologies, such as metals and controlled fire, that our ancestors didn’t have. Additionally, they have been adapting for tens of thousands of years to the depletion of megafauna, which forced humans to learn to hunt smaller prey and forage or domesticate plants, Ben-Dor says.
If we look at the evidence encoded in our own biology, a very different picture emerges, Ben-Dor and colleagues note.
For example, the human colon is 77 percent smaller than that of the chimpanzee, while our small intestine is 64 percent longer. The colon is where energy is extracted from plant fiber, while the small intestine is where sugars, proteins and fat are absorbed.
This means that after the human lineage diverged from the chimp line, around six million years ago, we progressively became more adapted at extracting energy from meat, and lost most of our ability to do so from plants.
The same progression toward a carnivorous diet can be seen in the evolution of our teeth. Australopithecines, who lived from four to two million years ago, had big jaws and large flat molars, necessary features to grind large amounts of plant material. With Homo erectus, mandibles and teeth shrink to a size comparable to that of modern humans, Ben-Dor notes. The inference is that by then we had migrated to softer food, like meat.
Multiple studies of isotopes in the bones of hominins have also shown that humans subsisted largely on an animal-based diet until the end of the Paleolithic, less than 20,000 years ago.
Og, pass the Tums
But how do we know that humans specialized in hunting large animals? Well, first of all it makes sense: a single elephant can provide sustenance for a group of hominins for weeks, while catching the equivalent amount of rabbits or birds can be harder and less efficient.
It also perhaps is not a coincidence that Homo erectus was not just the first hypercarnivore in our lineage, but was also the first hominin to leave Africa and populate Eurasia. Several researchers have suggested that erectus may have been following the migration of megafauna, Ben-Dor says.
There are also many clues of this specialization in our own biology. Humans, for example, have higher stomach acidity than herbivores and even most carnivores. In evolutionary terms, carrying around sack of corrosive acid inside your body is a fairly dumb idea, requiring much energy expenditure to produce the digestive liquids and maintain the lining of the stomach.
This adaptation can only be explained if we accept that humans hunted large prey, which they then butchered and carried back home to be consumed over the following days or weeks. In this case, high stomach acidity would have served us well in killing the bacteria that developed in the meat over time, in an era when humans had not yet mastered fire to cook their food.
Also, humans have much larger fat reserves than chimpanzees, and also compared with other predators. Again that looks like an evolutionary handicap, as carrying around fat requires energy and makes us slower – not a feature you want if you have to run after a gazelle to get your breakfast.
But large fat reserves make sense if you are a megafauna hunter, because large animals tend to be relatively few and far between, so the ability to store energy in lipid cells allowed our ancestors to fast for weeks in between finding prey, Ben-Dor and colleagues explain.
Big game, big brain
Fat is the rarest and most energy-dense micronutrient in nature, and hominins quickly became dependent on it (which is why we enjoy fatty foods so much). As a mean but not-so-lean fighting machine, humans required great skills and smarts to catch animals that could weigh up to a few tons and could probably smush them with a swing of a proboscis. Thus natural selection favored ever bigger brains for our hominin ancestors, which in turn required larger amounts of energy to function, making us ever more hungry for a juicy, fatty mammoth steak.
Fat was also crucial to our survival because humans can only derive between 35 to 50 percent of our energy from proteins. Any surplus protein beyond that poisons us, so the rest of our calories must come from fats or carbohydrates.
This suggests that Stone Age hunters preferred not just megafauna but probably targeted adult individuals in their prime over younger and older animals, who tend to be leaner. This means that our hunting habits may have put greater pressure on megafauna species by killing those animals who were in their prime reproductive years, Ben-Dor says.
The exact level of human contribution to the extinction of megafauna remains hotly debated. On one hand, animal size apparently began to decline in Africa already four million years ago, as a result of climate change, and had no correlation to humans, who had not yet evolved by then. On the other hand, the extinction of megafauna picked up significantly starting some 132,000 years ago, and several studies have linked the disappearance of outsize animals to the dispersal of humans across the globe.
“Climate fluctuates all the time and it does create pressure, but in the past these pressures didn’t cause such mass extinction events, so it was probably a combination of climate change and overhunting,” Ben-Dor says.
As megafauna declined over the last tens of thousands of years, humans had to use those big brains to find new sources of food. Projectiles, like the bow and arrow, were developed to target smaller prey; foraging for plants increased and, finally, some 12,000 years ago, we started to settle down and domesticate plants, triggering the Neolithic revolution.
The new investigation of the trophic level of prehistoric humans can lead us to several conclusions that are relevant to modern-day people.
On the lighter side, the very popular “Paleo diet” is, at best, tragically misnamed. Broadly defined, this diet eschews grains and processed foods, focusing instead on vegetables, fruits, nuts and seeds, lean meat and fish.
But if Ben-Dor and colleagues are correct, this diet comes close to what our ancestor hunter-gatherers ate only for a tiny fraction (relatively speaking) of the Paleolithic era after which this food fad is named.
For most of the Paleolithic, which lasted from 3.3 million years ago to about 12,000 years ago, the Homo species appears to have subsisted on an entirely carnivorous diet, becoming a reluctant omnivore only in the last few tens of thousands of years.
On a more serious note, this may explain why so many people suffer from intolerances to gluten, milk and other foods that only entered our diet recently (again, evolutionarily speaking), Ben-Dor says. Natural selection has not had enough time to eliminate those traits from our genetic code and give us all the necessary adaptations for a truly omnivorous diet.
And finally, this image of humans as all-consuming predators reminds us of the destructive effects of our behavior can have on our ecosystem and the massive effort required to curb what seem to be our most primal instincts.
“Humans are not a good caretaker of the environment, we are built to just go for the next animal and eat it,” Ben-Dor concludes. “We need a lot of cultural influence and constructs to overcome that.”