Extended Summary
The strongest evidence that humans evolved eating (almost) only animals is based on chemical elements in the fossilised bones and teeth of Neanderthals, our closest extinct relatives. Following the concept of “you are what you eat”, chemical analysis (stable isotope testing) of carbon and nitrogen in skeletal remains is compared to the chemical elements of foods, with the idea that we can estimate the long-ago diet of humans and pre-humans from todays presence of skeletal nitrogen and carbon.
Based on this type of chemical analysis, Neanderthals consistently show very elevated nitrogen isotope values in bone collagen, indicating a heavily animal-based diet. The analysis of bone collagen from many Neanderthals regions aligns with that of associated carnivores (e.g., hyaenas and wolves). A couple of similar studies in Spy Cave Neanderthals, using the most recent compound-specific version of stable isotope testing, even reported that plants contributed no more than 20% to the diet. Thus, in support of ‘optimal foraging theory’ — that hunted animals were prioritised in the diet due to high energy input relative to energy lost pursuing and processing the food — stable isotope testing moderately supports the claim. Plants were eaten in this human species but in small amounts.
Keep in mind, however, that Neanderthals are only one type of human species that existed, and researchers estimate that only up to 5% of the modern human gene pool is derived from the Neanderthal. 20 other types of human species also existed, so supporting the claim with only Neanderthal-specific data is less wise than it is common.
Moreover, stable isotope testing is not necessarily the strongest form of evidence here. In a review of stable isotopes as biomarkers of dietary intake, professor Diane O’Brien concluded that “more validation work remains to be done for these measures to achieve their potential as tools for nutritional epidemiology”. Other scientists have handed more criticism to staple isotope testing results. Paleoanthropologist Amanda Henry claims that “…nitrogen isotope values and faunal studies, provide minimal information about plant food”. She said the complex nonlinear relationship between food source and consumer means the stable isotope results are littered with potentially false assumptions, and that because foraging humans usually eat plants for their carbohydrate content, the signal from plants is “…relatively swamped by the meat protein signal”. The Head of the Palaeodiet Research Lab, Luca Firoenza, agrees, claiming that “Mounting evidence suggests that the nitrogen isotope record may underestimate the consumption of starch/carbohydrate rich plant foods by humans”. It is true that researchers can accurately distinguish between vegans and omnivores by evaluating stable isotopes in serum and urine; however, this method is not validated to estimate the proportion of animals and plants in mixed human evolutionary diets.
If we look at other lines of research, we find evidence opposing the claim. For example, insights into modern-day hunter-gatherers who live(d) similarly to our distant ancestors does not align with the notion that plants were minimal in the human diet. In 2000, Loren Cordain and colleagues brought together evidence from 229 hunter-gatherer groups and reported that most of them obtain 22 – 40% of their energy from plant-derived carbohydrate — with a small portion of the dietary fat energy sourced from plants such as hazelnuts and acorns. Moreover, in a review of hunter-gatherers as models in public health, Herman Pontzer and colleagues stated that “…nearly all hunter-gatherers populations today and in the recent past are known to have a mix of both meat and plant foods in their diet” and that the Hadza in particular eat 80 – 150 grams of dietary fibre (a component of plants) every day — aligning with reports in Palaeolithic populations and findings that half of hunter-gatherer tribes obtain less than 50% of their energy from game meat.
Also opposing stable isotope data is evidence of plant remain sediments dating back millennia. In living regions of the past 100,000 – 780,000 years, numerous plant remains and/or their residue on stone tools has been found in Israel, South Africa, Gibraltar, and France, among other countries. The type of plant remains in each location includes one or a combination of nuts, fruits, seeds, vegetables, plant stems (rhizomes) and underground storage organs. The same food remains have been found in early modern human (< 100,000 years ago) sites in Mozambique, Sudan, Germany, Italy, Russia, and the Czech Republic, suggesting a consistency of plant food intake over the last mega-annum. To much surprise, one study that examined 10,000 – 130,000 year old remains from 30 populations in the Near East, Europe and Africa, concluded that “Middle Paleolithic Neanderthals probably consumed as many plant species as modern humans did”.
Of course, none of these studies directly show what proportion of the diet was comprised of these plant foods compared to animal foods, but we know that plant foods must have been eaten regularly. Frequently enough to still be found in dental calculus residue, too. Detailed analyses of both Neanderthal dental microwear and dental mesowear suggest more variation within Neanderthal diets than expected from stable isotope data. However, region is important; researchers note that dental wear in Neanderthals in more northern and dry environments was similar to modern forager groups consuming predominantly meat, whereas Neanderthal groups living in southern and wooded environments had dental wear similar to that of modern forager groups consuming a variety of foods. The dental plaque DNA of some Neanderthal groups, such as in El Sidron, Spain, even suggest a mostly vegetarian diet, relying on mushrooms, moss pine nuts, and tree bark.
A final line of evidence opposing the claim is a simple one: the comparison of our digestive tract to that of true carnivores. Unlike what you would expect from a carnivore, who tend to have a well-developed acid stomach and a long small intestine, the human gut is a simple stomach with an elongated small intestine and reduced caecum and colon, which does not fit any one group but lies somewhere between the frugivore (an animal that subsists mostly on fruit) and faunivore (an animal that eats other animals). In other words, our digestive tract would suggest that we are omnivores.
Overall, then, as simply put by Nathaniel Dominy, a Professor of Anthropology, “humans had a mixed diet”. The claim that humans evolved to eat only meat is probably false.
