Did humans evolve to be carnivores

Evolution of the human diet

Natural selection indicates that as organisms evolve, the organisms that adapt to their environment to the greatest degree eventually reproduce, and organisms that are unable to act in congruence to their environment do not survive long enough, or healthlly enough, to reproduce. Thus, as organisms evolve they will adopt behaviors that allow for the most optimal health and vitality, and will terminate harmful and non beneficial behaviors. For example, throughout our entire evolution, humans have maintained the behavior of sleep. At face value, sleep is simply a period of time in which mammals are unable to carry out any productive behaviors. Furthermore, a sleeping organism is at a much greater risk of being killed by a predator due to unconsciousness and immobility. If sleep was not absolutely necessary for their health and vitality, the organisms that thrived throughout history would likely be the organisms that did not require sleep. Understanding natural selection allows one to look at the habits that have been consistent throughout evolution and determine the dietary behaviors that supplied humans with the most optimal mental, and physical function. The most accurate form of determining the diet throughout human evolution is through studying stable carbon and nitrogen isotopes of the bones of dead organisms. The results of isotope studies of late Neandertals indicates that humans evolved as carnivores who derived the majority of our energy from large herbivorous animals. Further, humans were likely at the top of the food chain, that is humans were the apex predators. (1) Simply put, when given the opportunity humans would hunt, kill, and eat animals to fulfill their energy requirements. It is estimated that Neandertals' diets were derived from no less than 80% of energy obtained from animals. Furthemore, the simple yearly availability of non animal products would fluctuate throughout the year. If humans were not capable of subsiding strictly off of animal nutrition, early humans would not have survived during times of extreme weather that reduced their accessibility to any nutrition other than animals. When large animals were available for consumption, that was the preferred source of energy. Fruits and vegetables were eaten for subsistence between successful hunts, and simply for entertainment. These actions were performed in the absence of scientific reasoning, rather based on what produced the most beneficial outcomes in the past. With our current availability of food, we are capable of having an abundance of animal food present whenever we need to fulfill our energy requirements, thus we have no need to use plant energy for subsistence until we can obtain animal products.

Human physiology

In addition to our evolutionary actions, the internal physiology of humans indicates that we have developed intestinal systems that allow us to digest and absorb animal products to a greater degree than animal products. Humans rely on enzymatic digestion of food in the foregut, dissimilar to a herbivorous animal that relies on bacterial fermentation to extract and curate the necessary nutrients they require for life. Humans have a large small intestine, and a small colon. Therefore our digestive system has evolved to ingest very nutrient dense food sources due to the limited amount of space for excess food volume. An decrease in the size of the human gut contributes to an increase in the size of the human brain. Scavenging and consuming animal products induced an increase in the size of the human brain, and development of our modern anatomy. (2,3)Humans hunted and killed extremely large ruminant animals that provided their energy requirements for multiple days. As a result, these meats began to develop large amounts of bacteria. However, as mentioned previously, these bacteria were unable to flourish in such a highly acidic environment, therefore humans evolved with a low stomach pH to facilitate their highly carnivorous diets. Modern humans still possess a low stomach acid, favoring a more carnivorous diet to obtain adequate nutrient absorption. The human stomach began to decrease in correlation with an increase in brain size. Due to the energy demands required by the brain, as the brain began to grow, selection pressure dictated that humans needed to decrease energy demands elsewhere. Thus our brains began to increase in size as our guts simultaneously shrank. A decrease in gut size is indicative of an increase in diet quality. Our brains began increasing in size as humans transitioned from a largely herbivorous diet, into an increasingly higher consumption of animal products, beginning with bone marrow and the scavenging of animal remains. The increased consumption of animal products dictated that humans did not require such an extensive digestive system in order to digest lower quality plant nutrients, thus we altered our gut morphology to support the consumption of animal products that were easier to digest, and provided a greater benefit. Selection pressure favored carnivorous humans because they were capable of expending less energy for digestion, allowing for more energy supply for brain growth. 30,000 years ago, as humans increased the consumption of grains, we have observed a thinning of the bones, deterioration of tissue strength, an increase in disease, a decrease in cognitive function, and a decrease in both height and brain size, as grain consumption increased. (4) The pH of a human stomach is estimated to be 1.5. Humans stomach acidity is most similar to that of a scavenger. The majority of a scavenger's energy is derived from decayed or rotting meat, thus requiring a very high stomach acidity in order to kill the bacteria that are present within the foods they consume. Conversely, herbivores have an average stomach acidity between 4.1-6.1, depending on the structure of their gastrointestinal system. Herbivores' stomach acidity developed to allow bacteria to live within the cecum of their stomach. These bacteria are responsible for the absorption and digestion of cellulose via the cellulase enzyme that is only produced by these bacteria. These bacteria turn fiber into acetate, propionate, and butyrate, which are fatty acids that can be used for the production of milk, and conversion to glucose in the liver of herbivores. Humans do not have the bacteria required for the digestion of cellulose. Having a low stomach pH is more energetically demanding than having the same stomach pH as herbivores, or other omnivores. Therefore there must have been a physiological advantage for humans with lower stomach pH’s, otherwise selective pressure would likely have favored humans with less energetically demanding gastrointestinal systems if it did not provide a benefit.(5) The most postulated hypothesis for the extreme acidity of a human stomach is due to our carnivorous evolution. Humans were estimated to have become apex predators between 2-2.5 million years ago. Human rise to apex predation correlates significantly to an increase in bone marrow and animal remains as a result of scavenging. Animal contained specific nutrients such as nicotinamide, that sparked the development of new neural systems on top of the primate brain. (2,3)Humans hunted and killed extremely large ruminant animals that provided their energy requirements for multiple days. As a result, these meats began to develop large amounts of bacteria. However, as mentioned previously, these bacteria were unable to flourish in such a highly acidic environment, therefore humans evolved with a low stomach pH to facilitate their highly carnivorous diets. Modern humans still possess a low stomach acid, favoring a more carnivorous diet to obtain adequate nutrient absorption.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6421459/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5417583/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6032294/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4892290/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4519257/