Meat Made Us Human

David Archibald

Another COP, another call to have meat removed from our diet. The further we depart from what we evolved to do, as individuals and as a society, the less efficient we become. We didn’t evolve to be vegetarian. Quite the contrary, humans are one of the most carnivorous animals on the planet, surprisingly so.

Eating meat is more efficient. Carnivores spend less time feeding than similar-sized herbivores. For example, one of our primate relatives, baboons (Papio cynocephalus), devote almost all their daylight hours to feeding while adult males of hunter-gatherer Ache (eastern Paraguay) and Hadza (northern Tanzania) tribes spend only a third of the day in food acquisition, preparation and feeding. Acquiring and consuming medium-size animals, at a return rate in the range of tens of thousands of calories per hour, is an order of magnitude more time-efficient than plant-gathering.  In nature, for humans, plant-sourced calories cost ten times the price of meat if it is available.

The most energy-dense macronutrient is fat (9.4 kcals/g), compared with protein (4.7 kcals/g) and carbohydrates (3.7 kcals/g). Plant proteins and carbohydrates typically contain antinutrients with functions in plant growth and defence. These antinutrients, such as lectins or phytate, limit full energetic utilization and nutrient absorption by humans. Phytates also limit iron absorption from plants. Up to 30% of the iron in meat can be absorbed while iron absorption from plants is limited to about 10%. That is why iron deficiency is a common symptom of vegetarianism.

Then there is the matter of stomach acidity. The acid involved is hydrochloric acid. Carnivores have a stomach acidity of pH 2.2 on average and omnivores are pH 2.9. Obligative scavengers, which feed almost exclusively on carrion, have a stomach pH of 1.3, while facultative scavengers, with a proportion of diet being dead animals they have found, have a stomach pH of 1.8. The human stomach acid level lies between the two types of scavengers with a pH of 1.5 which equates to a hydrochloric acid content of 0.115 percent. This is some 30 times the stomach acid concentration of omnivores.

Figure 1: The acidity spectrum by feed type

pH can be misleading as it is a log scale. Restated as percent of acid in the stomach, humans are well into scavenger territory.

Producing stomach acid and maintaining the stomach walls to contain it are energetically expensive. It seems that our antecedents found that it was easier to eat semi-rotten meat from a large carcass than to hunt for a fresh animal. Note that we would rather eat semi-rotten meat than increase the proportion of vegetables in our diet. We are hyper-carnivores – meat-eaters that keep going even as the meat starts rotting. Vegetables would take us backwards. This is instinctive knowledge.

Another attribute we share with carnivores is low insulin sensitivity which means that our muscles, fat and liver have a weak response to insulin. The role of insulin is to direct these organs to take glucose from the blood. It has been speculated that physiological insulin resistance allows humans on a low-carbohydrate diet to conserve blood glucose for the energy-hungry brain.

Most plant-eaters extract most of their energy from the fermentation of fibre by gut bacteria, which occurs in the colon in primates. For example, a gorilla extracts some 60% of its energy from fibre. The human colon is 77% smaller, and the small intestine is 64% longer than in chimpanzees, relative to chimpanzee body size. Because of the smaller colon, humans can theoretically obtain less than ten percent of total caloric needs by fermenting fibre, with the most rigorous measures suggesting that it less than four percent. That 77 percent reduction in human colon size relative to our closest primate relative points to a marked decline in the ability to extract the full energetic potential from many plant foods. The elongated small intestine is where sugars, proteins, and fats are absorbed. Sugars are absorbed faster in the small intestine than proteins and fats. A long small intestine relative to other gut parts is also a dominant morphological pattern in carnivore digestion.

Another thing about being human, relative to other primates, is the reduced size of our mouth and jaws. While chimpanzees spend 48 percent of their daily hours chewing, humans need to devote only five percent of their day in chewing. The shrinkage of the relative size of the jaw started with Homo erectus 1.9 million years ago.

Figure 2: Dentition change in response to meat – it started with Homo erectus

From left to right, the skulls of Homo erectus, Homo heidelbergensis, Homo neanderthalensis and Homo sapiens

Our energy-dense, high-meat diet also allowed humans to have a lower weaning age than other primates. In orangutans, gorillas, and chimpanzees, weaning age ranges between 4.5 and 7.7 years but is much lower in humans of hunter-gatherer societies at 2.5–2.8 years, despite the long infant dependency period.

The energy metabolism of humans is adapted to a diet in which lipids and proteins, rather than carbohydrates, make a major contribution to the energy supply. At the same time, humans are limited in how much protein can be converted to energy. This is a third to a half of normal caloric requirements, due to the liver and kidney’s limited ability to remove large quantities of the toxic nitrogen byproducts of protein metabolism – urea and creatinine⁵. Thus, depending on the relative energetic returns and abundance of plants and animal fat, humans have to obtain 50–65% of their calories from either animal fat or plant fat and carbohydrates. Thus, in the absence of readily available plant foods, humans have an important obligatory requirement for animal fat. Some hunters will give up on an animal once it is perceived to have a low fat content. The targeting of fat, at substantial energetic costs, could point to chronic maximal protein consumption.

Changing a plant-dominated diet to a meat-based one allowed the development of a larger brain. Once that process was underway it was self-reinforcing to the limit of the ability of the brain’s neurons to communicate with each other. The increase in brain size allowed humans to become more successful hunters which ultimately led to the Late Quaternary Extinction in which the megafauna of many continents was killed off. This was a problem for Stone Age hunters as they preferred to hunt larger, adult animals which have a high fat content. Hunting small and medium-sized prey takes more effort relative to the reward.

Dogs were domesticated not long after the Late Quaternary Extinction of the megafuana. As dogs can utilise a higher proportion of protein in their diet than humans, it has been proposed that dog domestication is a form of joint venture between humans and wolves/dogs. Under the arrangement, humans contributed surplus meat protein from relatively fat-depleted animals that dogs could utilize but humans could not. In return, dogs helped humans save energy by helping to track and chase smaller animals. In most ethnographic cases, dogs are employed to aid in hunting smaller animals. It is conceivable that dogs were domesticated as a behavioural adaptation to the increased energetic demands of hunting a larger number of smaller animals as prey size declined. Dogs aren’t the only animals with a symbiotic relationship with humans. For example, honeyguides are African birds that lead humans to bee hives.

When did the rot set in? A high caries prevalence, a sign of intensive carbohydrate consumption, first appears in Morocco about 15,000 years ago, together with evidence for starchy food exploitation. Caries prevalence in humans increased markedly after the transition to agriculture. The transition to agriculture though produced an explosion in human productivity as it removed the repressive effect of group food pooling behaviour which regulated the distribution of meat within a tribe. Group food pooling behaviour, necessary to even out the supply of meat for a family, punished hard work with a higher death rate while not rewarding it with an increase in the food received. The agricultural revolution flipped the scrip and suddenly working harder was rewarded by the ability to store excess food with a survival benefit.

Humans are now in the optimal position of growing grain to feed animals to produce meat. Meat made us human. Vegetarianism is a psychological condition on the spectrum, not as bad as gender dysphoria with its 50 percent suicide rate but self-harm is involved nevertheless.

David Archibald is the author of The Anticancer Garden in Australia.