Was cooking the crucial part of developing a human brain size or was its use of Stone Age tools, or whether adding starch-rich USOs (underground storage units) or meat to the diet?

What was the most crucial energy source that provided much-needed energy for the development of the brain size?

The debates are emotional in nature, and not as logical as science needs to be. It is because of our underlying desire to prove to ourselves that meat consumption is natural for human evolution so that we can justify large-scale meat consumption in the modern era. The scientific and archeological data can become a problem in this scenario if data don’t reflect the desirable way of looking at things.

Scientists are not immune to emotional bias. In order to have large-scale meat consumption on a daily basis, the two criteria must be met.

(1)    We need to have a viable option for acquiring the meat.

(2)   We need to have the physiological ability to digest it.

The first criteria for humans that are not anatomical hunters and are slow and weak and cannot compete with true anatomical hunters are to scavenge for it. That option will not support the calorie requirement and can only be an additional source of calories to a small extent. For the second option, we would need to have fire technology. Subsequently, anything before Homo erectus is excluded. Some scientists believe that even Homo erectus was not capable of controlling the fire. It is a big debate.

Currently, the earliest well-accepted instance of fire-burning in a controlled manner came from Israel’s Qesem Cave 400,000 years ago.

When you don’t hunt and you live on a tree and you need to forage for edible leaves and fruit you have a difficult life. What happened then is a process of natural selection that strongly favors traits that enhance the efficiency of foraging. Hence, as plant foods became increasingly important over time adaptation gradually gave rise to the group of characteristics presently considered the property of primates. Most of these traits are adapted to facilitate the movement and foraging in trees.

For example, adaptation yielded hands well suited for grasping branches and manipulating slender and small fruit and leaves. In order to detect ripe fruits and enable safe moving through arboreal habitats adaptation forced improvement of the optical capabilities (including depth perception, sharpened acuity, and color vision). Good vision is crucial for moving through the three-dimensional space of the forest canopy and quickly determining the appearance of ripe fruits or tiny, young leaves. Carnivorous species do not have full-color vision. They do not need to detect ripe fruits.

Moreover, such environmental pressures also favored the ability to learn and remember the identity and locations of edible plant parts and also to calculate the optimal foraging strategies to save energy thus increasing behavioral flexibility as well. Foraging benefits from the improvement of visual and cognitive skills. As a result, it promoted the development of unusually large brain sizes, a characteristic of primates since their inception.

Eating meat or bone marrow had nothing to do with the development of the larger brain size. Different plant foods will lack the different nutrients we need. For example, one plant may have some but not all amino acids and vitamins at an adequate level, or even if it is nutrient-dense and doesn’t have fiber it may lack energy in the form of carbohydrates (starch and sugar). Mammals that depend primarily on plants for meeting their daily nutritional requirements and are not adapted for one particular plant food source that is in abundance as a consequence must seek out a variety of complementary food sources from a different array of plants.

They have to combine different food types to get all of the nutrients they need. This demand greatly complicates food gathering. It is a tough life, and it is a constant struggle for food and requires constant use of thinking.

Most arboreal hominids and other primates concentrate on ripe fruits on one side and young leaves. They eat other types of food too, but these two are the main ones. Fruits tend to be rich in energy in the form of fructose and relatively low in fiber, but they might not provide all of the essential amino acids and tend to be the rarest of all plant sources. This kind of scarcity complicates things because if in a certain period of the year there are no fruits available. During that time period, the energy requirement is not met, and there is a need for supplementation with different plant sources. Leaves are full of protein and are everywhere, but they are of lower quality meaning there are no carbohydrates in them and we cannot live on them alone, and they tend to be filled with undesirable toxic chemicals.

Because primates are not adapted for digesting fiber they eat young leaves that are softener than the tough old ones that cannot be digested. When trees exhibit seasonal peaks in the production of fruits and young leaves primates have to eat them as much as they can and reliance on a single food choice is not sustainable.

From an evolutionary view, there are two basic strategies for coping with these problems.

One is to increase the efficiency of nutrient extraction from fibrous foods. This is a form of adaptation that we can see in mammals that are grazers.

For hominids in the past and also for primates, and humans fiber essentially go through their stomach unchanged.

Another biological adaptation that can facilitate survival on low-quality plant food is to grow larger over time. When an animal goes larger compared to smaller animals, it will consume greater overall amounts of food to feed its more extensive tissue mass. However, for reasons that science had not been able to entirely explain, the more massive the animal is the fewer calories it needs to sustain itself and attain adequate nourishment. In mathematical terms, larger animals need less energy per unit of body weight. What this means is that larger animals are able to eat less and can eat lower-quality food to meet their energy requirements.

However, growing bigger for primates is not an option because they are arboreal animals. For growing too massive, they risk falling to their death.

Another evolutionary strategy is open to arboreal plant eaters and is more behavioral than biological.

It is a foraging strategy. Because fruits are rare and very sporadically scattered in tropical forests, the strategy requires the implementation of practices that promise to reduce the energy of acquiring these resources. In order to survive the primates must use their brains more and more to form foraging strategies that are sustainable. A good memory would significantly improve the approach. Ability to recall the exact places of plants that produce desirable fruits and when these trees were likely to bear ripe fruits and to remember the precise directions to these trees would improve foraging profitability in energy expenditure sense by lowering search and travel energy costs by enlarging brain capacity to remember and to plan in advance.

In comparison, grazers do not need brain development because their food is all around them and all they need is to lower their head. Reliance on memory and foraging strategies have pushed for the selection and development of a bigger brain size that has a higher ability for storing information. As a group, primates have always depended on selective feeding and on having the brainpower to carry off this strategy successfully.

The growth of the brain size in combination with growth in body size and a decline in teeth size supports the notion of a high-quality diet. And this is an evolutionary adaptation that is universal to all primates in the last 66 Ma. Some have gone far like humans. We have a brain evolved enough to create pure refined white sugar.

Most other plant-eating species, in opposition, have tended to focus heavily on physiological adaptations for better digesting fiber in order to reduce the need to invest energy in searching for high-quality food. Behavioral adaptations, requiring increased brain power, enable certain species to choose high-quality food.

If we look calorie-wise, the brain is the most expensive organ to maintain. It takes over the vast amount of energy from food, roughly 20% at rest in humans. Natural selection is not going to favor the development of a massive brain size if it is not going to get any benefits from enlargement. The appearance of modern humans with big and capable brains occurred because natural selection favored adaptations that focused on the efficiency of foraging. That was the line of evolution that permitted primates to focus their feeding on the most energy-dense, low-fiber diets they could find, and find is a crucial word. 

Finding high-quality food in a scarce environment is what created modern humans. It had little to do with eating meat or any other form of energy. A form of energy is of lesser importance than the way that energy is obtained. In other words, if the meat had anything to do with the development of brainpower, then all of the carnivore species on this planet will be colonizing the outer reaches of the galaxy by now. There is no magic nutrient in the meat that was responsible for the rise of human brainpower. Meat is just meat, another energy source.

There is no absolute correlation between meat eating and intelligence. The manner of combining some amount of foraged meat with a predominantly vegan diet did not become a pivotal force in the emergence of modern humans. Also, it is not even correlated to brain size either. There is no particularly strong relationship between brain size and intelligence, with a correlation value between 0.3 and 0.4 out of a possible 1.0. it is the number of neurons in the brain no matter what that brain size is, that counts (1). The human brain has the largest number of cortical neurons (about 15 billion), despite the fact that the human brain and cortex are much smaller in size than, for example, those of cetaceans and elephants (with 10–12 billion or even fewer cortical neurons).

Sources:

Passages selected from a book: “Go Vegan? Review of Science: Part 1” [Milos Pokimica]

1. Neuronal factors determining high intelligence doi: 10.1098/rstb.2015.0180
2.  No sustained increase in zooarchaeological evidence for carnivory after the appearance of Homo erectus.  doi: 10.1073/pnas.2115540119

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