One of the greatest enigmas of human evolution is why we have such a large brain. We are very different from other animals, including our primate relatives. How and why did we end up with a brain so large in relation to our body size?
If a hypothetical extraterrestrial scientist were to look at photos of all primates, there would be one that would stand out for two obvious reasons: first, it goes naked, without fur; second, it is very big-headed, in proportion to the size of its body.
According to the encephalization quotient, which relates brain weight to the weight of an entire species' body, we are the winners. We have an index of 7.4, closely followed by dolphins with 5.3, and our closest primate relative, the chimpanzee, is far behind with 2.5, that is, three times less than humans.
It is known that the difference in size is related to intelligence, making us the most intelligent animal on the planet. However, it is important to explain how and why we came to have such a brain, since, despite the obvious benefits, it also has drawbacks. Something so large, in relation to body size, consumes a lot of energy.
Although the brain represents only 2% of our total weight, we allocate between 18% and 25% of our energy budget to it. Other primates, for example, barely dedicate 10%. As we can see, that great protagonist that helps to read these lines, understand them, and store them, needs to give many explanations about how and why it obtained such a large slice of the energy budget.
Where does it come from?
The brain has undergone a long evolutionary journey from its humble beginnings. To get an idea of what it was like in living beings about 1.5 billion years ago, let's think of marine invertebrates shaped like a sac, comparable to today's coelenterates, which barely have a set of nerve cells that allow for simple reactions.
The oldest evidence of something more similar to a brain dates back about 520 million years, belonging to a creature called Kerygmachela, only about 17 cm long, discovered in Greenland in 2019. It barely had the brain ganglion known as the protocerebrum with nerves branching out to the eyes.
The path towards a brain as specialized as ours began about 65 million years ago, after the disappearance of the dinosaurs, with birds and mammals. These are the two groups of animals that today have the highest encephalization quotient.
Within mammals, it would be primates that would take this evolutionary trend even further, with humans being the greatest exponent. Now, how and why did this happen?
Primates began colonizing trees about 55 million years ago. But a group of them returned to the land, and evolution adapted them to be increasingly efficient in this lifestyle. These are the hominoids, who, as early as 20 million years ago, began two evolutionary trends that would lead to us. One is walking on two legs, and the other is the increase in the encephalization quotient compared to other primates.
But thanks to the fossils discovered in Africa, we can observe the size of the brain of our fully bipedal ancestors, the australopithecines, and it did not differ much from that of a modern chimpanzee. However, from the australopithecines to us, in "only" 3 million years, the size of the brain tripled, going from 400 to the 1350 cubic centimeters of a modern human.
The leap occurred with a hominid who, for a reason, bears our surname, Homo erectus, which appeared about 2 million years ago. It already had a brain between 850 and 1000 cubic centimeters. The first explanation that emerged to understand this first leap, almost double that of the australopithecines, was related to the energy budget we mentioned earlier.
Homo erectus already mastered technology, a set of stone tools called Acheulean, very useful for processing the meat of animal carcasses. A series of changes in their body, compared to older hominids, also provide clues to the mystery of the brain: the reduction in the size of the teeth, the space inside the mouth, and the entire gastrointestinal system.
These indications in their fossil remains point to an important change in diet, since hominids more dedicated to a vegetarian diet needed large teeth, more efficient for chewing plants. Accompanied by spacious mouths, to be able to process more food at once, and finally a very large stomach and intestine, in order to digest a large amount of plants.
It is very different from how we could describe H. erectus, with reduced teeth and mouth, and much shorter intestines. The paleoanthropological evidence also suggests that meat and marrow were common in the diet of this ancestor of ours, which would partly explain how they could maintain such a large and energy-consuming brain.
Since, if they had continued on the exclusively vegetarian path, to meet those demands they would have had to dedicate about 14 of the 24 hours of the day to obtaining and eating plants. Let's think that between 2 and 1.5 million years ago, the times of Homo erectus, the fruits and vegetables they ate were not the ones we can get in the market today, which have undergone thousands of years of selection to be easier to digest and provide more nutrients.
Justifying Energy Budgets
In 1995, paleoanthropologist Leslie Aiello and colleagues proposed an evolutionary hypothesis suggesting that each increase in brain size should be balanced by a reduction in other energy-consuming organs, such as the gastrointestinal tract, heart, kidneys, or liver.
The solution to the energy crisis of the increasing brain in Homo erectus was a reduction in the intestines. However, this had to be accompanied by an increase in the quality of food, that is, high in nutritional value and easy to digest, as paleoanthropologist Daniel Lieberman relates in his book The Story of the Human Body.
Being a vegetarian without the help of today's technology requires large mouths, teeth, and intestines, and a lot, lot of time. A gorilla, for example, spends half the day chewing, and it is our closest fully herbivorous primate relative, with a brain of about 465 cubic centimeters.
On the other hand, a hunter-gatherer, on average, dedicates only about 5 hours a day to food, and most of that time is spent trying to obtain it, not chewing, digesting, and transforming it into energy.
But a large brain comes with a considerable cost. Ours costs us between 280 and 420 calories per day, while a chimpanzee's only consumes about 100 calories.
In today's modern world, a calorie-rich food is at our fingertips, but for a prehistoric hunter-gatherer it was not so simple. Even less so if we add to that the energy costs of feeding children. A pregnant woman with young children would need to obtain about 4500 calories in food. An enormous amount for the Paleolithic.
Seeing these numbers, and thanks to paleontological and archaeological evidence, experts have favored the hypothesis that Homo erectus was already obtaining enough energy, and more than anything, that the benefits of increased intelligence were already outweighing the cons of the energy cost. This would explain how the brain tripled in size in the million and a half years from H. erectus to us.
The Social Brain
The increase in size implied an increase in the number of brain cells. More neurons mean more connections, better cognitive capacity. This allowed for an improvement in the ability to cooperate. Thus, we arrive at one of the main drivers of the evolution of the human brain, which is the cognitive capacity to deal with complex social networks.
The vast majority of those extra neurons that we have are located in the outer layer of the brain, the neocortex, where almost all cognitive functions take place, such as memory, thought, language, and consciousness.
By increasing the size of the neocortex in humans, since the time of Homo erectus, the potential to perform complex tasks such as reasoning, remembering, thinking, or cooperating increased.
There is a general consensus among experts, as anthropologist Robin Dunbar recounts in his book Human Evolution, that complex social relationships are the main driver in the evolution of the brain among mammals, birds, and primates. In the latter, it went even further, with humans taking the lead by three positions.
For example, a chimpanzee has a brain of about 390 cubic centimeters, and an encephalization quotient of 2.5. Although it is far from our 1350 cm³ and 7.4, it is double that of any other mammal of the same body size. Chimpanzees often cooperate and maintain complex social groups, but at a much lower level than humans.
In a famous analysis, Robin Dunbar showed that the size of the neocortex, among primates, was related to the size and complexity of the groups and social networks that they could maintain (see box Anatomy of friendship). While other aspects of behavior, such as creativity, intelligence, and inventiveness, are related to how large the brain is, they are consequences and benefits, not the cause of its evolution.
The reason why evolution selected for an increase in brain size among mammals was because of how important it is for these species to be able to deal with the cognitive demands of group relationships. These relationships provide them with a significant advantage for survival in the environment.
Among primates, groups were transformed into social networks with alliances, friendships, couples, and relatives. In humans, the need to deal with complex networks in which cooperation was important would have selected for the evolution of an even larger brain, with more neural connections.
In the analysis that Dunbar carried out for decades, the cognitive capacity of each primate is equated with the maximum number of relationships it can maintain. Not only does brain capacity count, but also the time it takes to nurture and feed those relationships. Among chimpanzees, the maximum group size is 50 individuals, which is consistent with their communities in the wild.
In the case of humans, it is about 150 people, which in hunter-gatherer groups is known as a community, usually with a certain degree of kinship, even if it is not distant. In the modern world, those 150 are what we would call relatives and friends.
Being able to navigate within social networks triggered an evolutionary spiral that ended up leading to the Machiavellian intelligence that characterizes humans. In the ability to form complex kinship relationships, alliances, coalitions, federations, nations, which allowed them to thrive in a changing environment.
As an evolutionary byproduct, those extra connections and neural pathways that formed gave us an enormous power of association, which led to an increase in reasoning and intelligence, and with it the creation of technology, science, culture, societies, and civilizations. In short, to be human.
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