When we compare ourselves to other species, we all share a secret point of pride: We think, they don’t! Forget the PC notion that “they”—those other animals—are equal, just different. The fact is, unlike us, other species don’t plan for the long term. They don’t worry about how they are going to save to send their offspring to college in 20 years. And, they don’t make wedding arrangements for a year from now, let alone structure prenuptial agreements in case it doesn’t work out. For better or worse, human beings do spend time and energy thinking about these things. It takes brainpower and it doesn’t come for free. Let me explain.
The evolution of our thinking brain
The first agglomerations of neurons that could be called a brain appeared about 500 million years ago. For the next 430 million years, nothing really radical changed the basic structure and function of the primitive brain. In fact, we still possess these ancient structures and most of their associated functions deep in the architecture of our brains.
Then, about 70 million years ago, the first primates entered the evolutionary stage and after another 70 million years, the first proto-humans appeared. If you do the math, this timeline leaves no room for the human (including our brother and sister Neanderthals and Denisovans) to develop. This is because understandably, those dates are “plus or minus a couple of millions”, and our brains are only about 1.6 million years old! As Einstein famously observed, everything is relative, and compared to the older brains, ours is a newborn.
Bigger and better
The neocortex is smooth in rodents and other small mammals, whereas in primates and other larger mammals, it has deep grooves (sulci) and ridges (gyri). These folds allow the surface area of the outer layer of the cortex (neocortex) to increase far beyond what could fit in the same size skull, which necessitated an increase in its size. What prompted the increase in brain size? Probably adaptive genetic mutations that conferred reproductive advantages in new circumstances, such as the pressure of living in small bands, which require greater cooperation and competition in early ancestors.
But the change was not only in gross anatomical terms. Cognition, our ability to think, compute, make judgments, and plan, also made a great leap forward. At first, the process took a few hesitant steps, but then, it gained tremendous momentum when our first human ancestor who walked just like we do, Homo erectus, entered the stage of evolution.
With the size increase, there was greater voluntary inhibitory control of social behaviors resulting in increased social harmony. About 20% of our neurons are inhibitory, serving not only to help regulate such vegetative functions as the cardiac, respiratory, or gastrointestinal systems, but also social, suppressing antisocial thoughts (i.e., the impulse to kill or rape) so as to ensure propagation of our own DNA. Sounds good, but what are the costs?
The high cost of being brainy
As I have said, we have a bigger brain, all 3 pounds of it—triple that of our closest apish relative. But here is an astonishing fact: Despite making up only about 2% of our body mass, this blob of neurons consumes 20% of our metabolic energy output. Why?
The propagation of electrical activity along the axon membranes is energetically very expensive. There is tremendous electrical activity going on in the brain all the time. The brain doesn’t shut down even when we sleep. In addition to the regular vegetative tasks of keeping our heart beating, our lungs breathing, and our digestive enzymes digesting away, it performs other tasks, such as dreaming and consolidation of memories. All of this neuronal activity consumes loads of energy.
How could we possibly provide the brain with all this energy?
This question has consumed anthropologists for two decades. Several theories have been proposed, all plausible, and probably all correct as far as they go. One possible explanation was the radical shrinking of the gut of our H. erectus ancestor to consume less energy. Another proposal was that reduced muscle mass freed more energy for brain consumption (attention body builders!). Yet another energy conservation proposal focused on our gait: We walk and run more efficiently that our cousin apes.
On the supply side, there is the observation that we started consuming more energy-rich and protein-rich foods, such as tubers and meat. When we learned how to cook, we released even more energy from these foods.
All of these theories are probably correct. The selective pressure to provide more energy to the brain probably required an “all of the above” solution. But there is an additional problem: The bottleneck of metabolism. How can our metabolism handle such an increase in available energy sources? Think about it. If we fed chimps in captivity cooked meat and potatoes, are they going to turn into brainy wunderkinds? Of course not, they’d just get fat.
The solution to this conundrum was reported in a paper in Nature by a group of scientists who measured the total energy expenditure (TEE) of apes and human volunteers. The researchers fed 27 chimps, 8 bonobos, 10 gorillas, and 11 orangutans water labeled with certain isotopes of hydrogen and oxygen. Then, they measured those two isotopes in the apes’ urine to see how the ratio of the two molecules changed over time. The ratio reveals how much carbon dioxide the animal had generated which, in turn, reflects how many calories it had burned. The same method was used on 141 adult humans from five populations around the world.
After taking body size into account, they found that humans averaged intake of about 400 more calories per day than chimps and bonobos, 635 calories more than gorillas, and 820 calories more than orangutans. This meant that humans burned over 27% more energy per day on average than chimps. In other words, the metabolic bottleneck that keeps the apes from increasing their brain size does not exist in humans. We are simply higher energy apes!
Which leads to more questions…and surprising answers
Food resources in the days of our early ancestors were not as plentiful, nor availability as predictable, as they are today. With such an energy-guzzling metabolism, humans were in imminent danger of starvation when food was not available for an extended period of time. So, it should be no surprise that natural selection favored individuals who could store energy for use during lean periods. Nowadays, we are all too familiar with this energy storage tissue, a.k.a. fat.
But wait, the story doesn’t end here. As we have become more and more sedentary, our food resources more plentiful, and our diet larded with more fat and a generous sprinkling of sugar, our energy intake has far exceeded the requirements of even the most energy-hungry brain. The changes in our environment have overtaken evolution’s slow pace, and like the overfed chimp in captivity, we haven’t become smarter—just fatter. And, with the fat comes a whole slew of other problems…but that, my dear readers, is a story for another time.