This is one of the great paradoxes of evolution. Humans have shown that having a big brain is the key to success in evolution, yet this type of brain is extremely rare in other animals. Most animals have small brains and don't seem to need more neurons.
Why is that?The answer that most biologists agree on is that the energy needed for the brain to function is expensive. And, given the mode of action of natural selection, the benefits simply do not outweigh the costs.
But is it just a matter of size?Does the way our brains are organized affect its energy costs?A new study, published in Science Advances, offers some interesting answers.
All of our organs have an energy cost, but some have a low energy cost and some have a high energy cost. For example, bones require relatively little energy. While bones make up 15% of our body weight, they only consume 5% of our metabolism. The brain is at the other extreme, which weighs about 2% of the body's weight but consumes 20% of the body's total energy to function. And that's without particularly nervous thinking, even when we're asleep.
For most animals, the benefits of such an energy-intensive brain are simply not worth it. But for some hitherto unknown reason – and perhaps the greatest mystery in the history of human evolution – humanity has found a way to overcome the cost of the big head and benefit from it.
Of course, humans have to bear greater brain costs, but do these costs differ because of the particular nature of our cognition?Is it more expensive to think, talk, self-aware, or add than the typical daily activities of animals?
Answering this question is not easy, and the researchers started with a few known elements. The basic structure of neurons is roughly the same throughout the brain and in all species. Neuronal density is also the same in humans and other primates, so neurons are unlikely to be the driving force behind intelligence. If that's the case, certain animals with big heads, such as killer whales and elephants, may be more likely than humans"Clever"。
They also know that over the course of human evolution, the neocortex—the largest part of the outer layer of the brain, the cerebral cortex—developed faster than the others. This area includes the prefrontal cortex and is responsible for tasks involving attention, thinking, planning, perception, and incidental memory, all of which are required for higher cognitive function.
These two observations led the researchers to wonder if the energy costs of operating in different areas of the brain differed.
The team scanned the brains of 30 people using a technique that simultaneously measures glucose metabolism, an indicator of energy expenditure, and the amount of exchange between neurons in the cerebral cortex. They were then able to study the correlation between the two elements to see if different parts of the brain were consuming different levels of energy.
Neurobiologists will analyze and explore the details of these results, but from an evolutionary point of view, these results are already worth pondering. Researchers have found that energy expenditure in different areas of the brain varies widely. Not all parts of the brain are created equal when it comes to energy.
The most well-developed part of the human brain consumes more energy than expected. The neocortex requires about 67% more energy than the network that controls our movements.
This means that during human evolution, the metabolic costs of our brains not only increased as the brain grew, but also accelerated, and the neocortex developed faster than the rest of the brain.
Why is this happening?Neurons are neurons after all. The neocortex is directly related to higher cognitive function.
Signals sent through this area are mediated by brain chemicals such as serotonin, dopamine, and norepinephrine (neuromodulators), which form circuits in the brain that help maintain general levels of arousal (arousal in the neurological sense, i.e., wakefulness). These circuits regulate some areas of the brain more than others, and they control and alter the ability of neurons to communicate with each other.
In other words, they keep the brain active in order to store memories and think—a generally high-level cognitive activity. It is perhaps not surprising that the higher our level of advanced cognitive activity, the higher our energy expenditure.
At the end of the day, it seems that the human brain has evolved to such an advanced level of cognition not just because our brains are large or because certain areas of our brains are disproportionately developed, but also because of improved connectivity.
Many animals with large heads, such as elephants and killer whales, have a high level of intelligence. However, it seems possible to have a big head without developing human-level cognition"Correct"Circuit.
These findings help us understand why big heads are so rare. The big brain allows humans to evolve more complex cognitive abilities. However, it's not simply a matter of increasing brain size and energy at the same rate, but rather a matter of incurring additional costs.
This doesn't really answer the ultimate question: how did humans break through the ceiling of brain energy?As is often the case in evolution, the answer lies in ecology – the ultimate in energy. Growing and sustaining a huge brain – no matter what social, cultural, technological, or other activity it's used for – requires a reliable, high-quality diet.
To learn more, we need to explore the past million years, the period when our ancestors' brains really developed, to study the relationship between energy expenditure and cognition. Popular science