by It is one of the great paradoxes of evolution. Humans have shown that large brains are critical to our evolutionary success, and yet such brains are extremely rare in other animals. Most of them thrive on small brains, and they don’t seem to lose the extra brain cells (neurons).
Why? The answer most biologists have settled on is that large brains are expensive in terms of the energy they require to run. And, given how natural selection works, the benefits do not outweigh the costs.
But is it just a matter of size? Does the way our brains are set up also affect their costs? A new study published in Science Advances has provided some interesting answers.
All our organs have running costs, but some are cheap and some are expensive. Bones, for example, are relatively cheap. Although they make up about 15% of your weight, they only use 5% of your metabolism. Brains are at the other end of the spectrum, and at about 2% of a typical person’s body weight, they use to run about 20% of our metabolism. And this without any conscious thought – it happens even when we are asleep.
For most animals, the benefits of serious thinking are not worth it. But for some reason – perhaps the biggest problem in human evolution – humans found ways to overcome the costs of a larger brain and reap the benefits.
All this is well known, but there is a more tantalizing question. Humans certainly have to bear the greater costs of our brains because they are so large, but are there different costs because of the special nature of our cognition? Does thinking, speaking, being self-aware or doing sums cost more than normal day-to-day animal activities?
It’s not an easy question to answer, but the team behind the new study, led by Valentin Riedl from the Technical University of Munich, Germany, has risen to the challenge.
The authors initially identified several points. The basic design and structure of neurons is much the same across the brain – and across species. The neuronal density is also the same for humans and other primates, so it is unlikely that these are the drivers of intelligence. If they were, some animals with big brains like orcas and elephants would probably be smarter than humans.
They also knew that throughout human evolution, the neocortex – most of the outermost layer of the brain, called the cerebral cortex – has expanded at a higher rate than other parts. This region, which contains the prefrontal cortex, is responsible for tasks related to attention, reflection, planning, perception and episodic memory – all of which are required for higher cognitive function.
These two observations led them to investigate whether there are different costs associated with signaling across different regions of the brain.
The team scanned the brains of 30 people using a technique that could measure glucose metabolism (a measure of energy consumption) and the level of signaling throughout the cortex at the same time. Then they could look at the correlation between these two elements and see if different parts of the brain used different levels of energy – and if so how.
Surprising results
The details of the findings will no doubt be pondered and explored by neurobiologists, but from an evolutionary perspective, they are exciting. What they discovered is that the difference in energy consumption between different areas of the brain is large. Not all parts of the brain are equal, energetically speaking.
Not only that, but the most enlarged parts of the human brain had higher costs than expected. In fact the neocortex required about 67% more energy than sensorimotor networks per gram of tissue.
This means that during human evolution, the metabolic costs of our brains not only increased as they got bigger, but they did so at an accelerated rate as the neocortex expanded faster than the rest of the brain. brain.
Why should that be so? A neuron is a neuron, after all. The neocortex is directly involved in higher cognitive function.
The signals sent throughout this area are mediated by brain chemicals such as serotonin, dopamine and noradrenaline (neuromodulators), which create circuits in the brain to help maintain a general level of excitement (in the neurological sense of the word meaning to be awake , not to be crazy). These circuits, which control some brain areas more than others, control and modulate the ability of neurons across the brain to communicate with each other.
In other words, they keep the brain active for memory storage and thinking – a higher level of cognitive activity in general. Perhaps unsurprisingly, the higher level of activity associated with our cognition comes at a higher energetic cost.
Ultimately, the human brain seems to have evolved to such high levels of cognition not just because we have big brains, or even because certain areas of our brains have grown disproportionately, but because – at a cost – connectivity improved.
Many animals with large brains, such as elephants and orcas, are extremely intelligent. But it seems that you can have a big brain without developing the “right” circuitry for human-level cognition.
The results help us understand why larger brains are so rare. A larger brain can enable more complex cognition to evolve. It’s not just a question of increasing brains and energy at the same rate, but taking on additional costs.
This doesn’t really answer the ultimate question – how did people manage to break through the brain energy ceiling? As is often the case in evolution, the answer must lie in ecology, the ultimate source of energy. Growing and maintaining a big brain—regardless of social, cultural, technological, or other factors—requires a reliable, high-quality diet.
To learn more, we need to explore the last million years, the period when our ancestors’ brains really expanded, to investigate this interface between energy expenditure and cognition.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
The authors do not work for, consult with, or own shares in, or receive funding from, any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
