It is one of the great paradoxes of evolution. Humans have shown that having large brains is key to our evolutionary success, and yet such brains are extremely rare in other animals. Most live on small brains and do not seem to lack the extra brain cells (neurons).
Why? The answer most biologists have arrived at is that large brains are expensive in terms of the energy they need to function. And given the way natural selection works, the benefits simply do not outweigh the costs.
But is it just a matter of size? Does the way our brains are organized also influence their costs? A new study published in Science Advances has provided some intriguing 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 opposite end of the spectrum, and at about 2% of normal human body weight, using them requires about 20% of our metabolism. And this without consciously thinking about it – it happens even when we sleep.
For most animals, the benefits of serious thought are simply not worth it. But for some reason—perhaps the biggest puzzle in human evolution—humans have found ways to overcome the costs of having bigger brains and reap the benefits.
All this is quite well known, but there is a more tantalizing question. Certainly, humans must bear the higher costs of our brains because they are so large, but are there other costs due to the special nature of our cognition? Does thinking, speaking, being self-aware or doing sums cost more than typical daily animal activities?
It’s not an easy question to answer, but the team behind the new study, led by Valentin Riedl of the Technical University of Munich, Germany, has risen to the challenge.
The authors had some well-known points to start with. The basic design and structure of neurons is much the same across the brain – and between species. Neuronal density is also the same in humans and other primates, so these are unlikely to be the driving force behind intelligence. If that were the case, some animals with big brains, such as killer whales and elephants, would probably be smarter than humans.
They also knew that the neocortex – the largest part of the brain’s outer layer known as the cerebral cortex – has expanded faster than other parts over the course of human evolution. This region, which involves the prefrontal cortex, is responsible for tasks involving attention, thinking, planning, perception and episodic memory – all necessary for higher cognitive function.
These two observations led them to investigate whether there are different costs for signaling in different brain regions.
The team scanned the brains of thirty people using a technique that could simultaneously measure glucose metabolism (a measure of energy expenditure) and signal levels in the cortex. They could then look at the correlation between these two elements and see whether different parts of the brain used different energy levels – and if so, how.
Surprising findings
Neurobiologists will certainly ponder the fine details of the results, but from an evolutionary point of view they are thought-provoking. What they discovered is that the difference in energy consumption between different parts of the brain is large. Not all parts of the brain are energetically equal.
Not only that, but the parts of the human brain that expanded the most had higher costs than expected. In fact, the neocortex required approximately 67% more energy than sensorimotor networks per gram of tissue.
This means that over the course of human evolution, not only did the metabolic costs of our brains increase as they grew in size, but they did so at an increasingly rapid rate as the neocortex expanded faster than the rest of the brain.
Why would that be the case? After all, a neuron is a neuron. The neocortex is directly related to higher cognitive function.
The signals sent by this area are mediated by brain chemicals such as serotonin, dopamine and norepinephrine (neurommodulators), which create circuits in the brain to help maintain an overall level of arousal (in a neurological sense of the word meaning being awake, no pleasure). These circuits, which regulate some areas of the brain more than others, control and modify the ability of neurons in the brain to communicate with each other.
In other words, they keep the brain active for memory storage and thinking – a generally higher level of cognitive activity. It is perhaps not surprising that the higher levels of activity involved in our advanced cognition come at a higher energetic cost.
Ultimately, it appears that the human brain has evolved to such advanced levels of cognition not just because we have large brains, and not even just because certain parts of our brains have grown disproportionately large, but because – at a price – connectivity improved .
Many animals with large brains, such as elephants and killer whales, are highly intelligent. But it appears that it is possible to have a large brain without developing the “correct” circuitry for human-level cognition.
The results help us understand why larger brains are so rare. A larger brain can allow more complex cognitions to evolve. However, it is not just a matter of scaling brains and energy at the same pace, but also incurring additional costs.
This doesn’t really answer the ultimate question: how did humans manage to break the brain energy ceiling? As so often in evolution, the answer must lie in ecology, the ultimate energy source. Growing and maintaining large brains – regardless of what social, cultural, technological or other things they are used for – requires a reliable, high-quality diet.
To find out more, we need to explore the past million years, the period during which our ancestors’ brains really expanded, to investigate this interface between energy consumption and cognition.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The authors do not work for, consult with, 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.