Researchers have shown that human intelligence does not depend primarily on the size of our brain - there are animals with bigger brains (elephants, orcas) - but on the cognitive circuitry.
The team, led by Valentin Riedl of the Department of Neuroradiology at Klinikum rechts der Isar, TUM School of Medicine and Health, Technical University of Munich, Munich, Germany, have published their findings, open access, in the journal Science Advances. It is explained in an article in The Conversation by two Cambridge University professors who were not involved in the research.
Their article is reprinted here under a Creative Commons license, reformatted for stylistic consistency:
Human intelligence: how cognitive circuitry, rather than brain size, drove its evolution
Credit: wikipedia/Foley, CC BY-SA
Robert Foley, University of Cambridge and Marta Mirazon Lahr, University of Cambridge
It’s one of the great paradoxes of evolution. Humans have demonstrated that having large brains are key to our evolutionary success, and yet such brains are extremely rare in other animals. Most get by on tiny brains, and don’t seem to miss the extra brain cells (neurons).
Why? The answer that most biologists have settled on is that large brains are costly in terms of the energy they require to run. And, given the way natural selection works, the benefits simply don’t exceed the costs.
But is it just a matter of size? Does the way our brains are laid out also affect their costs? A new study, published in Science Advances, has produced some intriguing answers.
All our organs have running costs, but some are cheap and others expensive. Bones, for example, are relatively cheap. Although they make up around 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 typical human body weight, running them uses around 20% of our metabolism. And this without doing any conscious thinking – it even happens when we’re asleep.
For most animals, the benefits of serious thinking are simply not worth it. But for some reason – the greatest puzzle in human evolution, perhaps – humans found ways to overcome the costs of having a larger brain and reap the benefits.
All this is fairly well known, but there is a more tantalising question. Certainly humans 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-conscious or doing sums cost more than typical day-to-day 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, have risen to the challenge.
The authors had a number of known points to start with. 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 these are unlikely to be the driver of intelligence. If they were, some animals with large brains such as orcas and elephants would likely be smarter than humans.
Elephants have larger brains than humans.
venusvi/Shutterstock
These two observations led them to investigate whether there are different costs of signalling across different regions of the brain.
The team scanned the brains of 30 people using a technique that could simultaneously measure glucose metabolism (a measure of energy consumption) and the level of signalling across the cortex. They could then look at the correlation between these two elements and see whether different parts of the brain used different levels of energy – and if so how.
Surprising findings
Neurobiologists will surely ponder and explore the fine details of the results, but from an evolutionary point of view, they are thought-provoking. What they found is that the difference in energy consumption between different areas of the brain is big. Not all bits of the brain are equal, energetically speaking.
Not only that, but the parts of the human brain that have expanded most had higher costs than expected. The neocortex in fact demanded around 67% more energy than sensorimotor networks per gram of tissue.
This means that during the course of human evolution, not only did the metabolic costs of our brains go up as they became larger, but they did so at an accelerating rate as the neocortex expanded faster than the rest of the brain.
Why should that be the case? A neuron is a neuron, after all. The neocortex relates directly to higher cognitive function.
The signals sent across this area are mediated through brain chemicals such as serotonin, dopamine and noradrenaline (neuromodulators), which create circuits in the brain to help maintain a general level of excitement (in a neurological sense of the word meaning being awake, not having fun). These circuits, which regulate some brain areas more than others, control and modify 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 generally higher level of cognitive activity. Not surprisingly, perhaps, the higher level of activity involved in our advanced cognition comes at a higher energetic cost.
Ultimately then, it seems the human brain evolved to such advanced levels of cognition not just because we have large brains, nor even just because certain areas of our brain grew disproportionately big, but because – at a cost – the connectivity improved.
Many animals with large brains, such as elephants and orcas, are highly intelligent. But it seems it is possible to have a large 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 is not just a matter of scaling up brains and energy at the same rate though, but taking on additional costs.
This doesn’t really answer the ultimate question – how did humans manage to break through the brain-energy ceiling? As so often in evolution, the answer must lie in ecology, the ultimate source of energy. To grow and maintain a large brain – whatever social, cultural, technological or other things it is used for – requires a dependable and 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.
Robert Foley, Emeritus Professor of Human Evolution, University of Cambridge and Marta Mirazon Lahr, Professor of Human Evolutionary Biology & Director of the Duckworth Collection, University of Cambridge
This article is republished from The Conversation under a Creative Commons license. Read the original article.
AbstractCreationists might like to ignore the fact that the authors of this paper and the authors of the article about it in The Conversation explain how the human brain came to be the way it is as an evolutionary process and never once express any doubt that the Theory of Evolution by Natural Selection is anything less than fully adequate for explaining how this happened. Only by ignoring facts such as these can creationists continue to delude themselves that serious research biologists are turning away from the TOE and adopting their childish superstition, complete with unproven supernatural creative entities and magic.
In comparison to other species, the human brain exhibits one of the highest energy demands relative to body metabolism. It remains unclear whether this heightened energy demand uniformly supports an enlarged brain or if specific signaling mechanisms necessitate greater energy. We hypothesized that the regional distribution of energy demands will reveal signaling strategies that have contributed to human cognitive development. We measured the energy distribution within the brain functional connectome using multimodal brain imaging and found that signaling pathways in evolutionarily expanded regions have up to 67% higher energetic costs than those in sensory-motor regions. Additionally, histology, transcriptomic data, and molecular imaging independently reveal an up-regulation of signaling at G-protein-coupled receptors in energy-demanding regions. Our findings indicate that neuromodulator activity is predominantly involved in cognitive functions, such as reading or memory processing. This study suggests that an up-regulation of neuromodulator activity, alongside increased brain size, is a crucial aspect of human brain evolution.
INTRODUCTION
Over 400 million years, the brain structure of various species has evolved according to similar organizational principles (1, 2). Neurons, acting as the local signaling units, form a dense connectome with widespread signaling pathways through their synapses. Nonetheless, when compared to humans, certain mammals exhibit larger brain sizes (e.g., the Indian elephant), higher brain-to-body mass ratios (e.g., the mouse), or a greater number of neurons (e.g., the long-finned pilot whale) (3–8). This suggests that brain structure scaling is not the only factor that has contributed to the emergence of human cognition (6, 9–13). Here, our focus is on exploring the metabolic characteristics of the brain connectome. The brain depends on a constant supply of energy substrates and, in the case of humans, ranks among the organs with the highest energy demands (14–18). In comparison to other species, the human brain exhibits one of the highest energy demands relative to body metabolism (19). How does metabolic energy distribute across the brain? The fundamental design of neurons has been conserved throughout evolution, with the signaling costs of individual cells being comparable across different mammals (20–26). On a systems level, the human brain has the expected quantity of neurons and nonneuronal cells for a primate brain of its size (27). Furthermore, it maintains a similar distribution of neurons throughout its cerebral cortex as observed in other species (4). As a result, we hypothesized that regional energy demands will vary based on the degree of signaling within the brain connectome. In addition to the degree of neuronal signaling, researchers suggest that neuromodulation plays a crucial role in adaptive behavior and cognition in humans (28, 29). Topological analysis of the brain connectome even suggests a trade-off in energetic costs between signaling efficiency and modulation (30). Although our knowledge of the impact of neuromodulation on human evolution is still evolving, receptor autoradiography data from human donor brains reveal substantial variations in the distribution of neuromodulator receptors across the cortex (31). Comparative studies of the brain metabolome further demonstrate substantial differences in metabolites related to energy metabolism and synaptic modulation between the human brain and closely related primate species, with a notable regional variability (6). In summary, metabolomics and receptor data indicate regional heterogeneity in neuromodulation and suggest a potential link to energy metabolism. However, it remains unknown whether energy metabolism varies across the human connectome and scales with the presence of certain signaling mechanisms.
Gabriel Castrillon et al. ,
An energy costly architecture of neuromodulators for human brain evolution and cognition.
Sci. Adv. 9, eadi7632(2023). DOI:10.1126/sciadv.adi7632
Copyright: © 2023 The authors.
Published by the American Association for the Advancement of Science. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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