We humans are very proud and fond of our large and complex brains. We perceive ourselves as a lot more intelligent than other organisms here on Earth. Some people, including many creationists, would probably go as far as to say that we tower above all other organisms, seeing as we have a unique ability to think, reason, and communicate. I don’t share this point of view.
There’s no doubt that we humans are very brainy; however, that doesn’t elevate us above the rest of the natural world. Other organisms have other distinguishing characteristics and can do things we can’t do. Some can fly, some can run extremely fast, and yet others can survive under water for prolonged periods of time. It’s all just evolution at work. Also, as it turns out, the large human brain is not as big as it once was (1, 2). It has gotten smaller.
Here’s what a 2011 review paper had to say about this matter:
Less well known is that the terminal Pleistocene and Holocene (ca. 30,000 years ago to present) witnessed a substantial decline in endocranial volume. This decrease occurred within modern Homo sapiens, and has been observed in many parts of the world. The scope of this decrease is remarkable: for example, within the past 10,000 years the average endocranial volume in European females reduced from a mean of 1502 ml to a recent value of 1241 ml. This decrease of approximately 240 ml in 10,000 years is nearly 36 times the rate of increase during the previous 800,000 years. (1)
Perhaps we’re not so smart after all…?
Why has the human brain gotten smaller over the most recent millennia?
In order to understand why the large human brain has gotten smaller since the Stone Age, we have to go back and look at why and how it got large in the first place.
Imagine that several bands of hominin hunter-gatherers lived along the coast of Africa some 400.000 years ago. Like all other organisms, these foragers were concerned with their survival and reproduction. They spent their time looking for food, communicating with each other, having sex, and creating tools and weapons they could use to kill animals, cut and process food, and protect themselves from predators.
One doesn’t have to be a genius to understand that it was likely advantageous for these ancient humans to have a large brain. A large brain is very energetically expensive; however, it’s also a very powerful survival tool. A brainy and intelligent hunter-gatherer will likely have an easier time getting a hold of food, effectively communicating with his peers, assessing difficult situations, and staying alive in a challenging environment than an unintelligent hunter-gatherer with a small brain. The former may for example be able to create spears and other weapons that help him catch more fatty fish than the latter.
This brings us over to the key point of today’s article: The conditions under which the large human brain evolved to its peak size differ markedly from the conditions in which we – contemporary humans – find ourselves.
Our hunter-gatherer forebears were markedly more physically active than we are; they were exposed to different types of microbes and other small organisms, including certain types of helminths; their diets were more nutrient-dense, rich in polyunsaturated fatty acids; and they faced many dangers that are largely absent from modern human environments. Moreover, one could hypothesize that a modern man with a small brain doesn’t have the same disadvantage as a Paleolithic man with a small brain, seeing as one doesn’t have to be particularly brainy to survive and reproduce in a modern, energy-rich environment. All of these things can help explain why the human brain has gotten smaller since the late Stone Age.
The brain of the modern man is inflamed and malnourished
Has the human brain declined in size as a result of natural selection, or are other factors at play? First of all, it’s important to point out that the reduction in the human brain size over the most recent millennia can’t solely be explained by reductions in body size (1). Second, both selective and epigenetic processes have undoubtedly contributed to shrinking the human brain. In other words, we don’t just grow smaller brains because we express our genes differently than our larger brained ancestors did, but also because our genes differ somewhat from theirs.
There’s little doubt that one of the major reasons why early humans were able to grow larger and more complex brains is that they transitioned over from a plant-based diet to a more energy-rich diet higher in animal source foods. Animal source foods such as brain, marrow, and liver are rich in long-chain fatty acids, protein, and fat; all of which helped support the evolution of the large human brain (3). The introduction of more energy-dense, omega-3 rich foods into the human diet didn’t necessarily cause the human brain to grow bigger per se, but it provided the building blocks that were required so produce and sustain a larger brain. In other words, it provided a platform upon which the large human brain could develop.
The major changes in the human diet that accompanied the Agricultural and Industrial Revolution altered these ancient relationships between nutrient intake and brain growth. In particular the reduction in omega-3 fatty acids and various coenzymes and amino acids found in meat likely put a constraint on the growth of the human brain.
Besides being less nutritionally dense, agricultural and modern diets are also more inflammatory than preagricultural ones. In a recent paper, a group of researchers make the case that a lack of physical activity, coupled with other proinflammatory stimuli that accompanied our transition over to a more industrialized way of life, could have driven the reduction in the human brain size (3).
Both the human immune system and the human brain are very costly systems with respects to their energetic needs. Particularly a chronically activated immune system challenged by proinflammatory signals quickly gets very hungry. It gets cravings for sugar and protein. The authors of the above paper point out that increased proinflammatory activity shifts the energy balance between the brain and the immune system, something that could help explain why the brain of the modern man is not as big as that of the Paleolithic man. It’s an interesting hypothesis.
In some situations, it’s clearly beneficial to have a large brain; however, in others, it’s not. For example, if you find yourself in a pathogen-heavy environment and have limited access to food, having a big, energetically expensive brain may undermine your ability to survive and reproduce rather than enhance it. Inflammation has undoubtedly contributed to shrinking the human brain, likely both via selective and epigenetic processes.
As pointed out below, domesticated animals have also lost brain size over the most recent millennia. This clearly suggest that the recent reductions in brain size among humans and “tamed” animals can be traced back to clashes between old genes and novel environments.
The decline of human endocranial volume during the last 10,000 years is paralleled most obviously by the reductions of brain size in domesticated animal species, including dogs, cattle and sheep, compared to their wild progenitors. Nutritional, developmental, and functional issues are all possible explanations for these parallel cases of brain size reduction. Humans are different in many ways from these domesticated species, but exhibit other parallel trends such as decreased skeletal robusticity.
Do we have to consider changing our name?
Homo sapiens is latin for wise man. It could be argued that we’re not living up to our name. Not only are we the most destructive species on this planet (by far) and overestimate the quality of many of the infrastructures and systems we’ve created (e.g., the health care system), but we have lost – and seem to continue to lose – brain mass.
At least some of the difference in the brain size between modern humans and ancient humans can probably be explained by differences in gene expression, not differences in the genes per se. We’re not giving our genes the signals they need to build robust, well-functioning brain systems. We’re eating bad food and we don’t exercise. Moreover, we’ve largely replaced books, stories, and challenging activities with social media, reality shows, and other things that deliver information and entertainment that one doesn’t require much brainpower to absorb or make sense of. Our brains and cognitive abilities suffer as a result.
Unless we start using our brains, we may “lose” them…
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