Throughout our lives, we have heard countless times that "we are what we eat." Of course, this is true, and there is ample scientific evidence to support it. However, this post is not about nutrition but rather the relationship between diet and brain evolution throughout human evolution. Human evolution, such an intriguing topic with more questions than answers. We could go back around 7 million years and find those early representatives of our evolutionary lineage. Of our family (not in taxonomic terms, of course), some would say. And yet, if we were to look at their brain, we would see that it has little resemblance to ours, at least when it comes to size. ⚠ Caution! ⚠ When we talk about brain size, we are not directly talking about intelligence. That said, let's trace the path of human evolution to find out how we came to have the brains we do today.
First of all, let's define some terms that may be a bit complex but will help us better understand this process. The first term: encephalization quotient (EQ). It is basically the relationship between an animal's brain size and body size. This quotient has been calculated in many species, including humans, and a regression line has been plotted. A quick look at this line already shows that we are at relatively high values, meaning we have a larger brain than what would be expected based on our body size. At this point, we tend to ask ourselves "why?" Why do we have such a large brain? And here we fall into the trap because evolution does not have an objective or plan; it is about random mutations that, if they are favorable for survival, stay and get passed down from generation to generation. So instead of asking why, let's ask how. How did we end up with such a large brain?
Here comes the second concept to help us solve this mystery: the expensive tissue hypothesis (ETH). This hypothesis suggests that the energy required for an increase in brain size comes from a reduction of other tissues. In other words, the brain is an expensive organ to maintain because just by functioning it consumes around 20% of our energy. And this is only for basic reactions that keep the brain alive; if we perform more complex tasks, that percentage increases. So, the larger the brain, the more energy we will need. It's as if evolution were telling us: "look, if you want a bigger brain, you'll have to take energy from another organ because there isn't enough for everything, and then that other organ will become smaller."
There was a moment in evolution (which really spans thousands of years) when the brain size of human species was relatively small compared to their body. These human species had almost entirely herbivorous diets, so their digestive systems were long and complex (the digestive system does not fossilize, but we deduce it by looking at current herbivores), and much of the energy they consumed went into digestion. About 2.6 million years ago (Homo habilis), there is evidence of meat consumption in addition to plant-based foods; this species was omnivorous, like us. What happened? Meat, even raw, requires less energy for digestion than plants, so Homo habilis had an "extra of energy." Additionally, as the digestive system gradually became smaller over time, this reduction saved energy. The human species that came after were also omnivorous and gained this extra energy again. Over thousands of years, the digestive system becomes progressively smaller and less complex. And the surplus energy has been invested in brain growth because we could "afford it." This is how we have reached today with an average cranial capacity of 1200 cubic centimeters.
Of course, all these changes are gradual and slow, impossible to see within a single human lifetime. To conclude this article, we will say that although we have large brains, don't get complacent; what matters is the number of connections between neurons. So, let's train our brain!
Bibliographic references:
-Aiello, L. C., & Wheeler, P. (1995). The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Current anthropology, 36(2), 199-221.
-Martin, R. D. (1981). Relative brain size and basal metabolic rate in terrestrial vertebrates. Nature, 293(5827), 57-60.
-Mateos Cachorro, A., & Rodríguez, J. (2010). The diet that made us human.
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