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Mind and Body

The mystery of human brain evolution just got more complicated

New research illustrates some of the evolutionary factors associated with human brain size.

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One of the most consequential evolutionary truths of our species is the dramatic increase in the size of our brains.

Over the past million years, the brains of the Homo genus have tripled in size. What we don’t know for certain is why, despite theories ranging from encountering more information and calories to encountering magic mushrooms.

Our closest animal relative had no such brain boom. So what gives? And what does that mean for the evolutionary changes in our brains’ future?

On Thursday, scientists published a new theory — one that associates human brain evolution with changing temperatures, due to the influence temperatures have on access to resources.

What’s new — Our species, Homo sapiens, emerged roughly 300,000 years ago. Homo, the larger genus our species belongs to, has been around much longer. Other species in the Homo genus include Neanderthals, Homo habilis, and Homo erectus.

The skull of an early Homo sapien. It was found in Eastern Europe.Universal History Archive/ Universal Images Group via Getty Images

Researchers at the Universities of Cambridge and Tübingen measured the brain and body size of over 300 fossils from the genus Homo sourced from around the world.

They then reconstructed the climates of each region at the time to identify what the climate was like when those fossils were living humans.

The results, published in the journal Nature Communications, suggest climate had an enormous effect on the size of our bodies — and a smaller, indirect effect on our brains through the influence temperature has on the environment and, in turn, resources.

Andrea Manica, a researcher in the University of Cambridge's Department of Zoology, led the study. “Climate only explained a portion of the variability in brain sizes,” he tells Inverse. “There is still a lot that we can not explain by climate.”

How climate affects body size

The connection between climate and body size is strikingly clear, the researchers say. In regions with lower mean average temperatures, the body size measurements were larger. This is largely consistent with something called the Environmental Stress Hypothesis, which, according to the study, posits:

“Larger brain and body sizes are found in colder, drier, and nutrient-poorer environments as cognitive and physiological buffers against these circumstances. Environmental stress is countered by adaptive mechanisms to cope with greater environmental extremes over short-term scales by increased behavioral or cognitive flexibility (brain); by higher mobility and reduced vulnerability to predation, or phenotypic adaptation through plasticity or natural selection (body).”

When it came to brain size, however, the results were less clear.

Fossils showing the change in skull sizes over hundreds of thousands of years. via Will, Manica, et. al. Will, Manica, et. al.

How climate affects brain size

The researchers found no significant connection between brain size and mean average temperature.

They did find some connection to brain size and environment more generally, specifically its relationship to resources.

“Big brains are expensive, so they cannot be sustained with little resources.”

Stable environments, in which resources were consistent and plentiful, were associated with bigger brains. Extreme climates are not ideal for growing a variety of food, thus climates with long-term variability in precipitation were associated with larger noggins.

“Big brains are expensive, so they cannot be sustained with little resources,” Manica says.

When it came to the older Homo, Manica says, living in more open habitats promoted bigger brains.

“This was probably linked to the need of hunting bigger prey — in open habitats, there are fewer animals compared to forests, and they tend to be big — which probably required more cooperation and better cognitive abilities,” he says.

Despite these indicators of an association between environment and brain size, Manica says, they only account for a fraction of the observed variation in brain size.

Why this matters — Because the researchers specifically looked at climate, they can’t definitively say what the other factors affecting brain size are. But Manica has an educated guess:

“The cognitive challenges of ever more and complex social lives, more diverse diets, and more sophisticated technology were likely the main drivers of changes in brain size.”

It’s not just a euphemism: Learning to deal with new complex challenges really might make our brains bigger, just over a very long period of time.

What comes next — There is some evidence that our brains have started to decrease in size, and some have posited that our dependence on technology is to blame.

But despite what some inflammatory headlines suggest, scientists are neither concerned about this development nor convinced technology is to blame.

“Bigger brains might be linked to better cognitive capabilities, but it’s not a straightforward relationship.”

First, Manica says, when you’re looking at fossils, you only have skull measurements to work with. Thus, while we can clearly see skull size increase, we don’t know what parts of the brain are increasing to require that skeletal accommodation.

“Bigger brains might be linked to better cognitive capabilities, but it’s not a straightforward relationship, as it depends on what parts of the brain increase in size,” he explains.

As for the fears that your smartphone is shrinking your brain, Manica’s not worried.

“It has been argued that modern technology might require us to actually do less thinking, so maybe at some point that could shrink our brains, but it will take a long time — the changes that we studied occurred over tens of thousands of years,” he says.

Endless hours of smartphone use might not be great for your brain, but it’s not literally shrinking it. At least, not so much that you’d notice.

Abstract: Increasing body and brain size constitutes a key macro-evolutionary pattern in the hominin lineage, yet the mechanisms behind these changes remain debated. Hypothesized drivers include environmental, demographic, social, dietary, and technological factors. Here we test the influence of environmental factors on the evolution of body and brain size in the genus Homo over the last one million years using a large fossil dataset combined with global paleoclimatic reconstructions and formalized hypotheses tested in a quantitative statistical framework. We identify temperature as a major predictor of body size variation within Homo, in accordance with Bergmann’s rule. In contrast, net primary productivity of environments and long-term variability in precipitation correlate with brain size but explain low amounts of the observed variation. These associations are likely due to an indirect environmental influence on cognitive abilities and extinction probabilities. Most environmental factors that we test do not correspond with body and brain size evolution, pointing towards complex scenarios which underlie the evolution of key biological characteristics in later Homo.
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