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What If Human Brains Were Half the Size?

VO: Noah Baum WRITTEN BY: Mark Sammut
The human brain is packed with 100 billion neurons. It's responsible for everything we do, think and feel. It dictates our emotions, intelligence and overall mental health. But, what if our brains were 50% smaller? If the brain halved in size, would humans be half as effective at living their lives? Or, would we actually be more efficient than ever?
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What If Human Brains Were Half the Size?


While the size does vary from person to person, an average human brain weighs approximately 1,350 grams (that’s about three pounds) and measures in at around 15 centimetres long (or 6 inches). Divided into various sections and controlling the rest of the body, our neurological centre receives, interprets, and reacts to an endless stream of sensory information, and it is always working. It’s often described as the most complex of all our internal organs, if not one of the most complicated structures to have ever developed at all.

A typical adult brain houses around 85 to perhaps 100 billion interconnected neurons, responsible for turning sensory data into commands for the body. For any action to be completed, a signal is sent across the neural network. Trigger the same neural pathway often enough, and you form either habits or memories.

The thing with neurons, though, is that, unlike with most other cell types, replacements can be hard to come by. When we’re born, our brains are about one third the size of what they are as an adult. But, as we grow, countless new neurons are generated every minute of every day. However, eventually, and usually well into adulthood, neuron generation slows to a crawl. While the scientific jury is still out on whether adult neuron creation ever stops completely, with increasing numbers arguing that it doesn’t, the process drops off as we get older.

So, the window for dramatic brain growth closes at quite an early stage in our lives. But, does brain size even matter? You might assume that a larger brain equals more neurons, which equals more power to process information and store data. While some studies have claimed that a bigger brain is a better one, there’s just not enough evidence to unequivocally determine whether that’s really true. MRI research has shown a weak association between brain volume and intelligence in the past, but the important thing to remember is that intelligence depends on brain efficiency rather than stored data.

Compared to other primates, humans are pretty big-brained, with the size increasing throughout most of our ancient history. As our bodies grew and we developed new abilities, as well as travelling further afield, our cerebral engine expanded as well. Today, we have an especially well-developed neocortex, to handle high cognitive function, like reasoning, language, and motor commands – to set us apart from other mammals our size. And yet, in the past 30,000 years, our brains have actually gotten smaller, already. It’s said that we’ve lost about 10% (or a tennis ball’s worth) of brain mass, but it’s not thought that the change has reduced our capabilities. Instead, it’s been linked with our changing lifestyles, and has again been explained as an evolutionary trend toward making the organ more efficient.

If the size of our brains was somehow halved, however, we’d have to make do with only 40 to 50 billion neurons, packed into something that now weighs only a pound and a half. If the average size of a person remains unchanged, this new budget brain would account for less than 1% of someone’s total mass. It’d still outweigh our ancient ancestors' original model, but that was housed in a smaller body. As brain growth has mimicked body growth in the past, evolution should theoretically work to counteract a more compact brain by decreasing our average body size, but this process wouldn’t occur overnight.

That said, although logic dictates a larger mammal should have a proportionally sized brain, the ratio isn’t always linear. A mouse, for example, boasts a more favourable brain-to-body rate than an elephant, despite the enormous animal's reputation as an intelligent creature. This is partly because, regardless of the species, basic physiological functions tend to follow similar neural pathways, unaffected by body size. So, for simple acts like breathing or sleeping, a lion and a household cat require similar volumes of brain matter. Consequently, and as further evidence that size isn’t everything, the ‘king of the jungle’ lion actually weighs in with a relatively small brain for its bulky build.

At best, the brain-to-body mass ratio serves as an extremely vague marker for estimating actual intelligence. In the 1960s, H.J. Jerison developed the encephalization quotient to better explain the differences between human intellect and that of other species. Focusing primarily on mammals, Jerison's formula determined the ratio between actual and predicted brain mass – to give relative brain sizes between species. Using his model, and attributing cats with a normalized EQ of 1, humans have a high EQ of about 7.5. Chimpanzees – which share nearly 99% of their DNA with humans – clock in at less than 3 EQ. The quotient only ever suggests that humans are smarter than the average ape, though, without explaining why.

If human brains were half the size, it could trigger a fall in reasoning and analytic thought. But, with fewer neurons at work, the brain would naturally prioritize survival above things like creativity, prompting humanity as a species to adapt to its new scenario. We could also experience forgetfulness, and potentially dramatic changes in mood – although the specific changes would completely hinge on which cells are lost and which are retained. By Jerison’s quotient, though, even operating at half-power the human brain should remain ahead of any other land mammal. But, its dominance wouldn’t extend to the ocean, with dolphins potentially outperforming us.

Of course, the fact that human brains actually are decreasing in size seemingly shows that there wouldn’t necessarily be anything to worry about. Skipping the lengthy process of evolution and imagining that tomorrow everyone just wakes up with a shrunken brain, two outcomes are actually feasible: either half the neurons disappear, or all of the cells are crammed into half the space. While we’ve seen how the first scenario could lead to a lessening of brain function, the second could bring us to all-new levels of intelligence.

We’d now be holding and processing more data in less space. If the shift also halves our brain’s margin for error, then we’d be less prone to forgetting or misunderstanding information. That said, if the margin for error remained the same, then our processing speed could actually slow and we’d become more easily confused. Like a high-spec computer, we’d need the right balance between potential and performance.

So, if the human brain was halved, it might not be all that catastrophic, and it needn’t set us back on some sort of evolutionary timeline. Instead, if the decrease plays out over centuries of human history (as it already looks as though it is), then it could simply be our bodies adapting to the changing environment on Earth. But, even then, a bigger or smaller brain really means nothing in terms of how intelligent any one person or creature is – it’s all to do with how many neurons it carries, and how efficiently it uses them.
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