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For a long time, scientists considered the neocortex the brainiest part of the human brain – an obvious candidate for the thing that makes us unique. But in some ways, it’s not that different from other mammals’ brains. So researchers have started looking at other places to figure out what makes us distinctly human.

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This episode is sponsored by Fabulous, an app that helps you form  healthy habits that stick.

You can check out the link in  description to get a free one-week trial and 25% off a Fabulous subscription! [♪ INTRO]. Picture a brain in your brain.

That’s wild. So your mind’s eye is probably  showing you a pink wrinkly thing, and that wrinkly outer layer  is called the neocortex. For a long time, scientists  considered the neocortex the brainiest part of the human brain, an obvious candidate for the  thing that makes us unique.

It’s the biggest part of  our brain, and it has those lovely wrinkles that many  smaller mammals just don’t have. But in some ways, it’s not actually that  different from other mammals’ brains. So researchers have started  looking at other places to figure out what makes us distinctly human.

Move over, neocortex, it’s time to  talk about the underrated cerebellum. Now, there are good reasons why  scientists thought our neocortex was a major thing that made us stand out as a species. It takes up about 80% of the human brain’s mass.

Plus, it's divided into about 200 areas that  are specialized for different functions. For comparison, early mammals  probably had about 20. Those 200-ish areas are  responsible for a lot of functions we think of as characteristically  human, like making plans, speaking, and understanding language.

But as scientists dug deeper, they found  some things that weren’t all that unique. Like, many specialized regions in our neocortex perform functions we do share with other primates. For example, the prefrontal cortex is in  charge of planning and inhibitory control.

And the insula is involved in gut reactions. In fact, some functions seem to be  shared across nearly all mammals, like visual and auditory systems, and  areas for sensory and motor control. And even though our overall brains are 238% bigger than you’d expect for a primate of our size, our neocortex isn’t out of  proportion to the rest of our brain.

Which is what you would expect  if it was what made it special. I mean, even a mouse lemur has a frontal cortex that's about the size you’d expect  based on their overall brain. And humans like to fancy ourselves  as, well, fancier than a mouse lemur.

All of this means our neocortex might  not be particularly exceptional. And it might not be the only thing  that contributes to human intelligence. So scientists have started to look at  the second biggest part of the brain.

It's called the cerebellum, and it's  located at the back of your skull, just above your spinal cord. The word cerebellum means “little brain.” But even though it’s small, it contains  four times more neurons than the neocortex. In humans, the cerebellum  is largely responsible for fine motor control and implicit memory.

That's the kind of procedural memory  you don't memorize so much as practice, like riding a bike, playing an  instrument, or navigating home. Among vertebrates, we actually see the greatest size variation in the  cerebellum, not the neocortex. As humans and other apes  evolved, the cerebellum expanded much more rapidly than the neocortex, resulting in a cerebellum that is outsized  compared to the rest of the brain.

And when you look deeper, you  find even more differences. In 2021, researchers published  a study that looked at methylation of DNA in human and primate brains. Methylation basically means your DNA gets some chemical hop-ons that can  change how genes are expressed.

They found different levels of  methylation between humans and primates. But the difference was bigger in samples taken from the cerebellum than from the neocortex. What’s more, the genes that were likely  to have more methylation seemed to be related to neuronal growth,  plasticity and learning.

If those genes are being fine-tuned  through methylation, our brain’s growth and evolution might depend more  on the cerebellum than we thought. When we look at the evolution of  other animals, we tend to see an enlarged cerebellum associated  with more elaborate motor skills. Like, when primates and  squirrels get more practice using their forelimbs to climb trees,  that can favor a bigger cerebellum.

Which further enhances their motor skills. As humans evolved, the expansion  of our cerebellum likely increased our technical capabilities,  like our skill in making tools. That’s crucial, because a hallmark  of human intelligence is that we make tools better than any other animal.

And we couldn’t do that  without an advanced cerebellum helping us plan and coordinate  complex sequences of actions. In addition, our cerebellum-fueled  technical skills may have driven the development of language and syntax. Speech itself is an extreme fine motor  skill, and we do have neural connections between the cerebellum and areas  of the cortex involved in language.

Research suggests that the  human cerebellum expanded at the same time our language and  verbal memory abilities emerged. Which makes sense, because scientists  think human language may have evolved largely to help us teach each other how  to create tools from stones and plants. The more scientists investigate the cerebellum, the more central it seems to human abilities.

It helps us perform complex actions  in a smooth and coordinated way, whether those actions are crafting cooking tools or engaging in cognition and conversation. And it’s considered a  “learning machine” capable of predicting outcomes and  adapting after making errors. Now that’s not to say the  neocortex isn’t really important.

The neocortex and cerebellum are densely linked, and they closely coordinate to  produce thoughts and behaviors. So, to discover how humans operate,  we need to look at the whole brain. And remember that when it comes to  the neocortex, size is not everything.

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