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Go to CuriosityStream.com/SciShowPsych to start streaming thousands of documentaries and nonfiction TV shows. [♪ INTRO]. When it comes to the animal kingdom, humans have one of the longest childhoods around.  There’s no hard-and-fast definition of childhood, but if you consider humans children till at least puberty, that’s still a big chunk of our lives.

And there must be a good reason for that. Because, as much as it can be a happy time of learning and play, childhood also demands a lot from our parents or the adults who care for us. Intuitively, you might think that the reason has to do with the fact that we’re such advanced thinkers and need time to cultivate those skills.

But scientists think that a bigger reason why we have such long childhoods is to grow a part of our brains… and not the part you might expect. Of course, the answer isn’t perfectly straightforward. Any time you’re trying to explain why an organism evolved the way it did, there’s likely more than one factor.

And our long childhoods do give us time to learn things like social skills, language, and how to use tools, which could help us survive long enough to pass on our genes to the next generation.  But we couldn’t master all those skills without our big brains. And those take a lot of work to build!  That’s the main reason scientists think we have long childhoods. See, when we’re little, our bodies are hard at work developing our brains.

Like, children’s brains use up to 65 percent of the energy kids consume. That’s compared to about 20 percent in adults. So perhaps human bodies evolved to grow slowly so they could spare more energy for their developing brains.

On the surface, at least, that seems to justify the long childhoods. But, why are our brains so needy in the first place? We’re not the only animals with big brains relative to our bodies, but we are among the few that spend such a large fraction of our lives as children.  In fact, in other animals, scientists have compared the length of childhood with the relative size of the brain compared to the body.

And they haven’t found any real connection between large brains and long childhoods.  But among some primates, there is one specific part of the brain whose size relative to the animal’s body is linked with the length of childhood. And it may explain why kids’ brains need so much fuel. You might expect it to be the neocortex, which is the part of the brain that evolved most recently in mammals and gives us the capacity for things like self-awareness, abstract thoughts, and the ability to make sophisticated plans.  But research shows that the neocortex doesn’t have anything to do with the length of childhood.

Oddly, the one part that is linked with long childhoods in some species is a completely different part of the brain that’s been around since our early evolution: the cerebellum.  The cerebellum has traditionally been known as the part of the brain that’s responsible for motor control and coordination. You know, basic stuff. But more and more research suggests that it also plays a vital role in certain skills, like using language, forming plans, and making decisions.  The thing is, growing a human cerebellum is no small task. 80 percent of the brain’s neurons are packed into this region.   And it does a huge amount of growing after a child is born.

Like, it needs to grow nearly two-and-a-half times bigger in just the first year of a child’s life. And it keeps growing until we’re about 13.  After developing, the human cerebellum ends up with a huge number of neural connections. And those neural connections need lots of fuel!  That’s why kids’ brains are such gas guzzlers.

And in addition to consuming calories, the cerebellum also feeds on play! Research shows that, among primates, free play and exploration in a safe environment may help the cerebellum grow, which could explain why it’s such an important part of childhood.   While we don’t know exactly how play fuels the growth of the cerebellum, researchers have hypothesized that the neural connections developed when we play are essential for many important cognitive and behavioral skills later in life.  So, the fact that we evolved to spend so much time frolicking and exploring the world while adults look after us gives children the chance to fuel their brains with both the energy and experiences the brain needs to grow!  In the end, all this research shows that even though the definition of “childhood” varies a little from culture to culture, it’s also defined by our physiology . And the science backs up what many of us feel instinctively:.

Childhood matters, and the care we're given as we grow can affect all kinds of things. So, while it’s not the only way, a childhood full of play and care is a great way to make a human who’s resilient enough to survive and pay it forward to the next generation. This episode is brought to you by CuriosityStream.

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