Human brains are huge. You wouldn’t say so if you compared your head to, say, that of a cow, but in comparison to our body size our brains are much bigger than that of a cow. And it turns out we not only have large heads, but we also pack a lot more brain into these heads by folding it well. How do human brains grow differently? What parts of our DNA play a role in this? It is part of the puzzle of why humans are capable of language.
Those huge human brains…
Humans are born relatively early and in a relatively helpless state compared to other mammals. Whereas some mammals can walk after a matter of days, human babies take over a year to do so. Why would a baby be born so early, when it is so vulnerable? One of the main reasons, as you may have guessed, is our large heads. A more developed baby’s head would simply not fit through the birth canal of the mother during birth. Like most things in life, it’s a matter of logistics.
That human brains are unusually large becomes clearer when you compare them to those of our closest living relatives, the bonobo and the chimpanzee. On average a human has roughly three times the brain volume of a chimpanzee. This is a big change.
Packaging matters
But just more volume was not enough. Mammal brains are built like sheets. To pack brain into a skull, try thinking about fitting a piece of paper in a coffee mug. There are two ways to make it work: either you line it around the inner surface of your mug, or you make a really tight wad. The first solution is probably more tidy, but the second one gives you more bang for your buck if you do it well.
Because there is no perfect solution in biology, brains come in many shapes and are folded in a thousand different ways. The outer surface of a mouse brain, inside the skull, is as flat as the Dutch countryside. This is a bit like the first solution: lined around the surface of the mug. It is enough for the mouse and costs much less energy to maintain, because brains are very energy-hungry. A human brain on the other hand looks like the Alps, with big valleys and mountains: like the wad in the mug. Scientists call this phenomenon gyrification. The reason for this is, like the second solution, probably because it allows for more brain in our skulls.
So, humans have larger heads with more wrinkled brains compared to other primates, but even compared to our closest living relatives, chimpanzees. This gives us more brain power to do all sorts of things, like tool use, music, cooperation, and language.
Photo credits: https://zenodo.org/records/2538751
Want to know more about why your brain is wrinkly? Check this video.
References:
https://www.sciencedirect.com/science/article/pii/S0010945219301704
https://www.pnas.org/doi/full/10.1073/pnas.2200638119
https://karger.com/bbe/article-pdf/91/3/158/2265944/000489943.pdf
