Bigheads

The other day, I sat down to enjoy a giant grapefruit my coworker plucked from his accommodating tree. That’s the advantage of living in Florida: free fruit. As I admired the grapefruit's beauty and heft, it struck me that it was the approximate size of a newborn’s skull. I filed the information away, sliced into the fruit's meaty center, and feasted on its contents.

But it got me thinking about head size. Homo sapiens are a big-headed bunch. How we acquired such gargantuan gourds is a long and convoluted evolutionary tale, so I’ll stick to the highlights.

Natural selection is all about “advantage.” If a mutation bobs to the surface of the gene pool and happens to confer an advantage – whether it’s the ability to run a bit faster to elude a predator or a knack for exploiting a new food source – chances are he or she will leave more offspring and pass on that advantage. Through time and successful breeding, the advantage may become a staple, otherwise known as an adaptation. And the better adapted one is to his environment, the better his chances of survival.

So how did our heads get so big? 

Since our skulls, which serve as protective shell and handy carrying case, form atop the underlying tissues, we can blame our bigger brains.

Having a bigger brain is definitely an advantage. The more neurons you possess, the more complex the organ. But it’s not just overall size that matters. The brain of a walrus is about the same size as ours, but you won’t catch a pinniped performing calculus anytime soon. 

It’s the size of the brain in relation to body mass that really counts – what scientists refer to as the encephalization quotient (EQ). To give you an idea of EQs, take our hooved friend, the horse. Horses are pretty smart. They usually follow directions and can be trained to perform a number of nifty tricks. The typical equine has an EQ of around 0.8. Two of our other clever domesticates, dogs and cats, have EQs of 1.1 and 1.0, respectively. Humans, on the other hand, have EQs of around 7!
We are the true brainiacs. 

We’re still trying to tease out exactly when this advanced wiring emerged among our ancestors. By comparing skulls of our various kin, we can track brain changes over time. The neocortex, the outer region of the brain responsible for conscious thought, expanded by the time archaic humans emerged on the scene (around five hundred thousand years ago) and the temporal lobes, the regions on either side, are twenty percent bigger in modern humans compared to our predecessors. That’s important, since these lobes help us organize memories, aid in learning, and allow us to store information – all vital skills associated with the development of culture.

Our modern brains contain over a billion neurons and it’s this complex wiring that enables us to perform many intellectual feats that elude other animals. Reasoning, problem solving, forethought, and language are just a few of the impressive abilities made possible by our large brains (although some animals possess some of these skills to a limited extent). And it was the fine-tuning of these abilities that enabled Homo sapiens to invent things like complex societies and technology. You’d be hard-pressed to design a computer if you possessed the neurological complexity of a squirrel.

But our big brains come with a hefty price tag. For starters, they are metabolically demanding. About a third of the energy you produce each day goes to fueling that giant melon and without a constant influx of sugar and oxygen, it quickly dies. That’s why immediate CPR is so critical in sudden cardiac arrest. Ventilating the patient and performing chest compressions not only provide oxygen, but circulate it throughout the body to starving tissues. And the brain is at the top of the list, for without the brain, the rest of the body isn’t much use.

Another problem with a big brain is squeezing it through the birth canal. For this we can blame our mode of transport. Bipedalism (walking on two legs) places certain architectural demands on the pelvis. Our legs must be aligned below our trunks for efficient walking and running. If not, we’d walk like a crocodile, which can sprint for short distances, but will never win a marathon.

But a woman’s pelvis can only flex so far before things go wrong. That’s what can make childbirth such a dangerous endeavor. Hemorrhage is a common cause of maternal death, which is understandable when you consider the size of the newborn’s head in relation to the dimensions of the birth canal. Natural selection compensated by limiting pregnancy in humans to nine months. This way, the baby emerges before its head becomes too big to pass. The drawback: a defenseless newborn with an undeveloped brain, who is completely dependent on others for survival. Let’s face it… baby humans are basically helpless little blobs that can’t even lift their cumbersome noggins. Pitiful.

In closing, every physical attribute is a tradeoff. Bigger brains may make childbirth more problematic and gobble up much of our energy, but they enable us to do some amazing things. As I bang out this blog on my computer, I’m surrounded by the evidence of human ingenuity – all made possible by a big brain. The clothes I wear, the car I drive, the house I reside in, not to mention my electricity, phone, and medicines - none of these things would be possible without those demanding organs that sit atop our shoulders.

So hurray for the bigheads!

Here's a great read on the brain.