The blood supply to our ancestors’ skulls grew faster than our brains
Every minute, enough blood to fill three, 12-ounce cans of soda flows through your brain. That’s a considerable amount of fluid to be moved in and out of your skull, made possible thanks to three million years of evolutionary change. This increasing blood flow hasn’t been directly measured, of course, but has been preserved in the fossil record thanks to openings in the skull for internal carotid arteries. By comparing these small openings in a modern human to many of our extinct ancestors, scientists know that our brains are sucking up a lot more blood than older hominids, they’re just not completely sure why.
Boosting blood flow
Some of the more intuitive aspects of this system have been confirmed at this point. Bigger holes in the skull, close to the underside of the eye sockets, do correspond with bigger blood vessels. Those blood vessels have been confirmed to increase blood flow as you’d expect, which all makes sense. The obvious answer to all this is that with a brain that eats up 20 to 25 percent of your resting energy, bigger blood vessels simply corresponded to more active brains. However, this is where the rate of change doesn’t sync up completely.
Looking at the skulls of 12 different hominid relatives from the last three million years, the trend did hold up that bigger brain cases also had bigger holes for arteries to deliver more blood. The catch was that the increases weren’t directly proportional. A brain 10 percent bigger than its predecessor somehow demanded more than a matching 10 percent increase in circulation. This steeper growth-curve left researchers looking for something beyond straight resource consumption, including options like increasing brain cell density, or just more active brains, that would have been demanding more calories, and thus more liters of blood, per minute.
Cooling our craniums
There’s also a chance that our modern brain’s need for blood isn’t about consumption, but cooling. The high activity levels, plus the issue of a shrinking surface-area to volume for the organ itself, would mean that our enclosed brains likely needed a way to cool down as they grew. More blood flowing out of our heads would allow more heat to be carried out as well, which is a serious enough concern that it seems to have triggered some rather elaborate adaptations in other animals, and possibly… yawning in humans? More investigation is needed, since it looks like our brains’ need for blood is more complicated than previously realized.
Source: Brain’s blood appetite grew faster than its size by Laurel Hamers, Science News