There is universal agreement that homo sapiens, or humans, are at the top of the food chain because of our big brains. The downside is big brains are big drains. This fragile organ needs to be encased inside a hard and inflexible skull, and it also has very high fuel demands. The average adult human brain is only 2-3% of total body weight, yet it consumes about 25% of our energy, the highest caloric demand of any single organ in the body.

The combination of limited space and high energy requirements creates a unique predicament for the human brain. Because the skull prevents any expansion of brain tissue, there is no extra room for fuel storage near the working cells, the neurons. Consequently, the fuel (glucose) needs to be supplied in a “ready to burn” state, which demands optimal levels of oxygen. This oxygen and glucose requirement does not even decrease when we are daydreaming. While other tissues in our bodies can work for short periods of time without oxygen, brain neurons cannot do this. This means that our brain will quickly shut down (not a good situation) if an abundant supply of oxygen is not supplied along with glucose.