Your brain is in your head. At first glance, this seems like a dreadful place for your delicate brain to reside. Why is your brain located inside your head? It would be much safer hidden deep inside your chest. With very few exceptions, brains are always located at the front end of an animal’s feeding “tube” or digestive system that extends from the mouth to the anus. Bugs, worms, fish, birds, reptiles, dogs, and humans are simple feeding tubes with a “brain” sitting right up front, usually near the eyes, ears, and nose; thus, making it possible to find food by sight, sound, or smell and then to organize behavior so that the front end of the feeding tube can get close enough to “taste” the food and check its safety before engulfing it. Once the food is in your feeding tube, its nutrients are absorbed and become available to the cells in the rest of your body. The calories from a meal are not distributed equitably around your body. Imagine that the meal you have just finished is worth $1; the various digestive components of your feeding tube spend nearly 70 cents of your meal, with the remaining 30 pennies alone available to your brain and body to spend on its daily needs. Your brain and other organs that allow you to reproduce and move around your environment (including your muscles and bones) spend about 22 of those remaining pennies. As you can see, very few pennies are left over to spend on the other tasks in your body. These expenditures give you some idea of the priorities—thinking, sex, and movement—that billions of p. 2↵years of evolution have established for your brain and body (see Figure 1).
Human brains use a lot of energy. Under normal circumstances, the brain primarily uses energy in the form of sugar: the equivalent of about 12 donuts every day! (Now you can understand why there are so many donut shops located along your morning drive to work.) Your body spends nearly a quarter of its food budget on just the brain; five times as much as most other mammals devote to their brains. Your brain uses most of this energy to organize your behavior to find food, avoid danger, and socialize with others in order to find a mate with whom to reproduce. You know one manifestation of this p. 3↵imperative as dating, and it requires a very large and complex brain to pull this off successfully. Eating and sex are obviously excellent ideas if your purpose is to maintain and propagate your species; fortunately, your brain does an excellent job achieving both of these tasks.
The evolution of energy-guzzling bigger brains, like yours, depended on building longer feeding tubes in order to optimize the extraction of more energy from whatever entered the front end of the feeding tube. It is not surprising then that the length of the gut, when compared across many different species, correlates with the size of the brain. As brains became larger, however, the forces of evolution shifted strategies (after all, the length of the gut can be increased only until there is insufficient room in the body to contain it); animals developed a more efficient and shorter feeding tube that relied on a high-quality, nutrient-rich diet. Therefore, today we have a gastrointestinal system that is efficient at extracting energy for itself and its two principal customers, your reproductive system and brain. Due to the high energy demands of the brain and reproductive system, however, a surprising compromise occurred during evolution: as brains became bigger, human reproductive success failed. Now you can appreciate why humans do not give birth to litters. One might predict that having a larger brain would allow greater reproductive success. After all, you would expect that animals with bigger brains would find more food, avoid predators more successfully, and find more mates. This expectation is based on the assumption that bigger brains are always smarter, but this is not so. Animals with smaller brains and bodies, such as birds, often demonstrate impressive cognitive abilities, while some large-brained species, such as whales and elephants, do not.
Brains, regardless of how big or small, complex or simple, evolved to perform one basic function: survival of the individual and the species. Survival depends upon finding food and shelter, mating successfully, and avoiding predators; doing all of this requires the ability to learn how and where to hunt, p. 4↵to learn how to communicate, and to cooperate with others, while remembering what sounds and odors predict the imminent appearance of predators, as well as remembering whether it is better to fight or flee. The Spanish filmmaker Luis Buñuel famously stated that “…memory is what makes our lives. Life without memory is no life at all … Our memory is our coherence, our reason, our feeling, even our action. Without it we are nothing.” Thus, I will begin by discussing how memories are made and lost.