We now know that the brain develops from bottom to top and from back to front. This order reflects the evolutionary age of the areas of the brain. The oldest parts of the brain—the ones also present in our ancient ancestors and animal cousins—develop first, at the base of the brain near the spine. They control breathing, senses, emotions, sex, pleasure, sleep, hunger and thirst, or the “animal propensities” left intact after Phineas Gage’s injury. Roughly speaking, these areas are what we consider to be the emotional brain.
