Suppressing knowledge of the certain doom that awaits us all must have been a major factor in the evolution of what Freud calls defense mechanisms (are we the only animals with them? can a chimp suppress or repress?). These are processes by which the brain removes negative feelings, anxiety, guilt, unbidden thoughts, and so on from consciousness. Without such cleansing mechanisms, early humans might have become too transfixed by their ultimate fate to successfully dominate their niche. Perhaps clinical depression amounts to a loss of such defense mechanisms.
