Depression is a condition that arises throughout the world. As scientists begin to understand more about the causes of depression, a surprising discovery has been made with the help of fish, our primitive ancestors. Scientists discovered that under certain conditions, the part of the fish brain that works to protect them from natural predators goes out of control and causes depression. When mammals began living in groups, they developed traits that lead to a similar, poorly regulated response in the brain. This program reveals the evolutionary reasons behind why humans develop depression and introduces the latest treatments based on these discoveries.