Depression can be detrimental to good health, so why hasn’t the human body found a way, through evolution and adaptation, to eliminate it? Has depression somehow benefited mankind? These are the central questions asked by Emory University researchers in a new study that seeks to explain why humans seem so “hard-wired” for the mood disorder. The answer is intriguing — and very unexpected. Depression, and its related symptoms may be genetically tied to the body’s ability to fight infection.

The connection, it appears, may go all the back way to the earliest days of human history. As the research team comprised of Andrew Miller, MD, professor of psychiatry and behavioral sciences at Emory, and Charles Raison, MD, previously at Emory and now at the University of Arizona, explain, infection was the major cause of death in humans’ early history, so surviving infection was a key factor in determining whether someone was able to live long enough to pass on his or her genes.

If you think about the common symptoms of depression — fatigue, inactivity, and social avoidance — it’s easy to see how depressed people would have unknowingly avoided contact with those carrying germs and disease. Because they survived, so did their genes.

“The basic idea is that depression and the genes that promote it … [helped] people—especially young children—not die of infection in the ancestral environment, even if those same behaviors are not helpful in our relationships with other people,” Raison says.

Over time, say researchers, evolution and genetics have bound together depressive symptoms and physiological responses to infection. Studies continuously show links between depression and inflammation, or over-activation of the immune system. As researchers note, people with depression tend to have higher levels of inflammation, even if they’re not fighting an infection.

The theory provides a new explanation, for example, as to why disrupted sleep patterns can be seen in both mood disorders and when the immune system is activated. This may come from our ancestors’ need to stay on alert to fend off predators after injury, Miller says.

Miller and Raison’s theory could also guide future research on depression. In particular, the presence of markers for inflammation may be able to predict whether someone will respond to various treatments for depression. Miller and Raison are also involved in ongoing research on whether certain medications, which are normally used to treat auto-immune diseases, can be effective with treatment-resistant depression.

Source: http://news.emory.edu/stories/2012/02/depression_evolution/