Depression: an evolutionary by-product of the immune system

Each year almost 80,000 Australian adults will experience a depressive illness. Image: Shutterstock

Depression may have been a genetic evolutionary advantage.

Depression is common enough — one in four women and one in six men suffer from depression at some stage in their lives — and biologists predict that depression, or behaviours linked to it, can somehow offer an evolutionary advantage.

Depression is a medical condition that causes physical and psychological symptoms, including low energy levels, reduced motivation, low self-esteem, and poor concentration and memory. These symptoms often lead the depressed person to seclusion. But Andrew Miller, professor of psychiatry and behavioural sciences at Emory University, and Charles Raison, previously at Emory and now at the University of Arizona, have proposed that the genetic variations that promote depression arose throughout our evolutionary history because they helped our ancestors fight infection.

Other researchers have seen links between depression and inflammation, or an over-activation of the immune system. People with depression have higher levels of inflammation, even if they weren’t fighting an infection.

“Most of the genetic variations that have been linked to depression turn out to affect the function of the immune system,” Miller said. “This led us to rethink why depression seems to stay embedded in the genome.”

In early human history infection was the primary cause of death; so fighting infection was vital if one was to pass down their genetic information. The researchers propose that evolution and genetics combined depressive symptoms and physiological responses to reduce human interaction with infectious individuals. In this sense, fatigue, inactivity and social avoidance can be seen as adaptive behaviours able to contain infection.

This proposal provides an explanation for why stress is linked to depression. Stress is viewed as a by-product of a process that preactivates the immune system in anticipation of a wound. The change in sleep patterns could also be pinned on our ancestors, who needed to stay on alert to fend off predators.

Miller and Raison’s theory is important for further research in depression. It will help determine whether inflammation may be able to predict if someone will respond to certain treatment for depression.

Source: Science Daily

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