A new study has suggested that depression which clouds the prospects of millions of people worldwide may actually be 'hard-wired' into our brains to combat disease.
The radical new theory being proposed by a pair of psychiatrists suggests that genetic variations promoting depression may be a by-product of our ancestors' ability to fight infection.
"Most of the genetic variations that have been linked to depression turn out to affect the function of the immune system," said Andrew Miller, professor of psychiatry and behavioural sciences at Emory University.
"This led us to rethink why depression seems to stay embedded in the genome," added Miller, the journal Molecular Psychiatry reported.
"The basic idea is that depression and the genes that promote it were very adaptive for helping people -- especially young children -- not die of infection in the ancestral environment, even if those same behaviours are not helpful in our relationships with other people," said Miller's colleague Charles Raison, from the University of Arizona.
Infection was the major cause of death in humans' early history, so surviving infection was a key determinant in whether someone was able to pass on his or her genes, said a university statement.
Evolution and genetics have bound together depressive symptoms and bodily responses that were selected on the basis of reducing mortality from infection.
Fever, fatigue/inactivity, social avoidance and anorexia can all be seen as adaptive behaviours in light of the need to contain infection, they study authors wrote.
Similarly, a disruption of 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 said.