Around 15 million US adults suffer from depression. 6.9 million American adolescents have been diagnosed. The condition is common. 10% of the population may suffer from significant depression sometime in their life. While 30-50% of folks are bound to feel some sort of depressive symptoms at some point. This isn’t relegated to the West.

Depressive symptoms have been found in every culture on Earth and throughout history, though at one time it was called melancholia. Unlike other psychiatric disorders, which are rare, depression is fairly common. According to a San Diego University study, depression across the US has increased significantly over the last couple of decades or so, and people are experiencing more symptoms today too, such as difficulty sleeping or concentrating. Suicide is also at a 30 year high. This matches worldwide statistics.

More people in the world today die from their own hand than in wars and car accidents combined. In the US every age group, other than older adults, has been affected. But women and the middle-aged are particularly prone to suicide. Studies have shown that in most cases, depression or some other disorder was the motivating factor. Researchers blame economic anxiety such as financial trouble and job instability and the inability to save for retirement, or even an emergency.

Another reason may be increased social isolation, as the middle-aged have a high rate of divorce. One study even called it a byproduct of modernity. Here the author wrote, “Modern populations are increasingly overfed, malnourished, sedentary, sunlight-deficient, sleep-deprived, and socially-isolated.” Chronic diseases are on the rise too, and this may influence the depression rate.

Though effective for some, antidepressants don’t help everyone. And several come with worrisome side effects such as suicidal thoughts, leading to the need for a better understanding of depression.

Little is known for sure. But in the last few years, researchers have gained biological insights into the mood disorder and have hypothesized about its purpose. We think of depression as an obstacle to good health and in extreme cases, even survival. But what if at one time, it actually gave one an evolutionary advantage?

Some research points to neurophysiology as the basis for depression. 5HT1A is a serotonin receptor. This is the neurotransmitter that gives us a sense of well-being. In some studies, when this receptor was lacking in rodent models, they were resistant to depressive symptoms. Pharmaceutical companies are currently developing drugs that target this receptor.

Other studies have shown that the brain operates differently in those suffering from depression. An area of the brain known as the ventrolateral prefrontal cortex (VLPFC) shows remarkable activity in the depressed. This area is known to engage analytical thinking. The depressed often ruminate over their problems. When this occurs, the neurons in this region fire on a continuous basis. The 5HT1A receptor is thought to provide the fuel these neurons need to engage the rumination process.

Because of this, scientists posit that depression is a neurological process that helps the person avoid distractions, in order to concentrate on a difficult problem. Evolutionary psychologist Paul Andrews posits this “analytical rumination hypothesis,” which states that by promoting a period of rumination, one can come up with a solution and therefore, increase their chances of survival.

There is some evidence to support this theory. Research has shown that the depressed often get more REM sleep, which helps consolidate memories. Also, those with depression have been shown to be better at solving social problems. And social cohesion was extremely important to our early ancestors. If the band couldn’t cooperate, how could it survive?

Rumination may have given us an evolutionary advantage.

Another theory is that major depression is an immune response. Those genes which beef up our immune system to fight off infection also increase our risk of depression, according to Andrew Miller, MD of Emory University and his colleague Charles Raison, MD at the University of Arizona. The symptoms we associate with depression including fatigue, a lack of desire to take part in once pleasurable activities, and social withdrawal.

These would have kept a contagion away from the social group, helping to contain the infection. The person also conserves energy, which could be used to aid an immune response. Advances in medical science and public health may have made this reaction obsolete. But for our ancestors, infection was the topmost killer.

A gene mutation known as NPY is associated with an increased inflammatory response. Though this is helpful for fighting off infection, it's also associated with depression. This gene controls a neurotransmitter called Neuropeptide Y, which helps increase our ability to endure stress. Those with a mutated NPY gene saw a stronger immune response, according to researchers the University of Michigan.

But today, it merely causes a higher likelihood of developing depression. Those with this mutation may have an immune system that reacts much like a smoke detector. Though it lies inert, when a stressful event occurs, like an infection, this immune response is triggered, much like how the detector picks up smoke and signals the alarm.

Drs. Miller and Raison attempted to test the “Smoke Detector Principle,” by giving depressed patients an anti-inflammatory drug. Turns out, not every person dealing with depression has chronic inflammation. Only some patients responded, those with a certain genetic signature. But these participants saw dramatic improvements.

Knowing how the immune system and major depression are linked could help develop the new and growing field of immunotherapy, while offering a better understanding of the immune system, and more advanced treatments for certain kinds of depression.

A better understanding of the neurological and genetic aspects could reap further insights and novel treatments as well. The biological and evolutionary underpinnings are multifaceted and complex. More research is required, particularly since the need is so great.

To learn one novel approach to treat depression and other disorders, click here: