I like to believe that every modern-day affliction and illness once had adaptive and evolutionary value, or else it that affliction wouldn't exist in the first place. Well, in the spirit of the New Year, there is none better than depression, not only so we can understand what use it may have once had to our ancestors, but rather why, from an evolutionary standpoint, depression exists in the first place. As suicide kills more people every year than murder and war combined, I believe it is imperative for us to get to the bottom of one of the world's most devastating illnesses.

Depression is known to run in families, and it is no surprise that depression and mental illness have genetic components to them. To our surprise, however, researchers Andrew Miller and Charles Raison, physicians at Emory University and the University of Arizona, respectively, have proposed that the same genes that increase a person's risk for depression also enhance immunity to infections. "Most of the genetic variations that have been linked to depression turn out to affect the function of the immune system," Miller said. The hypothesis is that these genes were adaptive in helping people not die of infection in a world where life expectancy was significantly lower.

It's no surprise that afflictions like depression, anxiety, and mental illness have been more prevalent in our modern-day age. The current life expectancy in the U.S. is 78.7 years, compared to the approximate 40-year-old life expectancy of pre-industrial England. Without infections like tuberculosis and pneumonia to worry about, it is unsurprising that people have more time to be depressed in our modern era.

The genes that have a predisposition to depression are mutated variations of the "NPY" gene. Mutated versions of the gene allowed our ancestors to have increased inflammation, especially in early childhood when we are most vulnerable to infection. As individuals with the mutated NPY gene were more likely to survive, more and more copies of the gene were passed on to their offspring.

Recently, researchers at the University of Michigan found that people with depression were more likely to have a mutated NPY gene. Functioning normally, the gene codes for a neurotransmitter that increases tolerance to stress and made us less likely to be depressed. In ancestral environments, however, the trade-off for extended survival was often depression. According to Miller and Raison, acute stress was usually linked to physical harm or wounds, most of which led to death, or infections that easily led to death. As a result, evolution favored the selection of individuals with immune systems that operated under a "smoke-detector principle," a principle that reacted readily to false alarms of infection as well as actual infection.

"Immune responses to acute stress are typically not necessary -- not every stressful situation results in a wound and infection. However, if our ancestors became wounded even a single time and didn't experience a piqued immune response, they might die from an infection," writes Brian Gabriel of The Atlantic.

According to Miller, depressive symptoms like social withdrawal, and loss of energy were once advantageous to our ancestors. A loss of energy often was a symptom of the body using all of its energy to fight infection. Social withdrawal often was a symptom of avoiding exposure to additional infectious agents. Recently, the two researchers have worked on targeting inflammation to treat depression, with varying degrees of success. In patients with elevated inflammation, a drug that disrupted communication between immune cells worked.

I have said it before, but the findings in the Andrews and Raison study support the claim that depression is not a sign that something is wrong with you, even from an evolutionary standpoint. Like for our ancestors, depression, even now, is a sign that there is a situation or infection so serious that we need to devote all our energy to battling it and withdraw from others to avoid further damage or harm. We suffer when depressed, but we do not do so in vain.

Although it is a stretch to name any advantages of depression, two other researchers have shown that rumination, the foremost symptom and cause of depression, may lead to improved outcomes in our modern lives. Andy Thomson of UVA and Paul Andrews of VCU have found that the awful symptoms of depression "reduced the possibility of becoming distracted from the pressing problem." We need to pay attention to what underlies our depression. Say a family member just died or you went through a divorce: who wouldn't ruminate about those pressing life events? "Human attention is a scarce resource -- the neural effects of depression make sure the resource is efficiently allocated." The result is, because of our ruminations, we become much more analytical and deliberate in our thought processes.

Many people may see the two research papers' findings about the advantageous and evolutionary values of depression to be mere justifications for human suffering. It is true that these evolutionary and adaptive theories about depression only apply when an identifiable stressor is causing the illness. We know that sometimes depression can strike for seemingly no reason. But normalizing is not romanticizing, and I do not see a world where drugging away our problems is a feasible means of advancing the human species forward. I do not question that medication is the best way to treat some people afflicted with depression, but it is not the only way.

Sadness and unpleasant emotions do have their benefits: psychologist Joe Fargas has recently found that negative moods lead to better decisions in more complex situations. Paul Andrews later found that negative moods led to improved analytical abilities. Depressed people were found to think better. But "that is the paradox of evolution: even if our pain is useful, the urge to escape from the pain remains the most powerful instinct of all."

Thomson has one anecdote about how his theory of rumination has affected his medical practice. He has cut down significantly on antidepressant prescriptions because he believes the drugs can interfere with genuine recovery. One patient came in and requested a reduced dosage of antidepressants. The antidepressants were working great, and in fact, the patient told him that "I feel so much better." However, she also told him "I'm still married to the same alcoholic son of a bitch. It's just now he's tolerable."

There is usually a reason for depression and something behind it, even if that reason is not something we can see in the moment. It is a message to ourselves that our needs aren't being met. Part of fighting depression is by identifying a single problem, or perhaps the many problems we need to solve, and then making a long-term plan to solve them. It is a long, difficult, and tedious path, and at times it seems impossible, but the role of most therapists and counselors is so we know we do not face these problems isolated and alone, that we have guides who will walk with us on our journeys.

Maybe, today, after all, depression is adaptive, too.