What we call "Mental Illness" can be a positive adapation for certain environmental stresses

Is it possible that there are situations where "symptoms of mental illness" are positive adaptations?

Anxiety and Depression 

The two largest categories of mental illness in terms of number of diagnoses are Anxiety and Depression.

Anxiety: May I suggest that the ant in the story "The Ant and the Grasshopper" was driven by anxiety to store provisions for the winter?

Mass grave of Medieval famine victims

Getting ready for winter was not a trivial task in medieval Europe. The days of summer were long in terms of number of sunlight hours due to the high latitude. Peasants had to work as if the hounds-of-hell were chasing them every daylight hour. The name of that hound? "Anxiety".

Peasants who were not anxious starved to death before the next harvest.

Incidentally, people who are anxious tend to be great planners. They constantly play "What if..." in their minds and make plans to remediate those issues. Modern society sits on a foundation of What-if plans. 

Depression: Conversely, being able to sleep for 20 hours a day during the winter was an energy conservation strategy. Put the entire family into bed and throw all of the clothing and blankets over them and let the cooking fire drop to a slow-smolder. Relatively little energy was expended. 

If you charted the energy expenditures it would look like the flight of a woodpecker. A burst of beating wings and then a ballistic free-flight that was as long or longer than the burst of wings beating.

Not wanting to socialize was also a benefit. Lice spread typhus and those beds got lousy because boiling all of the bedding and then drying it all-in-one-go was beyond the means of most families. Socializing meant sharing lice with other families...lice which might bring typhus into your family. 

ADHD

It is notable that the surge in diagnosed cases of Attention Deficit, Hyperactive Disorder(ADHD) in boys approximately aligns with the removal of gym classes and recess from elementary schools? It also coincides with the time when kids transitioned from walking to-and-from school with busing being the predominant mode of getting to-and-from school.

A "good student" is one who sits passively and "doesn't make trouble".

There was a time when every person in the family had to contribute to the household economy. Those little boys would scour the area for twigs and sticks to cook the daily porridge. They would trap sparrows and catch minnows for the stew-pot. The child who sat passively and "didn't make trouble" was an economic dead-weight in terms of the family economy in a subsistence environment.

In older people, having a certain percentage of the population with ADHD meant that nearby tribes could not sneak up on your tribe. Nor were flash-floods and sudden wildfires likely to catch the tribe unaware if Billy was relentlessly scanning 360 degrees of the compass.

Finally, I had a conversation with a doctor who claimed to have ADHD.  His intellect was impressive as he quickly leapt from topic-to-topic. It is my impression that he was probably very good at diagnosing issues because he didn't get trapped or "overly-invested" in any one possibility. 

I have seen trouble-shooters in the factory get bogged down when they are "absolutely sure" they know the cause of a problem and then they try to bend the symptoms to match the illness. They would have been far more effective if they considered the four most likely diagnosis consistent with the main complaint and then looked for the best fit. 

Post Traumatic Stress Disorder

PTSD comes in a lot of flavors, but in some cases it keeps people alive on the battlefield. There are situations where actions must happen faster than conscious thought. Obviously, I am not talking about the PTSD where the person goes catatonic. I mean the PTSD where the patient explodes out of bed 210% awake and sprinting at the sound of whistling or a "pop". Maybe they smash the face of the person who is too close to them.

Summary 

Are these behaviors anomalies or is our current environment a transient anomaly? The subsistence agriculture in Europe lasted for roughly 3000 years. Warfare has been around longer than that. Industrialized education, depending on how you want to define it, has been around for fifty-to-one-hundred years.