There is ample evidence of the genetics of mental illness. Schizophrenia, an illness of disconnection from reality, occurs in one percent of all human populations, indicating a prehistoric timeline to explain its ubiquitous prevalence. Amish populations were the first to show a gene on chromosome 11 that is responsible for bipolar illness. Additionally, twin studies, and numerous family studies have shown a genetic correlation for the incidence of mood disorders (major depression, bipolar illness, and schizoaffective illness).
The rules of Darwinian evolution explain that a population will tend to strengthen a particular genetic variant if the variant improves the survival of an individual and its offspring. Interestingly, most mental illnesses are diseases of adulthood. For example, the average age of onset of bipolar illness in women is 29 years old. This implies that there is no Darwinian selective pressure either in favor or against mental illness.
That means we are left with our genetics, our biological predisposition for illnesses like depression. Evolution further notes that multiple generations are required to strengthen a particular genetic variant in a population. Let's imagine that there is a genetic variant that would improve our ability to deal with the stresses of modern life. How long might it take for this genetic variant to gather steam in the general population?
Some of our past inhabitants of the planet, the dinosaurs, can shed light on this question. Dinosaurs existed from about 225 million years ago until the comet smashed into the Gulf of Mexico about 65 million years ago. This is roughly 150 million years. During that time, dinosaurs came to inhabit every environment on the planet, including a challenging Antarctic climate of temperate summers and harsh winters. 150 million years is a long time for adaptation and evolution- a really, really long time. If we hypothesize five to fifteen years per dinosaur generation, this means that there were from 10 million to 30 million generations of dinosaurs constantly selecting for adaptive genetics that improved survival. That's a lot of generations.
How many generations of humans have there ever been? If we are generous, Homo sapiens have been on the planet for 100,000 years. At 15 to 20 years per generation, that means that all of human existence covers 5,000 to 6,000 generations. That means that dinosaurs had 2,000 times the generations humans have had for evolution. But humans live in a dramatically different environment than 100,000 years ago. Humans have been "civilized" (living in non-nomadic settlements) for about 6,000 years; that's several hundred generations at most. Humans have been living in an industrial age for over several hundred years; that's only a couple of dozen generations. Humans have been living in an age of technology for only a few generations.
A few generations are hardly the time necessary to adapt to a new environment. Therefore, we presently carry with us the genetic adaptations that were useful 10,000 or 100,000 years ago. As an example, being alert to the point of anxiety may have been adaptive for early humans. The more alert an individual is, the less likely the individual is to be eaten by predators or attacked by other marauding humans (sad, but that aspect of our history goes way back, too). But this level of anxiety is not always useful in the modern world; panic attacks, agoraphobia, and post traumatic stress disorder cause disability not adaptability.
If cognitive-behavioral or "talking" therapy does not control symptoms, then what do we have to use to adapt to our technological society? The answer is in technology, in the use of psychopharmacology, the use of medicines in mental health. We use technology to adapt while genetic selection takes its time to catch us all up to what we've done to our environment. In 1970, Alvin Toffler's "Future Shock" predicted the social difficulties of dealing with "too much change in too short a time". Our best way to cope in this future world is to use all the modern tools we have to adapt and function in this modern world.