Rather than subscribing to this broken deficiency model of depression, Rottenberg argues that affective science — the empirical study of mood — lies at the heart of understanding the condition. Defining moods as “internal signals that motivate behavior and move it in the right direction,” he argues that our bodies are “a collection of adaptations, evolutionary legacies that have helped us survive and reproduce in the face of uncertainty and risk” and paints the backdrop of understanding depression:

The mood system … is the great integrator. It takes in information about the external and internal worlds and summarizes what is favorable or unfavorable in terms of accomplishing key goals related to survival and reproduction.

[…]

Once a goal is embarked upon, the mood system monitors progress toward its attainment. It will redouble effort when minor obstacles arise. If progress stops entirely because of an insuperable obstacle, the mood system puts the brakes on effort.

Under this model, mood has an evolutionary function as a mediator of survival strategies. Rottenberg cites a number of experiments, which have indicated that negative mood incites one’s psychoemotional arsenal when a task becomes too challenging. For instance, when study participants are deliberately put in a negative mood and asked to perform a difficult task, their blood pressure spikes — a sign that the body is being mobilized for extra alertness and effort. But if the task is made insurmountably difficult, so much so that success stops being possible, the spike no longer occurs and the mood system dials down the effort. In that sense, mood — the seedbed of depression — isn’t an arbitrary state that washes over us in a whim, but a sieve that separates the goals worth pursuing from those guaranteed to end in disappointment.

(WOW!!)

so depression can be fixed without drugs and/or therapy?

I had a different take-away - that depression isn't a defect but serves a purpose.  Depression is a signal to make a change but in the case of major depression a person might be unable to initiate change without help.

Depression is useful to a population of people for group survival because it provides signal to a group.

Unfortunately depression is debilitating to the people who carry it, then and now.

Wow is right!  Imagine someone who has stressful things going on in their home life, and they have felt down for a couple months, doctors are pretty quick to hand out anti-depressants to treat the depression, when really maybe the person needs to address some issues with themselves or their family, and make some changes, but instead they mask the normal feelings, and just push on.

Maybe there are still a small amount of people that have a true in-balance of brain chemistry, but the majority we have been over-medicating for years.