I would
contend it's actually the other way around - that depression
is indeed a tool
for evolution, that it is
beneficial, and confers an advantage, one that we haven't quite
learnt how to use yet.
Speaking from personal experience, I don't know very many people who have never been depressed, and I go to university with a lot of rather clever bods, who are constantly being challenged and taught by a bunch of even cleverer bods, who also seem quite depressed at least some of the time. I have found that whenever I am depressed, it actually helps me do maths, or plan out programs, or solve complicated engineering problems.
In fact, I've found that when I'm cheery going into an exam, I do worse than when I'm depressed going in. I think more clearly, I sustain concentration longer, and my answers are more complete, and "astonishingly insightful". (To quote one of my maths teachers, Mr Gambel, who was quite upset to find out I was one of his better students that year, and it was a good year.)
The point is that when we are depressed, what is depressed is our range of emotions, which often interfere with our analytical abilities. Imagine walking over a landscape with hills, rivers, forests, and even mountains.
Interesting isn't it?
Now imagine if all those are gone, and you're walking over flatness. It may be boring, but you certainly will make better time, and have less to destroy your concentration.
The problems come when you aren't able to handle this analytical toolset, and you turn it upon yourself, trying to literally break yourself down into your component parts, and possibly doing intense damage to your own self-esteem in the process (we don't component well, us human beings). And after all, very few critical reviews are entirely positive (except Zaphod Beeblebrox's but that's another story).
Those people who are able to limit this introspection and guide their depressive, and detached mindsets towards constructive things usually don't commit suicide. They go on to prosper and have lots of kids. :-) Or at least give lots of ideas to other people which are beneficial. Anyways, it's just a theory.
BTW, if you are having problems with depression, SEEK MEDICAL HELP Especially if it doesn't lift in a couple of days.