I tend to agree with Thunder Bow. We are hunters-gatherers species. We used stress only in dangerous situations to deal with it occasionally (like when a tiger attacks) not on daily basis as today. We made huge effort to hunt once a while not on daily basis as today.

I think mental illness has become in effect during the agricultural revolution, when human settled in large communities, and have to work hard every day. I think, it is not a coincidence that organized religions appeared in this period, and the Church has confession because of misinterpretation of the human nature, which I view it as a kind of therapy. For example, there is something called the "the spirit of sorrow" which may be depression, and the Church has a prayer and a ritual for it.