Another thought from hypothesis land: maybe a little depression is a chance to retire and regroup, but in the original environment, there was more that would pull people out of depression-- social contact and work that obviously needs to be done.
This is about mild-to-moderate depression, though. Major depression seems to be different, and not good for anything.
I wonder what the highest rate of occurrence something can have and still be an evolutionarily detrimental spandrel, rather than an evolved response we just don't understand.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.