The article at The Atlantic website begins with this striking observation: "More people die from suicide than from murder and war combined, throughout the world, every year." From there, the author, Brian Gabriel, develops the idea that depression may be a consequence of evolutionary success: which is to say, there is something positively adaptive about the genetics behind depression, related to both immune response and other, more behavioral results. Basically, if you're a member of hominid species that mostly dies of disease in youth, there may be an evolutionary advantage to reacting to stress by isolating yourself and sleeping all day.
This actually makes some sense. I'm not savvy enough to judge the bits about immune response – and I also seem to remember learning that depression actually lowers immune response – so I'm not sure how that works or what the interaction is.
Regardless, it's a very interesting, brief article.