Considering that the period during which humankind has even been in existence is a bare blip in geologic time, the "sunny day" might actually be a "sunny minute" or even less.

But humanity has survived since well before that.... Granted, back then they didn't have computers, rifles, domesticated sheep, or much else, but they DID have fire, the knowledge of flint knapping, perhaps the bow, the atl-atl, and something we seem to have lost: a kinship with nature.