I stumbled across an article about Buckminster Fuller none too long ago. Apparently he claimed that purpose is innate, but follows a rule of "precession." The explanation being that one's purpose is achieved as a side-effect of one's goals.
Bees, for example, have individual goals along the lines of "go find pollen to make honey." As a side effect of their doing so, they pollinate plants, granting the rest of us the ability to eat.
Unfortunately, this means that there is no achieving anything in any lasting sense. Best to find a way to live while loving what you do, because there is no finish line. (Unless, as George Carlin jokes, we actually are here to produce plastic*.)
Perhaps, for those of us constantly hungry for meaning, the best we can hope for is positive feedback about the effects our lives have.
* Of course, if Carlin's approach is right, we're probably here to develop some kind of heavily armed super-consciousness.