you pose some great questions.
however in my many reflections as to the purpose of existence (god's goal, human's, the earth, etc.) i have come to only a few tentative and vague conclusions, though i wouldn't really call them conclusions persay, more like insights that tend to sound "right" and that just leading to more questioning/confusion, keeping in mind that confusion (in the existential sense is a very good thing, it is what keeps the brain functioning)anyway, here we go
that that is must have an opposite, that which is not. true purity or at the same time pure evil, are both inconceivable/unfathomable. man/woman
black/white sun/moon day/night in/out action/reaction. i guess if you wanted to be scientific you could relate this to newton, though he was still too specific and failed to see the bigger pictures.
these opposites are what define everything. religiously, there is the relative opposition of good and evil, in christianity you have God and the Devil. if one considers the earth as a whole living being, you have the things that nourish the earth and the things that destroy and pillage the earth. if the earth did not have all of these action/reaction sequences happening in infinite numbers all over the world, it would cease to be. in these terms, one may view the existence of humans to be that of a destructive one. presently the human race is polluting the earth with trillions of megatons of trash everyday, burning vast holes in the ozone layer, and killing off many of the other life species that, due to the fact that they cannot reason and that they do not have "free will", have been able to live in harmony with the this planet. yes there are viruses and plant killing parasites and animal killing diseases, but the problem with humans is that we are committing these destructive actions in great extremity. basically we as humans have severely disrupted that balance that this planet was living under before we evolved into being. so now i see the question as being how and did we, the human virus, the pathological anti-existence, the veritable disease of this planet occur? there are many other questions though, like you said, where do we fit in in the cosmos? how and why did the universe occur in the first place? is god simply that which facilitatesz life, the universe itself? to me, reason, self awareness, a reception/perception that is fueled by free-will, these are curses, mistakes, but that is all the more reason to utilize them in the same kind of philisophical mind-bending for which this site is meant. if i had to box it up and put it in religious terms and relate it to the conception of good/evil, i'd say we were created by the devil, or maybe we are exactly that "the angel/s that were ostracized from heaven and forced to live in a world of toil. anyway all this writing/thinking has made me want to go outside and smoke a cigarette and look up in the sky and just get wrapped up in this stuff all over again.
this is actually my first time to visit this site because my friend just told me about it, but its definitely not my last. for those of you who made it all the way through all of my ramblings i would love any sort of comments/feedback/arguments/contradictions that any body might have. thanks.