Survival of the species and self, which means passing on your genes to as many as possible, however, if you mean why are we here?, haven't a clue, but you'd probably get an answer in R&S.
woofgang- I was going to say that! Not long finished reading Hitchhikers... and loved the alternative question the mice come up with- "How many roads must a man walk down?" "42" hee hee
Well here is my explanation
We are not human beings going through a temporary spiritual experience.
We are spiritual beings going through a temporary human experience.