Humans comprise a naturally occurring species, so I would ask, "What purpose could any naturally occurring species serve?" We humans use some naturally occurring species, such as Oncorhynchus nerka (sockeye salmon), as food, but it doesn't follow that the purpose of that species is to be our food. Unless there is a god who created species for this or that purpose, naturally occurring species -- qua species -- have no purposes. Whatever has a purpose must be intentionally given that purpose, and I think that no being exists who could give humanity as a whole a purpose. So I agree with you that humanity as a whole has no purpose. But humans are hardly unique in that way.
Moreover, even if there were a being who created all humans for a purpose, I doubt that any humans (much less all of humanity) would thereby acquire that purpose, as I suggested in my answer to Question 27543. The only way I can see in which humanity as a whole could have a purpose would be if all humans collectively resolved to make some particular thing the purpose of our species, but even then I doubt that our unanimous resolution would do the trick. An organization can require that every member sign on to the purpose of the organization, but a species isn't an organization. No purpose that the existing members of our species might sign on to can bind past or future humans to that purpose, because past and future humans didn't sign on.