When the aliens see a human walking a dog in the park

Most likely the human. If the alien's race has the brains to travel to earth, they could deduce that it's the humans that build the architecture our societies consist of—that we're the ones who built this urban world, and that the dog is a domestic animal. In addition, nearly always the dog would be on a leash. If this was also the case, well, a master would never wear one.