What a good answer. I also think it relates a lot to what has happened in time to all the other species that were close. Think of a platypus. That is unique in so many aspects. We can easily create a dividing line around that and call it a species, genus and family. Then the nearest others are Echidnas. However at earlier points in time there were other species that were transitioning into platypuses. Back then defining the one species as platypus would have been difficult. It is the fact that the other transitional ones no longer exist at this point in time that make it easy in most cases and why we can put names to most organisms. The same applies to humans and everything else.
My earlier response was of course not an attempt to deny the concept of species, which works most of the time, but only that we would not find a viable and universally accepted definition.
Philip
