This is an argument from ignorance. Not knowing
an explanation
does not mean an explanation is impossible. And since we are barely
beginning to understand what consciousness is, it is not surprising
that we would not have its origin worked out yet.
In fact, preliminary explanations for the origin of consciousness have
been proposed, although they are too complicated to try to summarize
here (see Dennett 1991 and Minsky 1985). Much more experimentation and
refinement is needed before we have a full-fledged theory of the origin
of consciousness, but we have more than enough to know that such a
theory is possible.
A factor that likely contributes to the claim of consciousness's
inexplicability is the fact that many people do not want a
naturalistic explanation of consciousness, since a natural
consciousness does not fit easily with a divine soul. This threatens
people's desire for a divine origin and immortality (but see Dennett
1991, 430, for immortality of a naturalistic consciousness). An
examination of this point alone could fill a book. However, suffice it
to say,
There is much evidence -- from genetic predispositions of behavior
and personality, from brain injury studies, from brain imaging of
healthy people -- that consciousness is naturalistic now. A
natural origin would not matter much beyond that.
What we want has no bearing on what really is.
References:
Dennett, Daniel C., 1991. Consciousness Explained. Boston: Little,
Brown and Company.
Minsky, M., 1985. The Society of Mind. New York: Simon &
Schuster.