Or, to make matters a bit more attainable, what is the earliest occurrence of the term attested in writing?
According to Wikipedia,
the sense of "inhabitant of earth" is first attested in 1593
although no reference is given to back up this claim. However, the same entry also mentions a work from 1949 ("Red Planet" by Robert A. Heinlein), claimed to be the first work in science fiction literature to contain the term in its current meaning.