I assume you are trying to point out that only humans have 8 cranial bones because you believe this will verify that there is a sharp delineation somewhere in among the many fossils between our Pan-Homo common ancestor and Homo s. s. that will divide them in to two distinct groups?
If at that delineation point there are features other than the number of cranial bones that place the 'ape' in with 'Homo', which feature(s) takes precedence?
. . . which feature(s) takes precedence?
Human morphology undergoes dramatic change in the history of an individual. Comparative morphology, if it is dealing with specimens of the same age, should reveal consistencies in bone structure, but these consistencies need be no more rigid than comparisons between Shaq and a dwarf. I think number and shape of bones is a good rule of thumb. What else do we have to work with after decay has had its way?