History boils down to having some sort of primary evidence in order to gain a understanding about a civilizations life at any given time. This varies from civilization to civilization, however, without any written or oral accounts, all historians can do is speculate based on evidence such as archeology or remains.

One example of this is Neanderthals; we know from fossil and archeology findings that they certainly were capable of having thier own culture and customs, but since there is no way to know what said culture was we can not definitvly label anything as thier history, thus they fall into the prehistoric catagory.