Yes but there's some logic in us defining civilisations as cultures, which have left behind evidence of their existence. As example Denmark we have been a agricultural culture for 6000 years, but the first name we find are around 1500 years old, the first real history (written by our enemies) begins aroun 1200 years ago, we really only begin to have our own history (rather than myths which likely go back to late periode of the West Roman Empire) around 1100 years ago. While we can see they had complex societies for all those 4500 years, we have very little idea how they thought, what their belief was etc. It the same a lot of other non-literate people suffer under, we can only guess who they were. As example the Uffe (Offa) Hin Spage sage are from Anglish history before their migration to Britain, and as the Angles who stayed in Denmark was assimilated into Danes, that myth was kept alive.