Solus Prime lifted her mighty Forge and smashed the blade into five pieces. Our brother Nexus Prime, master of combination that he is, then volunteered to split into his individual components and spread the fragments across the Multiverse in hopes that Megatronus could never find them.
The Origin Matrix, the most powerful of the fragments, was sent to the distant, "prehistoric" as you put it, past. It landed on the Earth of Primax -408.24 Epsilon and I initiated a temporal barrier... A time lock, so to speak. The artifact could not leave that particular era in time, and no travelers could venture backwards to access it either. Consequently, this is the reason that the Vok were chosen as the Origin Matrix's guardians

-AVP