Stuff like this is why I like the Well of All Sparks being a combined thing from all the Primes. (Was it mentioned if it was their last act or something, like a gift to their descendants?)Would that mean that beast combiners came about when Nexus and Onyx got... cozy?
This way we get to have "the first __" without raising questions like "so how did those traits get passed on" or the possibility of tribes of just combiners or just beasts or just Mini-Cons and so on. (I don't mind those tribes being part of the history, just prefer they be factions rather than naturally-occurring dividing lines.)