Perhapse an alien race visited earth, and geneticly modified some dinosaurs to give them enough intelligence to be usefull slaves.

After a few millenia the slaves revolted, and a great war started, wiping most dino life out of existance (nuke winter 63 Million years ago). The suriving intelligent reptiles lived underground and had great technology.
They travelled to the stars.

Eventually they decided they needed a slave race, so the geneticly engineered some apes to be just smart enought to be usefull.

And the Apes did great labor in exchange for paper.


I'll let you know how it ends in a few thousand years.