Aliens: "Let’s create stronger but less intelligent creatures, and leave them to evolve over millions of years."
Millions of years later...
Aliens: "Now that we know which genes and codings work, i.e., survive, on this planet, we shall kill off all these creatures and use what gene-bases we need to bio-engineer a weaker but smarter species to serve us in this star-system.
Aliens make Humans, who wouldn't have been able to survive the initial stages of creation. Aliens evolve humans using select plant- and animal-origin DNA.
Et voilà, Pre-History is born.