The idea is gaining momentum in tandem with the widespread belief in visiting aliens today.
The Sumerian tablets mention in-depth supposed "wars" between planets (cataclysmic planetary events), and the corresponding wars between the ruling gods - the Annunaki - literally 'those who from the heavens came'.
It seems fairly clear genetic engineering took place from what's recorded in these tablets but this does not mean their scientists were alien. Could it in fact be they were those who figured things out through owning all the available resources and appeared like gods to the populace? After all - what a nice little earner it would be to say to your subjects, "We created you to work for us".....
The 'truth' of this to their subjects (or slaves) would have been made stark from their godlike ability to ascend to the skies in flying craft and have the most awesome of weapons and this seems almost certain from explicit descriptions.