When I was casting about for the origins of my evil galactic empire, I decided to make them human or at least partially human. What I based them on was a hominid species that had gone extinct on Earth long ago. In this way they would retain bits of the same DNA that we modern humans had. Which would make interbreeding, however unlikely, at least a possibility.
I’m a writer, not a geneticist or evolutionary biologist. I probably had things all kinds of out of whack when I imagined the early Votainions as being transplanted Neanderthals. Someone happened upon the Earth in ancient times and picked a dying branch of the human tree and took it hundreds of light years away and plopped them down on an alien world. These transplanted people eventually evolved into modern Votainions and started to viciously conquer the galaxy.
Time passed and I wrote two books in the Star Series. I added bits and pieces of the Votainion story in these books but when it came time to write the final book in the series, Starveyors, I had to ensure that the humans and the Votainions were genetically compatible. This is where modern scientists have helped me by their findings about Neanderthals.
It seems that while primitive human tribes were committing genocide against the Neanderthals they were also interbreeding with them. So modern humans with lineages that come out of Europe actually have some Neanderthal DNA in them. Fascinating. This fit into my plans perfectly. (Hands writhing, evil cackling.)
You can read some of the latest news on Neanderthals in this National Geographic article where they have once again created a sculpture of a lovely Neanderthal woman out hunting.