We've seen a number of human/singer hybrids (Unkalaki and Herdazians) and even human/Aimian hybrids (Natans). How is such a thing possible biologically or is there some outside influence there?
Brandon Sanderson (paraphrased)
He said that they (humans and parshendi/aimians) can interbreed because they were made by the same person, or rather that they were made with the same intent, that they were made that way on purpose. They don't have a common heritage or anything though.