They were robots at one time and then through observation of wildlife they came to understand the advantages of organic matter and began to use cloning to develop new body parts that could be controlled electrically. Over time, they slowly began to change their power source to an internal chemical system fueled by other organic matter and even began to offload computing power to organic components until they were eventually indistinguishable from organic lifeforms.
And maybe we were one of their prototyoes.
But enough about 2004 Battlestar Galactica.
Ha, did they do that? I've never seen the show. I just made that up off the top of my head to be absurd.
Basically. The Cylons, originally being entirely metallic, began producing organic/human Cylons that would slowly replace the metallic versions. These new Cylons were virtually indistinguishable from humans.