At least half of us is non-human cells, fungi, yeast, etc. so there's plenty of room to consider many retroviruses at work. Or, if you want something more spunky and soulful, maybe we're the psychedelic mushroom's dog and we became more like our master as we evolved. I'm sure Terence McKenna would have liked that one.
_________________
“Love takes off the masks that we fear we cannot live without and know we cannot live within. I use the word "love" here not merely in the personal sense but as a state of being, or a state of grace - not in the infantile American sense of being made happy but in the tough and universal sense of quest and daring and growth.” - James Baldwin