Well, since certain organelles within our cells (mitochondria) have their own DNA, it’s been suggested that they were once separate organisms that became the ultimate symbionts. Now, someone’s found that single celled amoebas may actually farm the bacteria they eat.
P.S. While looking for a picture of the guilty party, I came across this nice image of the amoeba, Dictyostelium discoideum, splitting into two on Wikimedia Commons.
