Nature is very fond of if it ain't broke don't fix it. It stands to reason they could have been around since the first digestive tracts formed.
These guys have the life as far as.... life goes I guess you could say.
Free food, free home, no animals trying to devour them (intentionally).
And if their host does gets eaten, hey its just circle of life they get a new one.
Cool none the less go , ya fossils even if is poo.
edit on 31-1-2013 by CitizenJack because: typo