In the Historical notes section of the Wikipedia article on category theory, it is mentioned that in 1942-1945, Samuel Eilenberg and Saunders Mac Lane, in the course of their work in algebraic topology, established the notion of category and functor in order to study natural transformations.
The article goes on to say that Stanislaw Ulam and some others have claimed that category-like ideas had existed since at least the late 1930s, and that category theory was, in a sense, related to the work of Emmy Noether during even earlier years.
I have not formally studied category theory, just done some reading on it, so it didn't occur to me that there are many trivial and early examples of functors. In light of this, I'll attempt a question that is perhaps easier to answer (if longer).
What exactly were these pre-algebraic topology notions of category theory developed in the 1930s? Also, even though particular classes of mathematical objects have existed for longer periods of time, presumably they weren't actually viewed as categories until later on. What is the earliest category mentioned in the literature (e.g. the category of groups or the category of topological spaces)?