Was Spender a Martian?

Or more specifically, was he taken over by one? It seems like Matheson may not want us to have a definitive answer, and it is strongly hinted during the "shape-shifting" scenes in episodes 2 &3 that he was taken over. But when he is killed at the end of episode one, Spender's body does not disappear. I took this as "proof" he was not Martian himself, but strongly influenced by the Martian culture. The counter-point to this is that when Spender approached the canal he introduces himself as "the last Martian", and he tells a strange anecdote of giving his boots and uniform to a Martian he met in the city, but it sounds allegorical. Any other thoughts on this are welcomed.