But I think this actually happens with normal life as well: Individual species of bacteria try to conquer a biological niche by shifting conditions such that on

But I think this actually happens with normal life as well: Individual species of bacteria try to conquer a biological niche by shifting conditions such that only they can survive. But by doing this they invariably create chemical by-products that can be harnessed by other life forms, thereby fueling their growth. It's an interesting question whether there are any biospheres on Earth that are colonized exclusively by a single species of life (I doubt it). So although it's a very valid point I'm not sure if it's a sufficient argument against the hypothesis of having two independently evolved and fundamentally different types of life on a given planet.