It's a different topic, but related. I was marveling at the diversity of life on Earth, which led me wonder: to what extent does sustained life on Earth depend on that diversity? I.e., what would be required or different for a planet to host and sustain a single species of life?

Given the nature of evolution, and that one subscribes to it, life on Earth started with a single organism that replicated. From that point until a replication modified the organism into a different species, there would have been one species. But was it necessary to have multiple species in order to sustain life?