Following up the analogy of individual to cell, species to organ, and biome to organism, and probably taking the metaphor to a place where it should never, never be allowed:
We might wonder what the functions of the human “organ” are in the system. If indeed the entire planet can be thought of as a single biome, the only way it could ever reproduce itself would involve seeding its particular biota onto other planets; so it could be that the urge for space travel is really a reproductive urge.
That’s right, we may well be the Earth’s gonads.
(This is not meant to detract from the actual serious intent of the previous post, and I am actually interested in comments and discussion on that one – this is just something that came up during a conversation last night)