Have a look at Our Generation Ships Will Sink over at Boing Boing, in which Kim Stanley Robinson tells us why, as a living, thriving species, we’ll never get off this planet (and therefore should look after it better).
We are always teamed with many other living creatures. Eighty percent of the DNA in our bodies is not human DNA, and this relatively new discovery is startling, because it forces us to realize that we are not discrete individuals, but biomes, like little forests or swamps. Most of the creatures inside us have to be functioning well for the system as a whole to be healthy. This is a difficult balancing act, and does not work perfectly even on Earth; but divorced from Earth’s bacterial load, and thus never able to get infusions of new bacteria, the chances of suffering various immune problems similar to those observed in over-sterile Terran environments will rise markedly.
Because we need a broad array of bacterial companions, one would want to bring along as much of Earth as you could fit into a starship. But even the largest starship would be about one-trillionth the size of Earth, and this necessary miniaturization would almost certainly lead to unknown effects in our bodies.