What if the human race is a parasite? The planet started with a natural equilibrium of water and biomass (plants) with healthy bacteria (animals) living on it, then the human parasite began to spread, devouring its resources and consuming the planet. What if we were seeded for this reason? Perhaps our forefathers, long ago, launched our genetic material into space in all directions, hoping that some small fraction would reach other habitable planets. Spreading the human race through the galaxy like a virus. Maybe we’re destined to die here, but not before launching our own genetic material back into space to keep the cycle going.
UPDATE 2015-02-08: This seems more plausible if life is exceedingly rare. An alien society seeding planets because it won’t happen naturally. Sure they expand themselves to various points throughout the universe, but probably not ALL the planets. Seeding hundreds, thousands, millions of them seems reasonable if the odds of life are so slim by default.
UPDATE 2018-09-10: Humans are not virulent. The planet has never been at a natural equilibrium. There have always been problems and humans are just as natural as other complex systems in the environment. It’s one thing to see similarities, but to reach a conclusion about humans being a virus or bacteria upon the earth is naive. Viruses don’t build cities, write books, make YouTube videos, or perform surgery. Comparing us to them is a vast oversimplification of the situation we find ourselves in. Sure, there are patterns, and comparisons like this can be useful, but it is not a conclusion.