Stardust

[cyber_indian]

But if life is truly ubiquitous in our galaxy, shouldn't it be settling through our atmosphere every time we pass through a comet's orbit?

There will be ... but it'll die quickly because it would need a hostle non-biotic enviroment to start from ... see to be transported in space and to be able to survive raw non-biotic planets it has to be constructed as extremophile that has to self upgrade after every new stage of teraformation is completed ... and in the end the original will either die off or will be verry scarce(like archaeas are) or will totaly disappear because no original is left in result of the upgrades(evolution).