Panspermia is the theory that life pervades the universe, and that it can be carried from one solar system to another. We know there are some spores that are tough enough to survive the radiation in space. Labs on earth have brought spores back to life after hundreds of millions of years, so we think they are good for the distance.
We used to imagine bacteria hitching a ride on meteors, but the newer idea is dust. There's a lot of it and it has an easier time riding an upwind to the top of the atmosphere.
If we met an alien, we don't know whether to expect his biochemistry to be carbon-based or ??? But if it turns out he has nucleic acids in his chromosomes and they're translated into proteins by a genetic code, chances are that we're related, and that life began not on earth but somewhere very far away and long ago, and perhaps our whole galaxy (and beyond) is seeded with life based on our familiar biochemistry.
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