Panspermia is the hypothesis that life exists throughout the universe, and is distributed naturally by meteoroids, asteroids, comets, and so on. Specifically, it states that life did not arise on Earth, but arrived here, from someplace else. (http://en.wikipedia.org/wiki/Panspermia)
Panspermia got a major theoretical boost in the last few decades, mostly after the Apollo missions to the moon: Once we had physical samples for comparison, a number of meteorite fragments, here on Earth, were conclusively proven to have come from the moon --- rocks blasted into space lunar impact events, and then falling to Earth.
Later, meteorites were similarly proven to have come from Mars, and asteroids. It turns out to be a fairly common process: Non-Earth stuff gets scattered into space, and lands here all the time.
Now, the reverse process has been shown to exist:
So, the mechanism for panspermia exists; that part of the hypothesis is, er, rock solid, so to speak.
Of course, panspermia sidesteps the basic question of how life arose in the first place. It had to start somewhere, before it hitchhiked to other places on ejected rock fragments.
And panspermia doesn't address the idea that life may be an inevitable emergent property of the correct local conditions; not requiring lots of transport from place to place.
After all, we've only just started looking, and already have found something like 3,500 earthlike worlds in our immediate vicinity; and over 100 of those are in the habitable zone. Extrapolating the math suggests there are something like 40 billion earthlike worlds in our galaxy alone; one of countless hundreds of billions galaxies.
Given all that real estate, and the vastness of time, the idea that life arose only once, and only locally, seems kind of silly to me.
But still: Once life arises anywhere, it's looking entirely possible that it gets shuffled around, at least on an interplanetary scale, by catching rides on ejecta.
And that's kind of cool.
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