Panspermia is the idea that life on Earth originated in space and was seeded here by some event. This covers a lot ofground sky: comets, Mars, Venus, aliens, and so on.
The idea isn't as far-fetched as it sounds. It's more medium-fetched. Mars is smaller and farther from the Sun, so therefore it cooled faster than Earth did after the period of heavy asteroid and comet bombardment a billion or so years after the planets formed. It may have had oceans and better conditions for the start of basic life before the Earth had cooled enough.
But if life started up first on Mars, how did it get here?
Asteroid impacts. The idea is that a smallish asteroid could have hit Mars and launched quite a bit of Martian territory into space. Eventually this could hit Earth. We know this can happen; we have samples of meteorites that are clearly form Mars; the isotope ratios of the chemicals in the meteorites matches what we know of Mars's atmosphere. Heck, I used to own a small Mars meteorite myself, until it fell out of my bag and I lost it, arrrrggggg!
Anyway, if some of that Martian ground had bugs in it of the protozoan kind, then they could make it to Earth.
Of course, there are hazards. They're in space a long time, so they have to survive that. They also have to survive the fall to Earth. It's not clear they could live through either event. And before that, they have to survive the enormous pressure of being smacked by an asteroid impact.
But a new paper that just came out in the peer-reviewed journal Astrobiology says that some bacteria could, in fact, survive the initial launch event. Amazingly, the enormous pressure generated in an asteroid impact on the surface of Mars may be survivable, if you're really really tiny.
The researcher made models of the Martian ground seeded with bacteria, then subjected these samples to the pressures expected in an impact event. Amazingly, many of the bacteria survived. Lichens and bacteriospores did the best, surviving pressures from 5 - 40 billion Pascals, which is about 50,000 to 400,000 atmospheric pressures. That's a lot. Cyanobacteria were the wussies of the lot, only surviving up to 100,000 atmospheric pressures.
Mind you, a human would be less than a greasy smear at that kind of pressure.
Anyway, this is pretty interesting stuff. It doesn't say that the buggers could survive the trip here (millions of years) and the entry into our atmosphere, but there are scenarios where those are possible.
On a personal note, I think panspermia is interesting and worth investigating, but some people think it's the panacea to everything. Notably an astronomer named Chandra Wickramasinghe, who has made the fun claims that interstellar dust is actually made of clouds of E. coli, and that the flu is really a virus from space.
Still, when the study stays scientific, it's worth a look. I have seen no evidence at all that we actually did start on Mars or a comet or Somewhere Else, but it's still possible such evidence will turn up. When we get to Mars, for example, what if we find DNA-based life? It's much easier to get a rock from Mars to Earth than vice-versa (due to orbital and gravitational mechanics), so a find like that would be pretty conclusive. Until then, we have to do what we can to figure out what it was like for such interplanetary interlopers each step of the way. And the first step appears to be viable.
Oh-- I cover some of this in my upcoming book, too. After all, if bugs might have made it here three billion years ago, they might make it today. Can we get wiped out by alien bacteria? Well, no, but read the book anyway.
Tip o' the Whipple Shield to SpaceRef.