There is this idea called panspermia, which says that life originated somewhere other than Earth, and was transported here by some means -- usually a comet or meteorite impact.
This isn't as silly as it sounds. For one, Mars is smaller than the Earth and farther from the Sun, so it cooled more rapidly and may have been more earthlike long before the Earth was. For another, complex molecules called amino acids, which are the building blocks of proteins, have been found in meteorites. Third, we know that chunks of other planets have made it here to Earth as meteorites.
The problems with this idea are legion. For one, two astronomers named Hoyle and Wickramasinghe who support panspermia have in the past made hysterical claims about it in the past, wielding it like a wildcard every time they can (when you claim bird flu and the 1918 flu come from space with absolutely no reason to think so, it's time to consider the source when new claims are made). For another, it's unknown how long a microbe, say, could survive in a chunk of rock or ice out in space. Research into this problem would be very interesting.
An article was just published on The Age's website saying comets cannot transport microbes to Earth, because cosmic rays -- subatomic particles in space moving at very nearly the speed of light -- would destroy them.
The problem is, the article is almost entirely content-free. The scientists found ancient microbes in some Antarctic ice and tested their genetic material for degradation. The article doesn't say how they went from this to saying microbes cannot survive in comets. It says the work has been published in PNAS, but I cannot find it. The article does quote other scientists who bring up legitimate objections, but this doesn't help if I can't find the (&*(*$*()^$ original article.
I'd love to say something scientific here about the study, but I can't. If anyone has any leads on this, I'd appreciate it.
Update: It figures. Within minutes of posting this, I coincidentally get an email from a list on I'm on pointing me to this article from the BBC, which clarifies the whole thing. The researchers revived old microbes and found the younger ones (100,000 years old) fared better than older ones (8 million years old). Radiation damage was the culprit. The ice they used was 3-5 meters below the surface, so it seems to me the claim that this is an objection to panspermia is hollow; comets can be very large, so microbes could be buried a mile deep in the cometary ice.