In a 1996 study of the Allan Hills 84001 Meteorite, strongly believed to be of Martian origin, NASA scientists announced the finding of "microfossils" indicating the presence of Martian bacteria. This was controversial, and scientists generally rejected it on the grounds the structures could have been formed naturally. (Not surprisingly, when you claim you have discovered Martian life, the bar for evidence is set pretty high.) A new analysis using high resolution electron microscopy has concluded the microfossils could not have been formed naturally. One of the team members said, "We feel vindicated. We’ve shown the alternate explanation is absolutely incorrect, leading us back to our original position that these structures are formed by bacteria on Mars."
COMMENT: As one cautious scientist observed, results from a single meteorite, however impressive, won't put a question as complex and monumental as Martian life to rest. The next step is to replicate the findings - either from other meteorites or on Mars.