A bacteria with an vary slow metabolism have been found 100 feet under the the sea floor of the Pacific Ocean. It was found in a location is isolated from both sunlight, and fresh nutrients. Their metabolism is so slow that if they reproduce at all its on the order of once every thousand years or so. This has lead to speculation the these bacteria could be millions of years old but that is pure speculation. What these bacteria really tell us is far more interesting.
The ability of these bacteria to survive in such low nutrient conditions show that bacteria could also survive in material ejected from a planet into space. One possible way this can occur is an asteroid impact however it has been shown that bacteria are constantly being ejected into space on dust particles. Regardless of how they get into space these space-borne bacteria could later get deposited, as stowaways on another planet. This could occur by them being carried on a meteorite or if the are being carried on dust particles they would have an even better chance of surviving reentry and landing since it would be a gentler process.
This discovery supports the idea that if we we to find life any place else in the Solar System that it most likely originated on Earth and got transplanted by way of the other processes. The reason why it is most reasonable to suspect the Earth as the origin is that the Earth is covered with life all over the place where if life exists on any other planet or moon in the Solar System it would be space at best.