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Extraterrestrial bacteria, although not intelligent life, are still (unconfirmed) aliens.
First discovered in 1996 by NASA scientists on a Martian meteorite, many of them began to think that they had proved that aliens existed, even if it was just unicellular and unintelligent. However, at the rate bacteria grows and the age that the scientists found it, it may have been contaminated upon contact with Earth.
Lately, there have been discoveries of bateria living in extreme environments on Earth, such as in glaciers, so this might nod to the fact that there could actually be living things on Mars.