I would say that given the number of stars/planets that we know of there is no doubt that some sort of life exists elsewhere in the universe. If there is a planet in a habitable zone with conditions similar to Earth, given enough time a carbon-based lifeform will emerge. It all comes down to the rate of change (how fast new carbon-based molecules can be created) and the age of the planet in that zone IMO. Since we never observed another type of life form (e.g. silicon-based) I would say that carbon-based lifeforms are most probable in Earth-like conditions. Given enough time anything is possible.