In the years since Miller's initial experiments, scientists have come to believe that the atmosphere of primitive Earth wasn't made up of the same gasses that he used in his initial study. But volcanic eruptions were likely very common during the early history of the planet and these volcanoes could have emitted clouds rich in hydrogen, methane and H2S, which would create conditions similar to Miller's experiments in limited geographic areas. The spark could have been supplied by lightning, which is commonly associated with volcanic clouds.'