It germinated from the spores of microbes that were floating around in the air.
Airborne microbes are everywhere.
But since we cannot see them, people do not realize that they exist. This was because invisible microbes floating in the air would settle down on food and then multiply and turn previously dead matter to life. So far, no known microbe lives its entire life cycle in the air. At some point they must come down to surface where there is water and nutrients to complete the life cycle. Some scientists speculate, though there is no proof, that certain microbes may grow inside of clouds, where there is enough water and some nutrients to support growth.