Yes, yeast occurs naturally in many environments. Yeast is a type of fungus that can be found in the air, soil, and on the surface of plants and fruits. It can also be found in the digestive tracts of animals, including humans.
In baking and brewing, yeast is commonly used as a leavening agent to help dough rise and to ferment sugars into alcohol. Yeast is also used in the production of many other food products, including cheese, yogurt, and vinegar.
So, while yeast can also be commercially produced, it is a naturally occurring organism that has been used for thousands of years by humans in various applications.