According to WikiPedia, "Yeasts are eukaryotic single-celled microorganisms classified as members of the fungus kingdom."
Yeast is, today, the most commonly used way to raise bread. Sourdough starters have wild yeast in them.
Reliable commercial bakers yeast became available in the mid to late 1800's. Yeast is grown in large vats filled with a molasses base. The yeast eat the sugars and multiply. When there are enough yeast in the vat, the yeast is harvested.
The liquid is concentrated a little bit, put in railroad tank cars and sent to large bakeries.
The liquid is dried a bit more until it forms a springy moist cake. This is sold as fresh, or compressed, yeast. Many bakers feel it is the only yeast to use. However, it is very perishable and has a short shelf life. You can often find it in grocery stores dairy section. You may have to ask the store staff where it is.
The compressed yeast is dried a bit more and becomes active dry yeast. Active dry yeast has a cell of live yeast surrounded by a number of dead cells of yeast. Active dry yeast won't begin to work until the dead yeast dissolve off the live yeast. The dead yeast have a minor dough conditioning effect. Active dry yeast is usually proved before use by making a sponge with it.
In the past few decades a new drying technology has been introduced that produces instant dry yeast. With instant dry yeast the powder is live yeast cells with no coating of dead cells. The yeast is ready to go to work with no sponge needed. It starts faster, but the overall rise time is not impacted very much. Since the live yeast is not protected by a layer of dead yeast, it is more easy to kill it. Water temperatures below 40F(4C) will kill the yeast, as will water temperatures above around 95F(35C). The way to handle this is to mix instant dry yeast with the flour and other dry ingredients in a recipe. This will protect the yeast from the temperature extremes.
At this time, no GMO bakers yeast is sold in the USA. The bakers yeast was naturally selected for performance considerations. You can make very good bread with yeast, and it isn't "unnatural".