Hi Atlas, you're totally correct that a healthy, active and ripe starter can be used to raise your bread without additional yeast. However, in a recipe like this, which allows for the use of less-active sourdough discard, the boost that yeast gives makes the rise more reliable within the designated time frame. And some recipes call for both sourdough starter and yeast because this type of "hybrid" recipe offers a unique texture and flavor profile as compared to an entirely naturally leavened recipe. 

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