I have Roodaka being able to mutate beings, so it makes sense for her to be able to do it. Since Roodaka is the only one able to mutate pre-existing living beings, she's the only possible candidate for developing the Emanates. In addition, it's possible she could be introducing outside materials to aid in mutations. And if she has a clear understanding of biology, it should be very easy for her to accomplish advanced mutations.
All the Precursors do is reproduce asexually (via seed pods and whatnot) and have those offspring develop independent personalities. Technically speaking, all Precursors are fragments of Annona that have become independent.
The Brothers are responsible for creating entirely new Kal, not specifically Emanates. I don't want to give all the credit for new beings to the Brothers, since it limits the number of explanations for origins to 1. Plus, it helps the "Family Tree" grow taller instead of wider.