A failry easy method would be to make one circle the edge of which matches the shape of one of the protrusions, filling a piece of the inside, then copy it around the shape edge, finally combine the circles into one shape with pathfinder and deleted the remaining inner shape.
You can place the original photo into the illustrator file on a layer below the illustration and set the "fade placed images to 50%" checkbox on. Then you can use that as a reference to match the placement exactly. There are some clear differences here. For example you have 34 "bumps" on the edge while the original has 28 of them.