As biologists know, nature can take its sweet time explaining itself. Andrew Gillis, associate scientist at the Marine Biological Laboratory (MBL), has been investigating how the paired fins of fishes evolved for nearly 20 years—ever since he was a Ph.D. student with Neil Shubin at the University of Chicago. A new study from Gillis and the MBL-UChicago Graduate Research Fellowship Program doesn't quite put the matter to rest, but it gives important insight to a broader question in evolutionary biology.