<p>by Caroline Schwaner, Tetsuya Nakamura</p> Dorsoventral polarity is a key feature of paired appendages, yet its evolutionary origin is enigmatic as median fins, which predate paired appendages, are symmetric. A new PLOS Biology study shows lmx1b is dorsal in paired fin mesenchyme but posterior in median fins, implying genetic rewiring of an ancient toolkit. Dorsoventral polarity is a key feature of paired appendages, yet its evolutionary origin is enigmatic as median fins, which predate paired appendages, are symmetric. A new study shows that lmx1b is dorsal in paired fin mesenchyme but posterior in median fins, implying genetic rewiring of an ancient toolkit.