Recent studies have shown IL11 to be pro-fibrotic, pro-inflammatory and anti-regenerative in heart, liver, lung and kidney disease in mice and humans. However, data also show that IL11 is specifically required for appendage regeneration following trauma in some species. In fish, tadpoles and axolotl, IL11 is uniquely upregulated in the regenerative organ, the blastema, following loss of fin, tail or limb. In this short essay I suggest that the pathobiology of IL11 in mammals is rooted in its deep evolutionary role for epimorphic appendage regeneration. In both blastema formation and mammalian disease there is robust IL11-driven fibroblast activation, extracellular matrix production, inflammation and epithelial cell dedifferentiation. While these cellular processes are critical for regeneration in lower species they cause organ failure in mammals. This hypothesis, if correct, may explain the apparent redundancy of IL11 for human health and suggest IL11 as a therapeutic target.