A joint research group from Kyushu University, Kumamoto University, and Hiroshima University has discovered the mechanism behind the diverse sternum shapes in birds. By comparing chicken (flying) and emu (flightless) embryos, they found that the heterochronic activation of TGF-β signaling determines the presence of the keel—a ridge on the sternum for flight muscle attachment. In flying birds, prolonged TGF-β activation promotes cartilage progenitor cell proliferation, forming the keel. In flightless birds, the signal weakens early, resulting in a flat sternum. This discovery, published in Nature Communications, sheds light on skeletal evolution and could help understand human thoracic deformities.