The evolutionary origin and diversification of feathers.

Abstract in brief

• The origin and diversification of feathers are better understood thanks to the developmental theory, which suggests feathers evolved through changes in developmental mechanisms of the follicle and feather germ, supported by the discovery of primitive feather fossils in nonavian theropod dinosaurs.
• These fossils show that feathers evolved and diversified in nonavian theropods before birds and flight, with primitive feather structures aligning with the predictions of the developmental theory, and the idea that feathers originated from functions like aerodynamics being proven false.
• Feathers are complex structures formed through evolutionary novelties involving duplication, hierarchy, and modular interaction of morphological components, and their evolution was driven by natural selection for a follicle capable of producing tubular structures, with weak support for the homology between feathers and scales.]