The appearance of feathers defines the appearance of birds. A number of changes defined, preceded or accompanied the event. The changes were hierarchical in nature and included revolutions in genomic organization (i.e., HOX and the feather keratin genes), protein sequence and shape, the large scale organization of proteins into filaments, and in the geometry of the cells and their roles in the follicle. Changes at each of these levels differ or produced different products than found in its analog in reptiles. They are essentially unique to birds and produced an evolutionary novelty. I used analysis of extant structure and information on development to reconstruct key events in the evolution of feathers. The ancestral reptilian epidermal structure, while probably a scale or tubercles, is still unidentified. The structural genes of feather proteins (φ-keratin) are tandem repeats probably assembled from pre-existing exons. They are unlike the alpha-keratin of vertebrate soft epidermis. Amino-acid composition, shape, and behavior of feather keratins are unique among vertebrates. The 3-dimensional organization of the follicle and the developmental processes are also unique.
Although we lack a complete understanding of the appearance and early role of feathers, they are clearly the results of novel events.