Limbs and tail as evolutionarily diverging duplicates of the main body axis

Contrasting hypotheses have been proposed to explain the pervasive parallel s in the patterning of arthropod and vertebrate appendages. These hypothese s either call for a common ancestor already provided with patterned appenda ges or body outgrowths, or for the recruitment in limb patterning of single genes or genetic cassettes originally used for purposes other than axis pa tterning. I suggest instead that body appendages such as arthropod and vert ebrate limbs and chordate tails are evolutionarily divergent duplicates (pa ramorphs) of the main body axis, that is, its duplicates, albeit devoid of endodermal component. Thus, vertebrate limbs and arthropod limbs are not hi storical homologs, but homoplastic features only transitively related to re al historical homologs. Thus, the main body axis and the axis of the append ages have distinct but not independent evolutionary histories and may be in volved in processes of homeotic cc-option producing effects of morphologica l assimilation. For instance, chordate segmentation may have originated in the posterior appendage (tail) and subsequently extended to the trunk.