The End of an Old Dogma with Crucial Implications for Medical and Biology Students and for Medicine: regenerative, Developmental, Paleontological and Evolutionary Studies Contradict the Fore‐hindlimb Serial Homology

Abstract

Most evolutionary and medical textbooks state that the tetrapod fore and hindlimb are serial homologues. Here we show how regenerative studies of axolotls, developmental studies of tetrapods, and comparative and evolutionary studies of all major vertebrate groups, including recent re‐analyses of the appendicular muscles of chondrichthyans, dipnoans and coelacanths contradict this old dogma. The integrative analysis of the data available strongly supports the idea that the similarity of the muscles and bones of the fore and hindlimbs of tetrapods such as salamanders and modern humans is not due to serial homology, but is instead the result of independent evolutionary changes (homoplasy) occurred mainly during the origin of tetrapods due to the co‐option of similar genes for the development of both limbs. It also offers new insights about the ancestral Bauplan and morphogenetic gradients of tetrapod limbs, including the striking similarity of the zeugopodial (forearm/leg) and autopodial (hand/foot) muscles of the two limbs and ventro‐dorsal symmetry of the zeugopodial muscles of a same limb, about the marked differences between pelvic and pectoral girdle structures, the homeotic transformations related to the number/identity of the digits and associated soft tissue changes, and the differences between limb ontogeny and regeneration, and about limb birth defects and their crucial medical implications.