Origin of Vertebrate Limbs

This article presents an original hypothesis of the origin of vertebrate limbs. According to this hypothesis, the tentacles and limbs of Bilateria originate from two circles of tentacles of the coelenterate ancestor, the labial circle and marginal circle. In Bilateria, the tentacles of the coelenterate ancestor gave rise to the metameric processes of mesodermal somites. In vertebrates, fin radials are established as metameric processes of somites. Taking account that chordates (including vertebrates) are inverted relative to other bilaterians, the homologs of labial tentacles are radials of unpaired median fin fold and the homologs of marginal tentacles are radials of paired lateral fin folds. Current data on Hox gene expression and paleontological finds indicating the homology of the digits of the Tetrapoda limb to the fin radials of cartilaginous fishes and lobe-finned fishes are discussed. This makes it possible to stretch the thread of homology from the tentacles of coelenterates to the digits of vertebrates.