The Fin to Limb Transition: New Data, Interpretations, and Hypotheses from Paleontology and Developmental Biology

After a brief historical review of the fin to limb transition and consideration of a theoretical “prototetrapod,” this article considers new ideas generated from recent fossil finds and from developmental biology that bear on the question of how limbs, digits, limb joints, and pentadactyly evolved. Among the first changes to take place were those to the humerus, in concert with those to the breathing apparatus, and these adaptations were acquired while the animals were still basically aquatic with the evolution of digits occurring during this phase. Studies from developmental biology of modern taxa can be integrated with information from fossils to produce a fuller picture. The acquisition of pentadactyly was among the last changes to occur in the modification of a fin into a limb. This vision differs radically from older theoretical ideas which perceived land locomotion as the prime evolutionary force driving the transition.