The evolution of initiation of limb development in tetrapods is best explained by the alteration of a gene to add a new function.
This syllogism is consistent with the behavior and abilities of fish and amphibians as well as with general zoological and paleontological knowledge. It implies that the tetrapod limb originated as a result of frequent, ongoing activities and stresses—escape from predators and obtaining food.
Since all tetrapods shared a common ancestor with limbs that had five digits, they all descended from one individual. Then, over the eons of evolution that followed, natural selection focused on pentadactyly variations rather than beginning from scratch to create tetrapods with another number of digits, whether it two, seven, or 17.
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