Evolutionary pathway and genetic mechanisms of fin-to-limb transition in vertebrates

The transformation of paired appendage structure from aquatic fins to terrestrial limbs represents a pivotal event in vertebrate evolution, underpinning the colonization of land by early tetrapods. This transition involved profound morphological and genetic modifications, particularly in the distal limb region known as the autopod and in the dorsoventral plane of paired appendages. Recent advances in paleontology, comparative and functional genomics, as well as evo-devo studies have shed light on several key events and evolutionary pathways and have improved our understanding of the direction of changes in regulatory mechanisms underlying the fin-to-limb transition. In this review, we aim to summarize current knowledge on limb evolution, with particular emphasis on studies of phylogenetically pivotal vertebrate groups - cartilaginous fishes and chondrosteans, which represent basally diverging evolutionary lineages of extant vertebrates, as well as sarcopterygians, the group of lobe-finned fishes most closely related to tetrapods. We consider the principal hypotheses concerning the prerequisites for vertebrate terrestrialization, key aspects in the search for structural homology between the morphological elements of fins and limbs, as well as the genetic mechanisms of spatial limb bud development described to date and the possible modifications of these mechanisms associated with the transformation of ancestral fins into pentadactyl terrestrial limbs.