Evolution of Vertebrate Limb Development

Abstract The origin and diversification of fins and limbs have long been a focus of interest to both palaeontologists and developmental biologists. Studies conducted in recent decades have resulted in enormous progress in the understanding of the genetic and developmental bases of the evolution of paired appendages in vertebrates. These discoveries in the areas of genetics and developmental biology have shed light on the mechanisms underlying the evolution of this key morphological innovation in vertebrates. In this article, I discuss recent advances in these fields and how they can provide a mechanistic explanation for the origin and evolution of paired appendages. Key Concepts According to the fossil record, single pair of fin‐like structures emerged in the bodies of certain ancestral jawless vertebrates, and two pairs of fins are unique to jawed vertebrates. There are two separate phases/waves of Hoxd gene expression in tetrapod limbs. The first wave precedes the formation of the proximal parts of the limb, whereas the second wave corresponds to the most distal part of the limb (digits). The earliest known amphibian fossils,AcanthostegaandIchthyostega, seem to have had more than five digits in their limbs. It has been proposed that regulatory changes in Hox and/or Shh expression modified the digit number during evolution. Regulatory modifications of specific gene expression appear to account for the evolution of paired appendages, such as the increasing length of bat wings, the loss of limbs in pythons, and pelvic reduction in sticklebacks.