The origins, scaling and loss of tetrapod digits

Abstract

Many of the great morphologists of the nineteenth century marvelled at similarities between the limbs of diverse species, and Charles Darwin noted these homologies as significant supporting evidence for descent with modification from a common ancestor. Sir Richard Owen also took great care to highlight each of the elements of the forelimb and hindlimb in a multitude of species with focused attention on the homology between the hoof of the horse and the middle digit of man. The ensuing decades brought about a convergence of palaeontology, experimental embryology and molecular biology to lend further support to the homologies of tetrapod limbs and their developmental origins. However, for all that we now understand about the conserved mechanisms of limb development and the development of gross morphological disturbances, little of what is presented in the experimental or medical literature reflects the remarkable diversity resulting from the 450 million year experiment of natural selection. An understanding of conserved and divergent limb morphologies in this new age of genomics and genome engineering promises to reveal more of the developmental potential residing in all limbs and to unravel the mechanisms of evolutionary variation in limb size and shape. In this review, we present the current state of our rapidly advancing understanding of the evolutionary origin of hands and feet and highlight what is known about the mechanisms that shape diverse limbs.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Keywords: development; evolution; limb.

Figure 1.

(a) Phylogeny of vertebrates illustrating the origin of the tetrapod autopod (Acanthostega) since a shared ancestor with modern fish (represented by the zebrafish). Digit loss has evolved repeatedly in tetrapods (six cases illustrated here) via mechanisms that affect the pre-pattern of digits or post-pattern chondrogenesis. For detail of alternate models of digit loss in Aves, see [25,26]. (b) Extent of Ptch1 expression, duration of Shh expression, and pattern of interdigital cell death in the mouse as a model of pentadactyl limb development.