This first lesson defines the general content and objectives of the lecture. It will deal with one of the most important morphological transitions in vertebrates, that which led to the appearance of the archetypal (chiridial) tetrapod limb (our arms and legs) from an ancestral fin-like structure (pterygian limb). This age-old question can now be revisited, thanks to the latest tools of molecular genetics and genomics, to easier access (thanks to technology) to the study of non-model species that are informative thanks to their phylogenetic positions, and also to the evolution of the conceptual tools used in this field. The aim is to take stock of the various theories and conclusions, and to attempt to reconstruct this critical evolutionary stage, which undoubtedly contributed to the colonization of the terrestrial environment.
The lecture begins with a reminder of the importance of this issue and the need for a multidisciplinary approach. Next, this evolutionary transition is placed in a paleontological context (the Upper Devonian) and the phylogeny of the animals involved in this transformation is discussed. The essential fossils are briefly presented, which leads directly to the fundamental question of "homologies", both between the bony limb parts of sarcopterygian fish, and between these and the bones that make up the chiridial limb of tetrapods.
