Résumé
The ability to walk is fundamental to human lives. Like all our biological features walking has a complex and deep history. It is most commonly thought that walking arose as fish made the evolutionary transition to land, shifting from an aquatic environment to a terrestrial one. In this view, the transition out of water meant that animals now had to evolve new mechanisms to deal with gravitational loads. As a consequence, they developed more mobile joints, arm and leg bones with robust connections for expanded locomotory muscles, and other structures to allow them to move about. Surprising, this very intuitive view is not supported either by comparative anatomy or the fossil record. The closest fish relatives to terrestrial vertebrates were capable of walking with four appendages, a fact seen in the structure of the bones and joints in their fins. Moreover, walking either on four appendages or two is commonly seen in aquatic fish ranging from sharks, lobe fin fishes, and diverse ray finned fishes. Indeed, many of these fish use alternating gaits and appendage motions in aquatic settings that are similar to terrestrial tetrapods. This observation leaves open the question of why fish walk in water instead of swimming. To answer these questions scientists have developed underwater treadmills, experiments training fish to walk, and robots that simulate walking behaviors. One major factor in the origin of fish walking, hence our own, is energetics: at slow speeds, and in certain environmental conditions is it energetically more efficient to walk than swim.
