Limb loss occurred concurrent with various other morphological and physiological changes associated with transition from land to water. Hind limbs likely regressed because their function was co-opted by the undulatory vertical axial locomotion made possible by the newly evolved caudal flukes. The continued existence of external hind limbs would have posed a handicap in a newly acquired swimming mode. We discuss developmental processes that may have driven limb loss in snakes, compare this process to the gradual loss of hind limbs in whales, and put forth the most likely scenario for the loss of limbs in whales. Limblessness in many snakes is associated with adoption of a burrowing life style and was driven by changes in development associated with elongation of the body.
Whale embryos form both fore- and hind limb buds.
Different processes of limb development and different patterns of Hox gene expression indicate that limb loss in whales and snakes may have involved different developmental mechanisms.