Most mammals have five fingers due to their shared evolutionary ancestry with early tetrapods, a group of vertebrates that first evolved from fish around 360 million years ago. These early tetrapods had varying numbers of digits, but the five-finger configuration became the standard and was retained through evolution.
This five-finger pattern is largely controlled by Hox genes, which regulate limb development during embryonic growth, and the Sonic hedgehog protein, which coordinates the formation of digits. Even animals without traditional limbs, like whales and seals, retain the skeletal remnants of five fingers, though these may be less visible due to adaptations to their environments.
Over time, this trait became stable due to a phenomenon called canalization, which reduces the likelihood of mutations affecting this feature, despite occasional genetic variations like polydactyly (extra fingers). Evolutionary pressures and genetic linkages have helped maintain the five-finger pattern across mammals.
