It is a consequence of the instructions dictated by the strict Hox genes, which also control the thickness and the separation between the fingers of the hand and the feet. This region of DNA contains specifications for building only five different types of appendages .
In fact, modern tetrapods – creatures with four legs – and their extinct ancestors count that number at most . Even the panda's thumb is not a true sixth finger, but is a modification of a bone in the wrist , the so-called radial sesamoid, which grew and lengthened to form the protrusion with which it strips the bamboo stalks.
This anatomical restriction appeared in vertebrates approximately 340 million years ago, in the Lower Carboniferous period, in parallel with the development of ankles and wrists with more sophisticated and complex joints. Before, there were terrestrial animals with six, seven and even eight toes , but their limbs were possibly less biomechanically adapted to walking and the evolutionary experiments ended.
