The pentadactyl limb has 5 digits on the hand and foot. It also has a specific pattern of bones.
The structure of a human arm includes a bone between the shoulder and the elbow called the humerus. Below the elbow are 2 other bones, the radius and the ulna, followed by a set of wrist bones and then the 5-digit fingers and toes. This is an example of a pentadactyl limb.
The pentadactyl limb is common to humans, other mammals (although whales and dolphins have lost their hind limbs), birds, dinosaurs, and other reptiles and amphibians.
Darwin noted how widespread it was when he wrote On the Origin of Species:
'What could be more curious than that the hand of man formed for grasping, that of a mole, for digging, the leg of a horse, the paddle of a porpoise and the wing of a bat, should all be constructed on the same pattern and should include similar bones and in the same relative positions?'
There are a few mammals that appear to have different numbers of digits from us. One of these is the panda, which looks like it has 6 digits.
However, close examination has shown that the extra thumb is actually an outgrowth of one of the wrist bones and not an extra digit at all. The same is true of moles.
The pentadactyl limb is common to most tetrapods (4-limbed creatures). It is evidence of humans' common ancestry with amphibians, reptiles and other mammals.
However, the basic pattern has been modified in different groups. For example, frogs have 4 fingers and birds have only 3 fingers in their wing skeleton.
Among extinct tetrapods, many dinosaurs had only 3 toes, and some marine reptiles had more than 5 digits in their paddles. But no animals living today have more than 5 digits that all developed in the same way.
The fossil record shows evidence of a few extinct species that had different numbers of digits. Examples are Acanthostega, Ichthyostega, and Tulerpeton.
These are all animals that lived during the late Devonian period, between 380 and 360 million years ago, when tetrapods first began to move onto land. Some had as many as 8 digits, before the common pattern of 5 digits became established that we have inherited.