The forelimbs have evolved from the pectoral fins of fishes. In tetrapods (terrestrial vertebrates), all the four limbs are used for supporting body weight, and for locomotion. In arboreal (tree-dwelling) human ancestors, the forelimbs have been set free from their weight-bearing function.
The forelimbs, thus ’emancipated’, acquired a wide range of mobility and were used for apprehension or grasping, feeling, picking, holding, sorting, breaking, fighting, etc. These functions became possible only after necessary structural modifications such as the following.
(a) Appearance of joints permitting rotatory movements of the forearms (described as supination and pronation), as a result of which food could be picked up and taken to the mouth.
(b) Addition of the clavicle, which has evolved with the function of prehension.
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(c) Rotation of the thumb through 90 degrees, so that it can be opposed to other digits for grasping.
(d) Appropriate changes for free mobility of the fingers and hand.
The primitive pentadactyl limb of amphibians, terminating in five digits, has persisted through evolution and is seen in man. In some other species, however, the limbs are altogether lost, as in snakes; while in others the digits are reduced in number as in ungulates.
The habit of brachiation, i.e. suspending the body by the arms, in anthropoid apes resulted in disproportionate lengthening of the forearms, and also in elongation of the palm and fingers.