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Since the brain knows what it is ‘commanding,’ it is significant that the area of the brain devoted to the thumb is much larger than that allocated to a finger. Recognizing this, Professor Wood Jones stressed, ‘The human hand is not a marvel of perfection, but the human brain can command it and control it so that its functional role is immeasurably greater than that of even the most intelligent anthropoid ape’ ( 5). Scientific American 1962 207:56–62.Ĭoncomitant with the development of our hand has been an increase in the size of the brain of our early ancestors. Reprinted with permission from the estate of the artist, Tom Prentiss, from Napier J. (bottom) The thumb lengthens, and the angle between index and thumb increases. (middle) The thumb's terminal phalanx increases in size. (top, left to right) The phalanges decrease in curvature.
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Hand bones of juvenile gorilla, Olduvai hominid, and modern man. They are rather short, and the thumb is fully opposable. The famous ‘Lucy’ is ap-proximately 3.5 million years old, and her hand does not have relatively long apelike fingers. Over time, the finger bones have straightened the curvature that was used to grasp tree branches, and the thumb has lengthened and enlarged the size and breadth of its terminal phalanx. An illustration in John Napier's classic Scientific American article ‘The evolution of the hand’ compares the hand bones of a juvenile gorilla, an Olduvai hominid, and modern man ( 4) (Figure (Figure1 1). The hominids living at the end of the Miocene about 15 million years ago began to develop bipedal locomotion, liberating their hands for independent use. This problem is compounded by the not infrequent occurrence of a triphalangeal thumb, in which the metacarpal has a distal and a proximal epiphysis-an oddity that adds the teratologists to the legions of puzzled anatomists.
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Napier settled this discussion when he wrote, ‘Indeed it is questionable whether it has ever possessed more than 2 phalanges, a conclusion that will no doubt infuriate the legions of anatomists who collectively have expended much time and energy trying to determine whether the missing element of the thumb is a phalanx or a metacarpal’ ( 3). The fact that the thumb has only 2 phalanges has bothered anatomists for many centuries. By contrast, the growth center for all finger metacarpals is at their head, while none is present in the thumb metacarpal head. Certainly, the position of the growth center, or epiphysis, at the base of the metacarpal matches the position of the epiphysis of all the phalanges. Vesalius in 1543 agreed, saying that the thumb metacarpal was really a phalanx. Galen (AD 165) thought the bones of the thumb were really 3 phalanges and that it lacked a metacarpal. It is known that almost without exception, the first ray of mammals, reptiles and amphibians, and humans had 2 phalanges. Its pectoral fin is the precursor of our hand, which has 4 fingers, each with 3 phalanges, and a thumb with only 2 phalanges. Our earliest known ancestor is the fish Rhipidistia ( 2), which has been extinct for 230 million years.