EVOLUTION OF THE LONGITUDINAL ARCH OF THE HUMAN FOOT

1. Comparative analysis of the human, anthropoid, and other primate feet clearly indicates that the foot of man was evolved from a grasping, arboreal type such as is peculiar to the modern great apes. Furthermore, the great ape or gorilloid type of foot presents an intermediate stage in structural and functional modification through which, of necessity, the evolutionary development of the human foot had to pass.

2. The results of this analysis conform with other extensive evidence pointing out the close relationship or man and the great ape group, and makes more convincing the correctness of Darwin's conclusion in placing man's origin in the ancestral great ape stem.

3. The erect posture is an extremely ancient character, developed by arboreal life, and it had been fully acquired by the ancient anthropoid group as its natural arboreal posture earlier than the time of the separation of the human offshoot. The erect posture was retained by the early human representative as the position of choice, from the very beginning of his terrestrial life, because the previous arboreal use of it had become firmly established by structural adaptations and modifications of his nervous system.

4. The human foot is a specialized organ for leverage in bi-pedal locomotion. Its characteristic longitudinal arch conforms in its external contour and in its trabecular architecture to the direction of the movements of force in such leverage action, thereby demonstrating the process by which its form was evolved. The elevation of the arch was greatly aided by the bowstring action of the flexor muscles against the sustentaculum tali, in raising the inner side of the os calcis.

5. The longitudinal arch is absolutely not a modification of the previous supinated posture of the arboreal foot, for had that posture been retained the outer border of the foot would have become hypertrophied as the weight-bearing portion, and the inner border would have become atrophied because it had been held in an unusable position.

6. The position of the line of leverage between the first and second metatarsal bones in the human foot is a distinct heritage from a previous grasping arboreal foot in which the movement of body-weight advanced between the two grasping portions, hallucial and outer digital. By such a previous grasping development the greater amount of function in weight-bearing was concentrated along the inner border of the foot and established conditions that were essential to the modification of this part to form the anterior pillar to the ultimate human arch.

7. The posterior pillar is the result of adaptation and hypertrophy of the os calcis for weight-bearing in association with the erect posture. It was entirely a terrestrial development.

8. Fixed adduction of the hallux in the developing human foot was acquired as lifting of the sustentaculum tali changed the abruptly inwardly inclined plane of the calcaneal facets (that characterizes the arboreal foot) to a plane which was more level and sloped more directly forward. The change in the position of the os calcis brought about the characteristic "balance" of the human foot and obviated the necessity of abduction of the hallux for inward lateral stability.

9. The position of the center of body pressure as it enters the human foot is not so important to foot balance as the degree of inward slope of the calcaneal facets which support the astragalus. The inward thrust of body-weight to overthrow the foot's normal balance is rapidly magnified with each degree of increased inward inclination of the plane of these facets.

10. Pronation, as a static disorder of the modern human foot, presents its particular danger in that the associated increased inward slope of the calcaneal facets cannot be counteracted upon by a divergence of the hallux as in the earlier developmental stages of the foot, because of the present fixed adduction of that member, and the added restricting action of shoes. Consequently, pronation creates a progressive vicious circle by which inward gravitation of body-weight depresses the inner border of the facets, which, in turn, by increased inclination magnifies the inward lateral thrust.

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