THE EVOLUTION OF THE HUMAN FOOT, WITHESPECIAL REFERENCE TO THE JOINTSBy HERBERT ELFTMAN AND JOHN MANTER

Columbia University

THE current divergence of views concerning the evolution of the human footis due largely to a lack of accurate knowledge concerning the functioning of theape foot, and a concentration of attention on morphological features lessintimately concerned with the action of the foot than arc the joints. By meansof an apparatus for studying the instantaneous distribution of pressure in thesole of the foot, we have been able to make an accurate comparison between themethod of function of the chimpanzee foot and that of man (Elftman andManter, 1935). In thelpresent paper we shall consider especially the jointsof the chimpanzee foot in comparison with Man, with regard to their bearingon the evolution of the human foot.

TIlE TRANSVERSE TARSAL JOINT AND THELONGITUDINAL ARCHI

The transverse tarsal joint (mid-tarsal, Chopart's) lies between the cal-caneus and talus on one side and the cuboid and navicular on the other(figs. 1, 3). Movement in this joint allows the forepart of the foot, which isquite rigidly attached to the navicular and cuboid, to move with respect to thecalcaneus and talus about an axis which is shown in fig. 2. These movementswe shall refer to as plantar-flexion and dorsi-flcxion about the transversetarsal joint. Since the axis of the joint in the normal position of the humanfoot is inclined at a considerable angle to the horizontal, plantar-flexion withthe calcaneus held imnmovable results in abduction of the forepart of the foot.Dorsi-flexion results in abduction.

When movement takes place in the transverse tarsal joint with the foot onthe ground, the tuber calcis and the anterior part of the foot must remain incontact with the substratum. Under these circumstances dorsi-flexion aboutthe transverse tarsal joint results, not in abduction of the forepart of the foot,but in a lowering of the joint and a flattening of the foot.

In the chimpanzee the transverse tarsal joint is freely movable, while in thehuman foot with a well-developed longitudinal arch the joint is relativelyimmobile. The human foot with its longitudinal arch (fig. 1B) displays thesame relationships between the forepart of the foot and the calcaneus and talusthat we find in the chimpanzee foot when it is plantar-flexed about the trans-verse tarsal joint (fig. 1A). We are justified in concluding that in the humanfoot the transverse tarsal joint has become fixed in a plh.ntar-flexed position.

The Evolution of the Human Foot 57Additional evidence for this concept is to be found in the prevalence of

mobility in the transverse tarsal joint in flat-footed humans. It seems likely thatmobility in the transverse tarsal joint, rather than being a result of flat-footed-ness, is more often a cause.

T T JOINT, ,w

A

Fig. 1. Chimpanzee and human feet in medial view. A, chimpanzee foot in an arboreal position;plantar-flexion about the transverse tarsal joint. B, human foot on the ground. C, chim-panzee foot on the ground: pronation with dorsi-flexion about the transverse tarsal joint.T.T. transverse tarsal joint.

The evolution of the longitudinal arch in Man is consequently due to animmobilization of the transverse tarsal joint in a plantar-flexed position. Toprovide a more complete explanation involves the determination of the

58 Herbert Elftman and John Mantermechanism of immobilization. This cannot be done at present. It might be dueto a shortening of such connective structures as the plantar aponeurosis, thelong plantar ligament and the plantar calcanco-navicular ligament. It couldalso be accounted for by changes in muscle action. It will be impossible todecide this issue until we are able to settle by experiment the question as towhether ligaments or muscles determine the limits of movement in the joints.

The position of the axis of the transverse tarsal joint which we have deter-mined differs from the position given by Fick (1911). There can be no doubtconcerning the position of the axis in the chimpanzee. Since our determinationson mobile and juvenile human feet agree with the results in the chimpanzee,we feel that Fick must have been misled by movements in other parts of thefoot than the transverse tarsal joint.

A B C

T 2

U~~~~~

Fig. 2. Chimpanzee and human feet in dorsal view. A, chimpanzee foot on the ground. B, humanfoot on the ground. C, chimpanzee foot plantar-flexed about the transverse tarsal joint, as infig. 1 A. T. axis of transverse tarsal joint. U. axis of upper ankle joint. L. axis of lower anklejoint.The importance of plantar-flexion or inversion about the transverse tarsal

joint as one of the factors in the evolution of the longitudinal arch has alreadybeen stressed by Keith (1923, 1929). The extension of his theory so as toassign prime importance to the tibialis anterior as an invertor of the foot has,however, been rendered doubtful by the studies of Fick (1911, 1931) and byour own determination of the position of the axis of the transverse tarsaljoint. The tibialis anterior may well be a postural muscle, intimately correlatedwith the evolution of the upright posture of man, due to its action about theupper ankle joint.

Attempts have been made to explain the evolution of the longitudinal archon the basis of changes in the position of the talus. It is true that plantar-

The Evolution of the Human Foot 59flexion in the transverse tarsal joint, with the heel and anterior support of thefoot in contact with the ground, does tend to bring the talar head moredirectly above the calcaneus, especially since this plantar-flexion is usuallyassociated with inversion about the lower ankle joint. But the change inposition of the talar head is a consequence, not a cause, of longitudinal archproduction. The arch is a subtalar structure, depending on the relationship ofthe calcaneus to the anterior part of the foot.

Weidenreich (1921) recognises that plantar-flexion about the transversetarsal joint has occurred in the evolution of the longitudinal arch, but considersthis as being subsidiary and subsequent to the upward inclination of the cal-caneus. It seems obvious to us that the upward inclination of the calcaneus isa necessary concomitant of fixation of the transverse tarsal joint in the plantar-flexed position and cannot be considered as a cause of the plantar-flexion.Certainly Weidenreich's argument that in Man the weight of the body isconcentrated on the calcaneus and thus is responsible for its tilt is not sub-stantiated by our researches on the path of the resultant of pressure in thefoot (Elftman and Manter, 1935). In the chimpanzee the resultant lingersfor a longer time in the tarsal region than it does in Man, but the chimpanzeegives no evidence of a calcaneal tilt.

Recognition of the evolution of the longitudinal arch as having been due toplantar-flexion about the transverse tarsal joint serves also to explain the factthat in the human foot the metatarsals are in line with the long axis of thecalcaneus, while in the ape foot, as seen resting on the ground (fig. 2A), themetatarsals diverge laterally from the long axis of the calcaneus. If the chim-panzee foot is plantar-flexed about the transverse tarsal joint, with the heeland anterior support of the foot still in contact with the ground (fig. 2 C), it willbe found that the metatarsals in their new position are parallel to the long axisof the calcaneus.

THE LOWER ANKLE JOINTIn the ankle there are two joints about which movement is possible. In the

upper ankle joint (talo-crural), the tibia can rotate backwards or forwards overthe trochlea of the talus. It is the lower ankle joint (subtalar), however, towhich we shall first direct our attention. In this joint the articulation isbetween the'talus on one side and the calcaneus, cuboid and navicular on theother. This gives rise, according to the usual description, to the possibility ofgliding movements in the joint. A more illuminating concept is that of Donitz,modified by Fick (1911), who demonstrated that, in spite of the bewilderingcongeries of articular surfaces involved, the movement in the lower ankle jointcan be quite accurately described as rotation of the entire subtalar portion ofthe foot about a line, the compromise axis of the lower ankle joint. The positionof this axis is shown in figs. 2, 3, 4 and 5. When looking at the right foot fromin front, a clockwise rotation of the foot about this axis would give rise toeversion, a counter-clockwise rotation to inversion.

It is possible to tell whether a foot is being held in an inverted or everted

60 Herbert Elftman and John Manterposition by noting the relations of the talus to the calcaneus, since the jointsurface passes between these bones. The talus and calcaneus of the chimpanzeeare represented in fig. 3A in the relative positions they occupy when on theground, but with the calcaneus tilted so as to make comparison with the otherfigures easier. Fig. 3C shows these bones in the positions they occupy in thearboreal position of fig. 1 A. It is obvious that when on the ground the chim-panzee foot is everted about the lower ankle joint, while in the arboreal positionillustrated a moderate degree of inversion is present. If the human relation-ships (fig. 3B) are now compared with the two figures of the chimpanzee, it isapparent that the usual human position in standing resembles that of thechimpanzee when moderately inverted.

A B C

Fig. 3. Anterior view of the talus and calcaneus, with the plane determined by the axes of thetransverse tarsal and lower ankle joints perpendicular to the plane of the figure. A, chim-panzee, showing the relationships of the bones in the terrestrial position of fig. I C, but withthe whole figure rotated outward so as to bring the calcaneus into a position congruent withthat of fig. 3C. B. human. C, chimpanzee, plantar-flexedl about the transverse tarsal jointwith accompanying inversion about the lower ankle joint, as in fig. 1A. U. axis of upper anklejoint. T., L. axes of transverse tarsal and lower ankle joint. A.L.P. antero-lateral process, itsextent roughly indicated by the dotted line.

The normal human foot can never achieve as highly everted a position asthat shown to be the common terrestrial position of the chimpanzee. One ofthe reasons for this is found by an inspection Of the anterior portion of thecalcaneus of Man. Both in the anterior and lateral views of fig. 5, and infig. 3 B. there can be seen Pa bony process, which we shall call the antero-lateralprocess of, the calc-aneus, which prevents extensive movements of eversion ofthe talus with respect to the calcaneus. No indication of this process can befound in the calcaneus of either chimpanzee or gorilla. It furnishes one of themost easily recognizable diagnostic cllaraclteristics of the human type ofcalcaneuls.

RELATIONSHIP OF THE AX~ES OF TIDE TRANSVERSE-TARSAL ANC LOWER ANKLE JOINTS

The articulation between the talus and the navicular presents a peculiarproblem, since movement in this articulation may take place either about theaxis of tigelower ankle joint, abouttcp e axis of the transverse tarsal joint, or

The Evolution of the Human Foot 61about both axes simultaneously. This would not be possible if the articularsurface were not essentially spherical, with the two axes mentioned intersectingat the centre of curvature of the sphere. Since the two axes do intersect at thiscentre of curvature, free movement can take place simultaneously in the lowerankle joint and in the transverse tarsal joint.

The extent of the articular surface on the head of the talus is intimatelycorrelated with the degree of movement possible. The greater range of ever-sion possible in the chimpanzee is correlated with the extension of the articularsurface (fig. 4) further posteriorly on the lateral side in the chimpanzee than inMan.

THlE UPPER' ANKLE JOINT

The talo-crural or upper ankle joint allows movement between the tibiaand fibula, acting together, on one side, and the talus on the other. We shouldconsequently expect that in the evolution of the human upright posture, withthe knees held near the midplane of the body during movement, there wouldbe a remodelling of the tibia, fibula and talus. The differences between thechimpanzee and Man in the tibia and fibula have been adequately treated byformer investigators.

A comparison of the talus of Man with that of the chimpanzee shows thatthe troclhlea in the human has been rotated upon the body of the talus. Thisrotation is clearly seen in the dorsal views of the bones (fig. 4) by concentratingattention upon the angle between the neck of the talus and the axis of the upperankle joint. This angle is larger in Man than in the chimpanzee. This differencein angle has been noted by many previous workers. They, however, haveinterpreted it as indicating a shift in position of the neck of the talus. That theneck has not changed can be seen by noting its relations to the articular surfacesas seen in ventral view. Further confirmation of our interpretation that it isthe trochlea which has undergone rotation emerges from a comparison, inposterior view, of the relation of the trochlea to the posterior calcancal articularsurface.

The medial border of the trochlea is higher in Man than in the chimpanzee,as can be seen in the anterior and posterior views (fig. 4). This is also correlatedwith changes in the tibia.

TIHE METATARSO-PIIALANGE'AL JOINTSThat the five metatarso-phalangeal joints are differently oriented in MIan

than in the chimpanzee and gorilla has been noted by many investigators,including Morton (1922) and Weidenreich (1921). In fig. 6 the heads of themetatarsals are shown in projection upon a vertical plane. It is apparent thatin the human there is not a well-developed transverse arch in this region, asthere is in the chimpanzee. In a previous paper (Elftman, 1934) the fact thatthere is no concentration of pressure, such as one would expect at the ends of afunctional arch, was taken to be an indication of a lack of a transverse archfunctional in weight-bearing in this region of the human foot. It is possible for

62 Herbert Elftman and John Manterthe human foot to distribute its pressure over this entire region of the ball of thefoot when the heel is lifted, because of the disposition of the axes of the fivemetatarso-phalangeal joints. In the chimpanzee this is not possible. The axesof the fifth and first metatarso-phalangeal joints are sharply inclined to theground. It is consequently impossible for the foot as a whole to bend in avertical direction in this region. When the heel of the chimpanzee and then thetransverse tarsal joint are lifted, pressure is transferred not to the region of themetatarsal heads but to the toes.

The evolutionary changes which have resulted in the human condition havebeen described as torsions of the metatarsals, the first in one direction, the fourlateral ones in the other. It is unquestioned that the distal portions of themetatarsals have undergone changes in which the bases have not participated.

THE JOINT AT THE BASE OF THE FIRST METATARSALThe opposability of the hallux in the apes contrasts greatly with its per-

manently adducted position in Man. Since this condition is apparent exter-nally it has been seized upon as the prime difference between the ape and thehuman foot. There is no doubt but that in the evolution of the foot otherfeatures are more important, but it is true that an adducted hallux provides astronger anterior pillar for the longitudinal arch than would an abductedhallux.

The articular surface on the first cuneiform of the chimpanzee is stronglyconvex and faces medially. In Man the articular surface is more nearly flatand faces more anteriorly. Some specimens of human cuneiform do, however,possess a markedly convex surface, the axis of curvature being oriented as inthe chimpanzee, but the articular surface faces forward. There is no doubtwhatever that the human condition could have evolved from that found in theapes. Detailed evidence for this has been put forward by Schultz (1930).

THE BONES OF THE FOOTThe evolutionary changes in the joints have inevitably been accompanied

by changes in the bony elements. It is advisable, therefore, to compare brieflythe more essential features of the bones of the human and chimpanzee feet.

The talusIn comparing the talus of Man with that of the chimpanzee it is necessary

that the two bones be oriented in similar fashion. The usual method is to usethe trochlea as a basis of orientation. This is highly unfortunate, since we haveseen that the trochlea changes in the course of evolution. We shall thereforeorient the two bones with the axes of the lower ankle joints parallel and theventral articular surfaces in congruent positions.

When this is done, many of the differences which have been described asexisting between the two bones are seen to be due to faulty orientation. Thegeneral similarity of the ventral articular surfaces (fig. 4) is apparent. It is truethat the radius of curvature of these surfaces is shorter in the chimpanzee than

The Evolution of the Human Foot 63in Man. There is difficulty in estimating the importance of this, however, sincethe radius of curvature of these surfaces is shorter in small human feet than inlarge ones.

The rotation of the trochlea with respect to the rest of the bone and theincreased height of the medial border of the trochlea in Man have already beendiscussed. The latter feature is of importance in studying the head of the talus.In the anterior view of fig. 4, it is noticeable that the greatest length of thetalar head in the human makes a larger angle with the dorsal surface of thetrochlea than it does in the chimpanzee. This has been interpreted as due to atorsion of the talar head. It is obvious, however, that here, again, it is thetrochlea and not the head of the talus that has changed. The difference in

k~~~~~~~~~~71 _ u,X.

DORSAL VENTPRAL ANTERIOR POSTER/ORFig. 4. The talus of man and chimpanzee oriented with the axes of the lower ankle joints parallel.

Upper row: chimpanzee. Lower row: human. U. axis of upper ankle joint. L. axis of lowerankle joint.

extent of articular surface on the head of the talus when the chimpanzee iscompared with Man contributes to this apparent torsion. But this we havealready found to be due to greater possibilities of eversion in the chimpanzee,not to a change in the axis of the joint.

The variations in the anterior extent of the trochlear surface in primitivetribes of Man have been extensively discussed by anthropologists and need notbe considered here.

The calcaneusWhen the calcaneus is oriented by means of the axes of the lower ankle

joint and the transverse tarsal joint (fig. 5), there is a striking similarity betweenthe chimpanzee and human bones. One of the chief differences to be noted isthe appearance in Man of a new process, the antero-lateral process of thecalcaneus, which may be seen in anterior, lateral and dorsal views. Thesignificance of this in limiting eversion in the lower ankle joint has beenmentioned.

Herbert Elftman and John ManterThe similarity in contour of the facets for articulation with the cuboid is

worthy of mention. Except for the additional area due to the antero-latcralprocess, the human facet illustrated is characteristically like that of the chim-panzee. In some other human calcanei examined, the transverse groove whichindents this facet is not so well developed. It is likely that variations in thisgroove arc correlated with the variability in the mobility of the transverse tarsaljoint.

The shape of the tuber calcanci, seen in posterior view (fig. 5), is extremelyvariable in Man. It is frequently quadrate in appearance, but varies from thiscondition to one of ovate contour resembling more closely the chimpanzee.The calcaneus of Man is relatively longer and stouter than that of the chim-panzee. In this character the gorilla resembles Man more closely.

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The Evolution of the Human Foot 65

Other tarsal elementsThe navicular, cuboid and second and third cuneiforms of Man are longer

than those of the chimpanzee, when compared with the combined length oftarsus and metatarsus. This lengthening is balanced to a certain extent by theshortening of the four lateral metatarsals. The euboid of Man has a largersurface for articulation with the calcaneus, covering in part the anterior face ofthe antero-lateral process of the calcaneus. The differences in relative pro-portions of the second and third cuneiforms cannot be discussed here; they maybe correlated with a more important feature, the shape of the first cuneiform.The surface on the first cuneiform which articulates with the first metatarsalhas already been discussed.

Fig. 6. The heads and the articular surfaces of the bases of the metatarsals, projected upon avertical plane. The articular surfaces of the bases are shown in dotted lines. A, human.B, chimpanzee in normal terrestrial position. C, chimpanzee with foot plantar-flexed aboutthe transverse tarsal joint.

Metatarsals and phalangesIn addition to the torsion of the heads of the metatarsals with respect to

their bases, there is another difference of importance between Man and thechimpanzee. This is the shortness of the four lateral metatarsals and theirphalangeal series in the human. Schultz (1930) has demonstrated that in thecourse of evolution the first metatarsal and its phalanges have undergone nomarked change in length, when measured with relation to the rest of the foot,but that there has been an actual reduction of the four lateral metatarsals andtheir phalanges, especially the second phalanges. It is significant that in thegorilla, which is more terrestrial in its habitat than is the chimpanzee, thelateral phalanges are shorter.

THE MECHANISM OF FOOT EVOLUTIONThe contribution of the present paper lies chiefly in a clarification of the

structural differences between the ape foot and the human. An analysis of themechanism of foot evolution must interpret these structural differences fromtwo points of view: first, the changes in the developmental process which lead

Anatomy LXX 5

66 Herbert Elftman and John -Manterto differences in adult structure; and secondly, the differences in function of theadult structures which determine whether they shall be judged fit or unfit by theenvironment in which they struggle for existence.

The first of these problems has been studied by numerous investigators,notably in recent years by Straus (1927). It would be well worth while to re-analyse the descriptive embryological data available in terms of the structuralchanges noted in the present paper, with a view to finding, not evidence forrecapitulation, but some clue to the developmental factors involved. We fullyrealise that an analysis into such factors as differential growth rates is but apreliminary step toward a definitiv-e analysis in more fundamental terms, suchas genie differences.

The second problem, that of selection by the environment for procreationof those structures which are functionally advantageous, is in some ways moreeasily studied. We can determine, in a fashion, the changes which have takenplace in the environment. In the case at hand, we at least know the environ-ments in which the chimpanzee and Man live. We can tell, by studying thestructures experimentally, why they have survival value in the forms now living.If we knew more definitely the environments which surrounded the stagesintervening between the ape ancestor and Man, we could make a fair estimateas to which types of structure would have had survival value during thiscritical period.

SUMMARYThe fundamental similarity in architecture demonstrated in the comparison

of the foot of the chimpanzee with that of Man leaves no doubt as to the evolu-tion of the human foot from that of an ape. But superimposed upon thisfundamental similarity are important differences which allow us to make thefollowing list of the most important changes which the foot has undergone in itsevolution from ape to Man:

1. Stabilisation of the transverse tarsal joint in a position of plantar-flexion. This accounts for the longitudinal arch and partially for the alignmentof the lateral metatarsals with the long axis of the calcaneus.

2. Prevention of extensive eversion in the lower ankle joint, partly by theappearance of a new process, the antero-lateral process of the calcaneus.

3. Rotation of the trochlea of the talus with respect to the body of thebone, and raising of the medial border of the trochlea, correlated with thechanged position of the tibia in upright locomotion.

4. Torsion of the metatarsal heads, allowing flexion in the metatarso-phalangeal joints perpendicular to the ground. This eliminates the anteriortransverse arch as a functional factor and allows the distribution of pressureover the ball of the foot.

5. Permanent adduction of the hallux, forming a strong anterior supportfor the longitudinal arch.

6. The following changes in proportion: shortening of the four lateralmetatarsals and their phalangeal series; increase in relative length of the

The Evolution of the Human Foot 67cuboid, navicular and first and second cuneiforms; and increase in stoutnessand length of the calcaneus.

Although the gorilla resembles Man more closely than does the chimpanzeein the relative shortness of the lateral digits, it shows no indication of the morefundamental changes, such as those in the transverse tarsal joint, which areessential for the development of the human condition.

The fact that the human foot, adapted as it is for walking on the ground,bears a closer resemblance to the ape foot as used in arboreal than in terrestriallocomotion may be regarded as another evidence of Man's arboreal ancestry.It would also suggest that the essential features of Man's foot were acquired atan early stage of his terrestrial existence, rather than after long apprenticeshipon the ground. It would be perfectly possible, however, for a human type offoot to evolve from either the chimpanzee or gorilla foot as now constituted,since the structures which, by modification, can give rise to the human structure,are still intact.

REFERENCESELFTMAN, H. (1934). "A cinematic study of the distribution of pressure in the human foot."

Anat. Rec. vol. LIX, pp. 481-91.ELFTMAN, H. and MANTER, J. T. (1934). "The axis of the human foot." Science, vol. LXXX, p. 484.

(1935). "Chimpanzee and human feet in bipedal walking." Amer. J. phys. Anthrop.vol. xx, pp. 69-79.

FICK, R. (1911). Handbuch der Anatomie und Mechanik der Gelenke. Dritter Teil. Jena: GustavFischer.(1931). "tber die Bewegungen und die Muskelarbeit an den Sprunggelenken des Menschen."

S.B. preuss. Akad. Wis8. S. 458-94.KEITH, A. (1923). "Man's posture." Brit. med. J. pp. 669-72.

(1929). " The history of the human foot and its bearing on orthopedic practice." J. Bone JtSurg. vol. XI, pp. 10-32.

MORTON, D. J. (1922). "Evolution of the human foot. Pt. I." Amer. J. phys. Anthrop. vol. v,pp. 305-36.

SCHULTZ, A. (1930). "The skeleton of the trunk and limbs of higher primates." Hum. Biol. vol. II,pp. 303-438.

STRAUS, W. L., Jr. (1927). "The growth of the human foot and its evolutionary significance."Contr. Embryol. Carneg. Instn, vol. XIX, pub. 380, pp. 93-134.

WEIDENREICH, F. (1921). "Der Menschenfusz." Z. Morph. Anthr. Bd. XXII, S. 51-282.

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