Popular Science Monthly/Volume 65/September 1904/The Evolution of the Human Hand
|THE EVOLUTION OF THE HUMAN HAND.|
NEW YORK UNIVERSITY.
THE succession of organic modifications which resulted in the formation of the human hand is part of the general process of evolution by which in the animal series the means of progression and of the taking of food were shaped by the environmental conditions under which life was carried on. Antecedent to the appearance of vertebrate limbs a series of manifold devices had originated by which the body could be transported from place to place and appropriate foodstuffs seized and carried to the mouth. These consisted of more or less permanent extensions of the body substances, naked or clothed in protective shields of denser material. In some types the limbs were created in the act of extension itself and were retracted by absorption and disappearance into the general body mass; in some they were formed of erectile tissues which could be protracted or withdrawn as occasion demanded; in some the whole body was thus contractile, and alternately elongated and shortened as the animal progressed; in some the organs of locomotion consisted of definitely formed limbs, which, while subject to loss by violence or even sudden shock, might be repeatedly and perfectly regenerated in the course of the individual life. In the forms to which they are molded and the mechanical principles upon which they depend, these organs of movement present the utmost variety, including ameboid extensions, flagellate cilia, pulsating bells, contractile stalks and bodies, suckered tentacles, swimming fins and tails, wings and articulated legs. They appear as a great series of adaptive levels through which the evolution of this particular mechanism passed toward more highly integrated and developed types.
The functions of life which call into service the bodily limbs are chiefly two—locomotion, an activity which has arisen in connection with the search for food and flight from enemies; and prehension, which is concerned primarily with the grasping and tearing of food, but secondarily also with processes assistive of locomotion and other biological functions, such as sexual congress, the care of the body, burrowing and climbing. Of these two functions, if we regard the vertebrate class only, the former is the more primitive. Upon the office of locomotion the prehensive and manipulative activities of the limb have been superposed as subsequent and more specialized adaptations. In vertebrates of less modified types the food is seized and manipulated by the mouth parts directly. Fish, reptiles and birds feed in this way. In these as well as in mammalian forms which present relatively slight limb specialization, the mouth parts have in many cases undergone modifications which render them effective instruments for grasping, rending, digging, picking and the like. Such adaptations are shown in the snouts of the mullet and pig; the beaks of the paddle-fish, the duck-billed otter, the humming-bird and the secretary; the tusks of the boar, the horn of the rhinoceros, the proboscis of the tapir, the tongue of the chameleon and the trunk of the elephant. In all these cases the specialization of the limbs which accompanied such modifications of the mouth parts consists in an adaptation of the function of locomotion in connection with the particular conditions under which the life of the species is carried on; in consequence of which their general features have diverged very widely from those of manipulative organs.
The earliest form of locomotion which vertebrate limbs fulfilled was propulsion through the water. The problem to be solved did not include the support of the body, which was buoyed up by the dense medium in which the animal moved. The same dense medium afforded a sufficient resistance to allow of a relatively slow and weak movement on the part of the locomotive organs. The earliest vertebrate limbs, or the body extensions which foreshadowed them in times still earlier, needed neither the strength and rigidity of the terrestrial leg nor the expanse and velocity of stroke of the aerial wing.
If we conceive the progenitor of the limbed vertebrate to have progressed by means of an undulatory motion of the whole body, brought about by a peristaltic wave of contraction passing from front to rear of the animal, it is not difficult to infer the advantages which would accrue to those individuals in which a modification appeared in the form of flexible extension parallel to the longitudinal axis of the body, by the independent undulations of which progression became possible. The economy resulting from reduction of movement in the whole body mass would be accompanied by a decrease in the likelihood of attracting notice, a greater control of movements in taking food, and a more exact process of perception in adjusting the body to surrounding changes.
Though the series of limb forms is obscure in its earlier parts, the whole group is generally supposed to have its prototype in the lateral fold of the primitive fishes, in which locomotion took place through a wave-like movement passing backward along the length of the web. Out of this primitive lateral fold the various fin-formed limbs which characterize the aquatic progenitors of the land vertebrates arose by a series of modifications in which the following stages may be noted: In the undifferentiated swimming folds first developed a system of parallel rods extending from the body surface to the margin of the web, which probably both served the purpose of increasing the resistance of the locomotive organ and was accompanied by muscular and nervous developments which allowed greater definition and force in the reactions produced. Among these rods certain members outgrew the rest, a development which from mechanical causes alone would tend to survive in a bilateral form. The number of such points of origin of increased growth in the rods was finally reduced to two on each side of the body, after a series of forms which we may conceive to have presented a diminishing series of rods, as the lamprey and shark present numbers of gill arches intermediate between those of the lancelet and the perch. With the definition of these fore and hind pairs of axial spines a concomitant modification of the adjacent members of the system of parallel rods took place, in consequence of which, first, a differentiation in size arose among them, those in proximity to the axial spine increasing, those remote from it decreasing in length; secondly, changes in the points of their attachment to the body occurred, the system of secondary rods moving from the median regions in either direction toward the axial spines; and finally, these accessory rods arranged themselves in a radial relation to the central rib, thus giving anterior and posterior fan-like extensions connected by the remnants of the degenerating fold and rods in the intermediate body regions.
Further differentiation of the axial and neighboring spines, in which the latter were progressively affiliated upon the former and there appeared a definite point of articulation of the whole system with the body mass, gave rise to the bipinnate fin, a roughly symmetrical organ in which the main spine occupies a central position and is flanked by a group of supplementary rods on either side. From this form structural modification proceeded, first, by the reduction and disappearance of the accessory spines on one side of the main axis, giving the unilateral fin; and secondly, through a similar degeneration of those on the remaining side, by which the limb was reduced to a prong-like form represented in the lepidosiren. The limbs at this stage of development were in a condition which in general was more adapted to progression upon land than through the water, since the expansion upon which their propulsive action depended had ceased to be an element of importance, and all that was needed for terrestrial locomotion of a crude sort was a condition of sufficient rigidity in the limbs to allow of their use in dragging or pushing the body along, as the turtle does, but not necessarily of supporting its full weight as do the common quadrupeds. Before this final stage was reached, however, the animal had begun to practise land travel, using fins which were in the bipinnate condition as terrestrial limbs, as is the case with the Australian salmon, ceratodus.
From this primitive terrestrial vetebrate limb, through a series of cleavages of, or buddings from, its extremity, giving successively two-, three-and four-toed forms, arose finally the five-toed generalized type of mammalian limb. The subsequent modifications of this organ, if we omit the divergent series of adaptations which gave rise to the pterosaurians and finally to the birds, present forms of specialization connected with the following modes of progression, namely, swimming, running, leaping and climbing. The first, exhibited in different degrees by the whale and the dolphin, we may pass by, both because it follows a process of adaptation unlike that of the group of animals to which man belongs, and because the change may be regarded as degenerative, inasmuch as the animal returns to a medium which makes less demand upon the structural resistance of the organism than did that which was relinquished. Adaptation to running finds its extreme form in the hoofed animals, in which the body is poised upon the extremity of the limbs, thereby conserving their full length for the purpose of rapid movement by employing the utmost length of stride; and in which the number of functioning toes is progressively reduced until, as in the horse, only a single massive and horn-shod central digit forms the body of the so-called foot. In adaptation to leaping, which is presented both by animals which have passed through an arboreal stage, as the kangaroo, and by others which have always been terrestrial, like the hare and jerboa, the structural modifications consist primarily in an increase in the size of and strength of the posterior limbs, with a concomitant degeneration of the fore limbs as they are less and less called upon to share in the function of supporting the body. Along with this primary modification goes a greater or less degree of specialization in the extremities of the limb, by which, as in the case of the running animals, one or more of these take upon themselves the chief support of the body and the rest suffer functional atrophy. In the jerboa, for example, one toe only is thus degenerate, while in the kangaroo three are rudimentary.
It is with the modification of the five-toed limb for the purpose of climbing that we are here especially concerned, since it is in the arboreal group of animals only that the specialization of the fore limb in the form of a hand appears, and since it is to the adaptations fostered by this form of existence that man owes the early development of his own dextrous and accomplished manipulative organ. This modification consists, first, in the modeling of the extremities of the limbs to a form which made the act of grasping possible; secondly, in the separation of the whole system of limb terminations into two opposable groups, by which primarily a more perfect grasp was secured, and later the refined manipulation of objects was made possible; and finally, in the differentiation of hind and fore limbs, by which the former were made to provide secure and rapid locomotion and the latter were left free for specialization controlled by the sole condition of prehension and manipulation.
The first of these functions appears to be essentially connected with the habit of walking on the sole of the foot—plantigrade locomotion—and not on the knuckles or toes—digitigrade locomotion. Another method of climbing exists which is common to the rodents and the cats. In these animals the act of climbing depends upon the development of claws sufficiently long, strong and sharp to be attached like hooks to the roughened surfaces upon which the animal climbs and thus to support the body. In such forms the modification is of a superficial feature of the body structure and is probably a secondary function, the claws having been developed in connection with habits of seizing prey rather than of climbing. There is here no essential modification in the anatomical relations of the various parts of the limb, and it is inconceivable that any such subsequent development should be connected with this form of climbing organ as is presented in the limbs of the anthropoid apes and man.
In the plantigrade animal, on the other hand, the disposition of the limb is such that when the weight of the body is thrown upon it the toes tend to be thrust apart even when the foot is resting on a flat surface, and to be forced into a concave shape when pressed upon a rounded object. It is probable that a fair degree of development in the joints of the limbs had taken place in both flexion and separation before they were used for the purpose of climbing. But flexibility and separability of the digits form only the intial step in the process by which adaptation to an arboreal life was perfected. The second—and beyond all other changes important—modification consisted in the structural opposition of one digit to the remaining group. This differentiation occurs also in the lizards, e. g., the chameleon, and in the birds, under similar conditions of climbing and perching; but in connection with such specialization of the limbs in other regards and such modification of the body system as a whole that important service in the evolution of intelligence was precluded.
In the production of opposition changes took place in the hind limbs first and most generally, since all species in which the thumb is opposed possess opposable great toes also—except in the single case of man—while many species occur in which opposition is presented by the hind limbs alone. In this adaptation of the foot to climbing three structural changes were effected—the parts of the limb became more flexible, the joints more widely separable, and the great toe, as has been said, opposed to the group formed by the remaining digits. All these are important features in rendering the limb a more efficient tool.
For the development of those peculiar functions which characterize the human hand, however, a further change in the use of the fore limb was necessary, by which it was relieved from participation in the support of the body and in primary locomotion. This relief must have taken place by a process which involved simultaneous changes in both fore and hind limbs. The support of the body, hitherto laid upon all four limbs, could not have been taken over at once in its final adequacy and security by the legs alone, unless we conceive of a spontaneous variation of improbably large extent. The animal at first raised itself hesitatingly upon its hind limbs, supporting its weight in part by the grasp of the hands upon higher portions of the trunk and branches, thus distributing the function as heretofore among the whole set of limbs, but in such a way that the fore limbs were adapted to their new specific use while performing their old generic function. The body, in this stage of development, was sustained in part by support from beneath and in part by suspension from above. Either of these factors may be conceived as appropriating a chief place in the locomotive function; and in different animal species these divergent directions of development are both presented, progression by swinging from limb to limb in the long-armed apes, and by the sole use of the legs in man.
It is probable that the progenitor of man, together with the whole group of anthropoid apes to which he belongs, maintained the quadrupedal position longer than those types which, like the Cebidæ, e. g., the tee-tees and Capuchin monkeys, present no opposition in the members of the fore limbs. If we conceive the semi-upright position to have been assumed at a time anterior to the development of opposition in the hind limb, say at the beginnings of arboreal existence, so that from the outset each pair of limbs was modified under different conditions of function, it will be found difficult to imagine the causes which under these unlike circumstances brought about a similar modification in each set of limbs. If, on the other hand, both fore and hind limbs were used to support the animal in a quadrupedal position upon the branch beneath it during the early period of arboreal life, it will be as difficult to imagine a reason why both sets of limbs should not present the same type of adaptation. The condition which predisposes to conservation of the phenomenon of opposition is support, not suspension; it is peculiarly a modification of the foot. All that is involved in successful adaptation to the function of suspension is the existence of sufficient elongation in the digits, flexibility in the joints and strength in the muscles—the development of a strong and supple member, but not necessarily one possessing an opposable thumb. Even a single series of joints may form an efficient instrument of suspension, as in the case of the prehensile tail of the monkey tribe. For support upon the rounded branch beneath, on the other hand, some sort of forking is almost the only modification which could give security, and in the man-like ape this has taken the form of an opposition between a single member and the rest of the group.
We may therefore conceive that the progenitors of the Capuchins and other parallel fingered species soon after their adoption of the arboreal habit—or at least before the appearance of any important modification of the earlier structural relations of their limbs—took to a form of locomotion in which the body was partly supported from beneath by the hind limbs and partly steadied or suspended from above by the grasp of the fore limbs; so that the peculiar modification which the arboreal form of life contributed to the animal type was incorporated in the hind limbs alone. The anthropoid apes, on the contrary, which show this specialization in fore as well as hind limbs, we shall conceive to have persisted in the quadrupedal habit during a period the continuance of which was sufficiently protracted to allow of the appearance of similar modifications in all four limbs. Only subsequent to this process of adaptation should we imagine the progenitor of man to have arisen from the quadrupedal position and to have used the fore limbs for the secondary support of the body by grasping the upper branches.
In this new function the limb specialized by opposition had probably little advantage over the more primitive hand of the monkey, in so far as suspensional support was concerned. In respect to those other uses upon which the subsequent development of man in all kinds of mechanical skill depends, this new structural variation was of the highest significance. The monkey tribe gave up the habit of walking on all-fours too early and is suffering from the consequences to the present day.
This stage of development, however, represents a condition in which the factors of further evolution are confused and the various parts of the organism imperfectly adapted to the functions they are hereafter to perform. Hands and feet conform to the same architectural type. Both share in the unitary process of locomotion; the hands are capable of supporting the fore part of the body in moving, the feet are still prehensile organs. There is no exclusive functional specialization by which fore and hind limbs may be set off from each other. This subdivision of labor must come about through a development of the lower limbs by which they become capable of the sole support of the body at rest and in progress. In other words, the hands can not be released from their office of steadying and supporting the body until sufficient skeletal changes and muscular growth have taken place in the lower limbs to enable them to carry on the function of locomotion alone. The freeing of the hand for exclusively manipulative purposes thus depends upon the replacement of the semi-erect posture by a fully erect one, in which process the calf develops, the joints are straightened and the whole limb rotates upon its point of attachment to the body until the main axes of the two are parallel and each is vertical in position.
These changes could hardly have taken place during the continuance of an arboreal habit of life. The means of support afforded by the branches is too precarious, the form of locomotion which practical conditions impose upon the animal too restricted and interrupted to make the development of such a limb as the human leg possible. The need of supplementary support to which an unstable balance must give rise and the facility with which the arms can come to the aid of the legs as the animal makes its way from tree to tree are likewise factors which retard the development of efficient bipedal locomotion. The freeing of the hands may therefore be regarded as a concomitant of the return of man's progenitor to a terrestrial habitat, in which free, large and continuous movements of locomotion were both possible and necessary. Only on the wide, open spaces of the ground can we conceive the apeman to have become a swift and sustained runner, holding the body upright and the arms free.
At the same time with the changes in the leg already described the habit of traveling over the level surface of the ground would tend to produce a closer knitting of the ligaments of the foot and a greater compactness and rigidity in its general structure. The opposition of the great toe, no longer necessary to preserve the animal's equilibrium—since this is sufficiently secured in a lateral direction by the relation of the two legs—becomes a distinct impediment to land travel, owing to its interference with the movements of the fellow limb and its liability to injury by striking upon the objects among which the animal walks. With the further development of the foot, however, whether degenerative or other, we have not here to do.
As regards the special causes which led to the adoption of a terrestrial habitat in preference to the earlier arboreal life, it is probable that the change was intimately related to the development of the opposable thumb. The platyrrhine monkeys have the same type of foot as that possessed by the man-ape and do not progress predominately by swinging as do the long-armed apes. Anatomically, therefore, they differ from the progenitor of man chiefly in the fact that, unlike him, they have retained the parallel-fingered hand. In this differential feature resides their disability. The monkey form of hand is adequate for seizing and clinging to branches, but deficient in adaptability to all other mechanical purposes. For grasping and pulling, for digging and tearing, for handling stones and sticks the human hand with its opposable thumb is incomparably superior. Among the uses for which, in virtue of these capacities, it is especially fitted are the employment of weapons, the construction of means of defense from attacks by carnivorous beasts and later the use of tools.
The relinquishment of an arboreal habit involved the giving up of an important refuge and the assumption of a mode of life assailed by many new and grave dangers. The tree is a place of safety; it affords a secure retreat from some enemies and concealment from many others. A life amid its branches is compatible with a condition of weakness or defenselessness which would be fatal to the species under the circumstances of a ground habitat. To descend from the trees and venture that mode of life implies one of three possible resources: the animal must either he Heel of foot enough to distance his pursuers, or he must possess weapons of defense sufficient to repel attack successfully, or, finally, he must supply deficiencies in these regards through a cunning which enables him to escape his enemies by artifice. The apes are not swift of foot as compared with beasts of prey. They are poorly provided with natural weapons or means of defense. They have neither tusks nor claws, neither hoofs nor horns, neither great mass and strength nor impenetrable hides. If they are to take the aggressive or even to repel attack successfully it must be by the invention of artificial weapons whereby their deficiencies are made good; but as recourse to such instruments is a purely mental resort to obviate actual physical difficulties, it may be said that the ape-man met his difficulties in only one way, namely by cunning—escaping his enemy by retreat to strongholds of his own devising; meeting him, when battle was unavoidable, not with bare hands but with weapons, and taking his prey by traps and snares. But the schemes of his cunning brain could become practicable only as the result of a distinct mechanical constructiveness. Stones must be gathered and dropped or thrown with accuracy; clubs must be selected and wielded; traps must be put together after they have been devised. In all this the manipulative hand is essentially linked with the resourceful mind. With any other known type of limb the problem would have been insoluble. The specialization of the hand, therefore, with its opposable thumb and its wonderful adaptability to mechanical uses we may conclude to have been the single indispensible condition, so far as regards gross anatomical features, which determined the widely divergent subsequent fortunes of the monkey tribe and man-ape respectively.
For the principle of separation between this type and the rest of the anthropoid apes we must look to the different directions of development taken by the central nervous system in the two cases. Henceforth no important structural changes are to occur in the general features of the hand. Development is to take place chiefly through an increase in the facility and precision with which a variety of relatively simple movements are made, and the substitution, in ever increasing grades of complexity, of mechanical instruments for the use of the hand itself as a manipulative and constructive agent.