Man And His Ancestor: A Study In Evolution

by Charles Morris

The most distinguishing characteristic of the apes and of the nearly related lemurs has not hitherto been definitely pointed out. This is that they form the only group of strictly arboreal animals. The tree is not alone their native habitat, but they are specially adapted to it in their organs of motion, a fact which cannot be affirmed of any other animal group. If we consider, for instance, the squirrels, one of the best-known groups of tree-living animals, we find them to be members of the great order of rodents, whose native habitat is the land surface. Though the squirrels have taken to the trees, there has been no adaptive change in the structure of their limbs and feet. The same may be said of almost all tree-dwellers except the lemurs and apes. The sloth, indeed, is specially adapted in organization to an arboreal residence, but this change is individual, not tribal, this animal being an aberrant form of the ground-dwelling edentata. In the apes and lemurs, on the contrary, the ground-dwellers are the aberrant forms, stray wanderers from the host. Nearly all the species live in trees, to which they are specially adapted by the formation of their feet. It remains to inquire how this deviation in structure arose, what were the steps of development of the grasping foot and hand, the special characteristic of this group.

In considering this question, the first fact to appear is that the apes and lemurs are plantigrade animals. Their natural tendency is to walk on the sole of the foot, a habit which few other tribes of animals possess. Most of the larger animals walk on the knuckles or the toes, and develop claws or hoofs, but the ancestral form of the ape, ages in the past, was doubtless a sole-walking quadruped, its toes apparently provided with nails instead of claws. What the story of this very ancient quadruped was we are quite unable to say. It may, in the exigencies of existence, have come to a parting of the ways; a section of the group, drawn by a love of fruit, developing the climbing habit; the remaining section continuing on the ground and following a separate line of evolution. Perhaps only a single species took to the trees; for it is quite possible for a single form, in a new and advantageous habitat, to vary in time into a great number of species.

Of all this we can know nothing: but of one thing we may feel assured, which is that the plantigrade foot is the only one that could have developed into a grasping organ; such a development being impossible to the digitigrade or the hoofed animals. One can readily see how the habit of walking on the sole might tend to a spreading of the toes, in order to obtain a wider and firmer footing. And it is equally easy to see how a free and wide motion in the great toe would aid in this result. The animal may have been at first light in weight and able to support itself on its unchanged foot, but as it increased in size and weight it would need a firmer grasp, and the final result of spreading its toes for this purpose may well have been the opposable great toe.

It must be borne in mind, in this consideration, that the apes differ from the other tree-dwellers in being destitute of claws. The squirrels, the opossums, and other arboreal animals have sharp claws, by whose aid they can easily cling to the surface of the bark-covered boughs. The nails of the apes are incapable of affording them this service, and it is not easy to perceive how a foot like theirs could become adapted to locomotion in the trees otherwise than by the gaining of mobile action and grasping power in the toes.

The existing habits of the ape tribe lead us to the conclusion that the ancestral animal may have soon begun to seek support from upper limbs. The plantigrade foot is one capable of readily curving into an organ of support, and in the case of the forefoot the toes would tend to spread and gain flexibility of motion, and the first toe to become opposable to the others and yield a more complete grasping power. It does not seem difficult to comprehend, from this point of view, how the feet of a five-toed plantigrade animal may in time have developed into grasping organs, since there would be required only an increased flexibility of the joints, and a wider and fuller movement of the great toes. That such a change took place in this instance the facts appear to indicate, the most simple and probable explanation of the development of the grasping power in the hands and feet of the ape being seemingly that given above.

The relation of the lemurs to the apes is not clearly defined. It may be an ancestral one, or the two animals may represent distinct lines of descent. In the latter case we would have two lines of animal evolution in which the grasping power was gained and adaptation to arboreal life completed. Whatever their relationship, they both possess the opposable thumb as the hall-mark of their arboreal habitat, and whenever found walking on the ground they may be looked upon as estrays from their native place of residence.

Once the grasping power was gained, the first step of change from the quadrupedal to the semi-erect attitude was completed. The process may have begun in the effort to fit the sole of the foot to the rounded surface of boughs; or its first stage may have been in the seizing of overhead branches with the flexible hand; or both influences may have acted simultaneously. We see the result only, we cannot trace the exact process; but we have as an outcome the adoption of a method of locomotion different from that of all other tree-dwellers, the forefoot developing into the hand with its opposable thumb, and the hindfoot gaining a similar grasping power in the toes.

The power of walking on a lower limb and grasping an upper one once attained, a succeeding step in evolution quickly appeared, and one of prime importance to our inquiry. The animal had ceased to be in a full sense a quadruped, while not yet a biped, and a variation in the length of its limbs was almost sure to take place. This is an ordinary result when animals cease to walk on all fours. In the leaping kangaroo and jerboa a shortening of the arms and lengthening of the legs appear. Here the arms are relieved from duty and a double duty is laid on the legs, with the consequence stated. In the ancient dinosaurian reptiles, upright walkers, the same was the case. Those varied from quite small to very large animals, but in all known instances the fore limbs were greatly reduced in size. A similar condition may be seen in the birds, the bones of whose fore-limbs have largely aborted from lack of employment as walking organs.

In the case of the apes and lemurs, while a similar effect has taken place, an interesting difference appears, due to the difference in conditions. In these animals the fore limbs are not freed from duty as organs of locomotion. In many cases, on the contrary, they have an extra duty put upon them, with the result that they have grown longer instead of shorter. Very likely these animals differed considerably in the past, as they do to-day, in the degree of use of their legs and arms. Many of them walk in the quadruped manner, either on the ground or in trees. Others make much use of their hands and arms in grasping and swinging. Great differences in the use of the arms and legs may have arisen in different species. In some, the legs may have been mainly trusted to for support, and the hands used for steadying. In others the arms may have been the chief locomotive organs and the feet have given steadiness. Here the legs may have grown the longer, there the arms, the limbs developing in accordance with their degree of employment. In the lower monkeys and the lemurs, the bones of the pelvis are altogether quadrupedal in character. This is not the case in the higher forms, and in the highest apes the pelvic bones approach those of man.

Highly interesting examples of these varied results may be seen in the existing anthropoid apes. In all of them it would appear that the arm was a prominent factor in locomotion, for in each instance it is longer than the leg,—but it differs in proportional length in every instance. It is shortest in the chimpanzee, somewhat longer in the gorilla, still longer in the orang, and remarkably long in the gibbon. In all these instances the fact that the arms exceed the legs in length indicates that they must have played a large and important part in the work of locomotion, and especially so in the case of the gibbon. It is well known, in fact, that the gibbons progress very largely by the aid of their arms, swinging from limb to limb and from tree to tree with extraordinary strength and facility. The legs lend their aid in this, but the arms are the principal organs of motion, and seem to have developed in length accordingly.