Man And His Ancestor: A Study In Evolution

by Charles Morris

These considerations bring us to an important question: Why did the man-ape gain a length of arm not the best suited to its arboreal habitat? Why, in fact, do changes in physical structure ever take place? How does an animal succeed in passing from one mode of life to another, when during the transition period it is imperfectly adapted to either, and therefore at a seeming disadvantage in the struggle for existence? The study of animal development has given rise to certain difficult problems of this character, some of which have been solved by showing that the supposed disadvantage did not arise, or that it was balanced by some equal advantage. In this way a considerable gap in life conditions has perhaps occasionally been crossed. Small gaps have doubtless been frequently passed over in the same manner.

In the case of the anthropoid apes, we perceive a considerable variation in the length of the arms, from the very long arms of the gibbon to the comparatively short ones of the chimpanzee. These differences are probably the result of some difference in their life habits, and accord with the possibility of a still shorter arm in the man-ape. There is, however, some reason to believe, as we shall show later on, that the arm of this animal was longer and the leg shorter than in man himself, their comparative length perhaps not differing greatly from that of the chimpanzee. Aside from all other considerations, the use of the legs as the sole organs of locomotion could not well fail to produce this result, the legs growing longer and stronger in consequence of the increased duty laid upon them, and the arms growing shorter and weaker through their release from duty in locomotion. The case does not differ in character from those of the dinosauria and the kangaroos, in both of which instances a release of the arms from duty in walking was followed by a considerable decrease in length and strength, while the legs grew proportionally stronger.

If any disadvantage attended the shortening of the arms of the man-ape, to the extent that this may have taken place in the tree, it was probably correlated with some advantage. In the various instances of short-armed animals cited this appears to have been the case, and it was probably so in man's ancestral form. While the hands continued useful in grasping and enabling the animal to maintain its place on the boughs, they may have been gradually diverted to some other service, with the result that the animal found the tree less desirable than before as a place of residence and sought the ground instead. This would be particularly the case if the new duty was one best exercised upon the ground.

Shall we offer a suggestion as to this new use? Such changes are usually the result of some change of habit in the animal, frequently one that has to do with its food. Change of diet or of the mode of obtaining food is the most potent influencing cause of change of habit in animals, and the one that first calls for consideration.

The apes are frugivorous animals, though not exclusively so. Carnivorous tendencies are displayed by many of them. They rob birds' nests of their eggs and young, they capture and devour snakes and other small animals. In zoölogical gardens monkeys are often observed to catch and eat mice. It is evident that many of them might readily become carnivorous to a large extent under suitable conditions. The large apes are usually frugivorous, but some of them eat animal food. This is the case with both the chimpanzee and the gorilla. The latter, while living usually on fruit and often making havoc in the sugar-cane plantations and rice-fields of the natives, also eats birds and their eggs, small mammals and reptiles, and is said to devour large animals when found dead, though it does not attempt to kill them for food. The young gorilla which was kept in captivity at Berlin became quite omnivorous in its diet.

With all this readiness to eat animal food, none of the existing apes are carnivorous to any large extent, but the fact of this inclination makes it not improbable that some of the apes of the past may have been much more so. It is quite within the limits of probability, for instance, that the man-ape at an early date became omnivorous in its diet. Its change in structure may well have been the result of a decided change in diet, such as that from fruit to flesh food. Such a radical change as that from vegetable to animal food would certainly demand a more active employment of the arms as agents in capture. Fruits and nuts wait to be pulled; animals must be caught before they can be eaten. The former is an easy matter to an arboreal animal; the latter might prove a difficult one, especially if large animals were to be captured.

In short, the pursuit and capture of any of the larger animals for prey could not fail to modify to a great degree the use of the arms. Their employment in locomotion would interfere seriously with their utility in this direction. To succeed in capturing nimble prey by an animal with the ape form of hands a considerable freedom of the arms would be necessary, and the feet would have to be mainly, if not wholly, depended upon for motion. The ape has not the sharp claws of the carnivora with which to seize and hold its prey. It must have been obliged to use its palms for this purpose, and this it could not well have done unless they were free in their action.

It is conceivable, indeed, that the man-ape may have run down its prey, or sprung upon it from covert, and seized it with the hands, but there is good reason to believe that this was not its mode of capture. The organization of the ape tribe gives it a characteristic action which is not to be found in any other group of the vast animal kingdom, that of handling and throwing missiles. In this it necessarily stands alone, since no other animal has a grasping palm. The power is one of prime importance, for without it we cannot perceive how man could ever have emerged from the general animal kingdom. The use of missiles is by no means uncommon with the monkeys. We cannot safely accept the story that American monkeys will throw cocoanuts from tree-tops at those who hurl stones at them from below, from the fact that the cocoanut seems too heavy and too firmly fixed to its support for the strength of those small species, but it is not uncommon for them to throw lighter objects. Yet in doing this they usually seem to have no idea of aim, but toss the missile aimlessly into the air. Of the large apes, the orang will break off branches and fling them at its tormentors, or will throw the thick husks of the durian fruit, but with similar lack of aim. The most skilful in this exercise are some species of baboons, which can hurl branches, stones, or hard clods with much dexterity.

It is of interest to find existing apes availing themselves of their grasping power in this manner, since it leads us irresistibly to the conclusion that the man-ape may have done the same thing. The species which use missiles fail to take aim for two reasons, one that they employ them only occasionally, often in imitation of human action, the other that their arms are ill suited to this motion from their constant employment in another duty. In the case of the man-ape we may justly look for a more effective result, since if the arms became relieved from duty in locomotion they were free to gain facility of action in other directions.

If in addition to this the man-ape began to use missiles with a definite purpose in view, that of striking down animal prey, so that the use of such weapons became habitual instead of occasional, it would soon gain some power of aim and a growing strength and skill in the throwing motion. It is quite probable, also, that an early use of weapons was in the form of clubs, which were retained in the grasp to strike down the prey when overtaken. In this case, we may imagine our primitive biped running swiftly after its prey, club in hand, striking at it when within reach; or, if it should prove too swift, hurling the club or a stone through the air with the hope of bringing it down in this manner. Such a flinging action, if now and then successful, would be likely soon to become habitual; while the arm would grow accustomed to this new motion, and attain skill in taking aim. We may reasonably infer, also, that the club would be used for defence as well as for offence, in case the man-ape were in its turn pursued by larger animals. Instead of fleeing to the nearest tree, it might now stand its ground and beat off its enemy.