There is some reason to think that man, when he institutes comparison between his own structure and carriage with those of other animals, is rather prejudiced in his own favour. It is very apparent from his literature that he has come to regard his own body as the most perfect organization yet created, and not without reason, for he has proved himself the conquering and universal species. The feature of his body which he prizes most is its erect posture ; in that he sees a distinctive mark of superiority, and yet were a visitor from an outside world to appear as an impartial judge it would be the anomaly of this character rather than its superiority which would impress him. The earliest vertebrates adapted for life on land trailed their horizontal bodies along the mud or marsh on four weak limbs. In birds the two fore limbs have been converted into wings for an easy and swift progression in the air, while the two hinder limbs, with the thighs bent under the body, have been reserved for locomotion on land or water.
In mammals generally, the four limbs have become adapted for rapid carriage of the body; in some they have become modified for swimming, in others for climbing. Amongst mammals there are two forms which carry their bodies in a really peculiar manner, these are the kangaroo and man. In both the hinder pair of extremities have become specialized for locomotion, but there is no real resemblance in their styles of progression nor in the manner in which the body is carried. In man only have the lower limbs been brought into a line with the body, so that thigh and trunk form a pillar perpendicular to the ground on which he treads. Our posture and manner of walking never strike us as in any way peculiar; we are rather proud of them, because they are characters which mark us off from all other animals. If, however, it had been one of the ordinary four-footed mammals which had been endowed with our powers of observation and criticism, it is much to be feared that those peculiar features which we are so proud of would have been the subject of joke or criticism.
How then has man come by his remarkable posture of to-day ? Many who have written about this phase of man's evolution have not fully realized the difficulty of the problem. They have assumed that at a far past stage of the world's history a form of monkey had abandoned a climbing life in the forests and become adapted to a life on the open ground. In place of running on all fours, a form of ape, they suppose, was gradually evolved which waddled on its two lower limbs. These were bent towards the belly at the hips and flexed at the knees, while the upper extremities were used as crutches to assist its unsteady gait. Such a method of progression is adopted by anthropoid apes in a state of captivity. It is further supposed that the lower extremities became by degrees straightened out at the hips and knees, gradually gaining in strength until they were able to support the head and trunk erect. The manner in which the crawling infant learns to walk gives some support to this hypothesis.
The evolution of our posture is not nearly so simple a problem as those writers and thinkers have supposed. The changes implied are much greater than a mere modification of limbs and of backbone; such a change entails a complete revolution in the organization of the body. As an animal runs along on all fours its viscera are supported on the flat muscles which pass along the lower surface and sides of its body; turn such an animal up into a human posture .and all the viscera tend to sag down to the lower part of the abdomen. With the evolution of the upright posture a new method had to be evolved for keeping the viscera in place. The mechanism of respiration must also be changed. Monkeys, like dogs and cats, have their chests flattened from side to side; when the animal stands or runs its chest is slung between its forelegs. When such an animal is turned upright and made to support itself thus, all the muscles of respiration are disturbed in their action and act with difficulty. Hence we find that the human chest is shaped quite differently to that of four-footed animals, and its chief muscles have undergone an alteration in their disposition. The human chest is flattened, not from side to side, but from before backwards. The automatic nervous mechanism which regulates the distribution of the blood in the body of a four-footed animal has to undergo a complete adjustment to serve the same purpose for an erect animal such as man. The backbone which in a four-footed horizontal animal forms an arch between the fore and hind limbs, in an upright one is not only altered as regards its curvature, but also in the length and the shape of its various segments. The musculature of the backbone has to be revolutionized. The head in a monkey, as in a dog, is so fixed to the body that the base of the skull continues the line of the backbone; in man the head is poised on the spine so that the base of the skull is almost at a right angle to it. Nay, the closer one studies the matter, the magnitude of the structural transformation required by a change of posture becomes more and more apparent. There is not a bone, muscle, joint or organ in the whole human body but must have undergone a change during the evolution of our posture.
Our present task then is to see what evidence can be produced as to how and when man came by his posture. Such evidence does not come to the man who sits in his study and reads books; it is to be found only in the jungles of the tropics, and in the rocks and strata, where there are found fossil remains of animals which lived in tropical forests when the earth was much younger than it is now. It was the writer's good fortune to spend some time in a part of the Malay Peninsula many years ago. The region was hot, moist, malarious, and thickly covered with forest and jungle, In which monkeys of several kinds abounded. In the course of a morning's walk many families or troops might be encountered. As they scurried off, springing noisily from branch to branch, without ever coming to the ground, it soon became apparent that although each kind of monkey or ape had its own peculiar mode of making its way along branches, and springing from tree to tree, the gibbon or small anthropoid, which differed very little from other apes in size, was altogether peculiar in its gait. While the ordinary monkey ran along on all fours with its body parallel to the branch, using its hinder extremities as the chief means of gaining impetus for springing from tree to tree, the gibbon moved deftly along the larger branches on its legs, with body erect, and with its long arms stretched above its head to seize in passing the overhanging twigs for support. When seeking to escape it used its long and powerful arms as the chief means of progression. It was marvellous to see how it could swing itself from branch to branch and from tree to tree, often bounding thus across an interval of 40 or 50 feet. The ordinary monkeys were horizontal in posture of the body but the gibbon was upright. Later, when investigating the anatomy of the animals native to the jungle to ascertain whether or not they suffered from the effects of malaria, the writer had his attention arrested by the remarkable manner in which the gibbon and ordinary monkey differed in structure of body. In the gibbon the viscera of the abdomen were fixed and arranged much as in man; the muscles of the belly wall had the disposition seen in man; the thorax was flattened from back to front; the spinal column had the chief features seen in man's skeleton, with the exception that in the loins the column was straight instead of being curved forward as in the human body. As regards the features just enumerated the monkey resembled the dog rather than the gibbon. There, then, is a most important fact —a certain stage in the evolution in the upright posture has been attained in the gibbon. The gibbon is an ape with a body adapted for an upright posture, amidst arboreal surroundings.