The reader may well ask, " What has the upright posture in the gibbon to do with man's posture ? " In answering that question a number of considerations must be kept in mind. The first and most important is the fact that the gibbon occupies a position in the animal scale between the great anthropoids— the gorilla, chimpanzee or orang, and the common monkeys of the New World and of the Old. It stands between the higher and the lower. Secondly, we know that the erect posture was attained by gibbons at a very remote period, for in the lower Miocene strata of the earth a form of gibbon, not unlike the present type, is known from its fossil remains. It is very probable that a type of ape similar to the gibbon was evolved at the beginning of the Miocene period, or even before that period began. That is certainly many millions of years ago. We know nothing of the early stages of the evolution of the upright posture in the gibbon, but we can realize that it implied a gradual and complete alteration in its structure and in its method of locomotion.
In the great anthropoids we recognize the same posture as in the gibbon. They are adapted for an upright posture on the trees. Men who have studied the attitude and gait of those animals in confinement have been misled by watching them waddling across the floor of their cage, using their long arms as crutches. They are different animals when at home in tropical forests. They all hold their bodies approximately upright, but differ materially in their methods of locomotion. The orang, like the gibbon, has extremely long arms, which it uses much more than its legs, in swinging itself from branch to branch and from tree to tree. In the chimpanzee the arms, although longer than the lower limbs, are employed to an almost equal extent in locomotion. In the gorilla we see a strengthening of the legs ; the foot has become an organ for supporting the weight of the body, and has lost some of the features which make the typical anthropoid foot a grasping organ. Even among the great anthropoids we see a specialization, not in posture, but in manner of locomotion.
If I have succeeded in carrying the reader with me in my argument he will now realize that the erect posture is a very old one, evolved with the appearance of the gibbons. In the great anthropoids we see a stage which takes us appreciably nearer man; they have the erect posture and also man's bulk of body. Man does not differ from them in posture, but in his manner of progression. He differs from them in being adapted for progression on the ground—an adaptation which allowed him to escape beyond the limits of forests and occupy the whole world. The problem which confronts us is not how man came by his upright posture —that was in existence at an early date— but how and when did he come by his foot, his leg and his thigh ? Apparently a long time ago, for although we do not know the foot of Pithecanthropus, the oldest fossil man yet discovered, we may infer from his thigh-bone, which is absolutely human in character, that his foot was also like ours. No human foot has ever been seen, either in human foetus or in primitive native races, in which the great toe was separated like a thumb, as is the case in all anthropoids, yet from appearances to be found in the human foot itself, the evidence is overwhelming that the great toe was once set like a thumb, and that the human foot was at one stage of evolution a grasping organ. The stages leading from the anthropoid to the human foot are unknown. The foot was evolved before the brain, for in Pithecanthropus— belonging to a very late Pliocene or very early Pleistocene date—the brain is little more than half the size of the modern human brain. It is very likely that the human form of foot and leg appeared when the great anthropoid forms were evolved—probably in the long Miocene period—which was three times as long as the Pliocene period which succeeded it.
It was evolved with the appearance of ground-living anthropoids—one of which we believe stands in the ancestral line of man.
No doubt when we come to know the treasures of " missing links " and of extinct forms of animal life which are at present hid in the more superficial strata of the earth we shall be able to trace step by step the various stages which culminated in mankind. As regards his upright posture we know enough to say that it is an old feature, one which appeared with the evolution of the gibbons. Indeed, the greatest blank in our knowledge relates to the manner, and the time, in which the posture of the gibbons was evolved ; yet so close is the structural relationship between the monkeys and the gibbons that there can be no doubt that such a transformation was effected. The next blank in our knowledge relates to the appearance of the human foot, leg, and thigh. There, again, the structural resemblance between the anthropoid and human foot can only be explained by supposing they have been evolved from a common form. Thus, in the body of man there are certain features which are new, some not so new, some old and others older still. His large brain appears to be his latest acquisition; his foot, leg, and plantigrade gait is older, his size of body older still, and his erect posture quite an ancient character—one which probably dates from the beginning of the Miocene period.