If we compare man with these apes, which certainly of all animals resemble him most, the following principal differences may be noted. Instead of holding himself in a bending position, and walking supported on his arms, man walks in an erect attitude—the truly biped mode of progress. In harmony with this attitude, his vertebral column presents three curves, cervical, dorsal, and lumbar, very definitely indicated, while they are only faintly marked in the anthropoids, and almost absent in the monkeys. This character, moreover, is graduated in man; in civilised man the curvature in question is more marked than among savages. There is no need, however, to see in that any “character of superiority.” It is quite simply an acquired formation; it is more marked in civilised man just because it is one of the conditions of the stability of the vertebral column, a stability so essential in sedentary life, while a curvature less marked gives much more flexibility to the movements, at once so numerous and varied, of the savage.[11]
But to what does man owe this erect and biped attitude? Professor Ranke has put forward on this subject a very ingenious hypothesis.[12] According to him, the excessive development of the brain, while conducive to enlargement of the skull, would at the same time determine the change of attitude in a being so imperfectly and primitively biped as was our progenitor. In this way would be assured the perfect equilibrium on the vertebral column of the head, made heavy by the brain. Without wishing to discuss this theory, let me say that several peculiarities in the anatomical structure of man, compared with those of anthropoid apes and other mammals, give it an air of plausibility.
In fact, while with the majority of mammals the equilibrium of the head is assured by very powerful cervical ligaments, and with anthropoid apes by very strong muscles, extending from the occiput to the spinous processes of the cervical vertebræ, twice as long as those of man (Figs. [1] and [2], a), which prevent the massive muzzle from falling upon the chest and pressing on the organs of respiration,[13] we see nothing of a similar kind in the genus Homo—no cervical ligament, and no powerful muscles at the nape of the neck. The very voluminous brain-case of man suffices to counterbalance the weight of the much reduced maxillary part, almost without the aid of muscles or special ligaments, and the head balances itself on the vertebral column (Fig. [2]).
This equilibrium being almost perfect, necessitates but very thin and flexible ligaments in the articulation of the two occipital condyles of the skull on the atlas. The slight muscles to be found behind the articulation are there only to counterbalance the trifling tendency of the head to fall forward.
In connection with this point, we must remember that Broca and several other anthropologists see, on the contrary, in the biped attitude, one of the conditions of the development of the brain, as that attitude alone assures the free use of the hands and extended range of vision. Somewhat analogous ideas have lately been put forward by men of science of the first rank like Munro and Turner.[14]
FIG. 1.—Skull of Gorilla, one-fourth actual size.
a, spinous processes of cervical vertebræ;
b, cranial crests, sagittal and occipital.
In any case, let us remember in regard to this point, that at birth man still bears traces of his quadrupedal origin; he has then scarcely any curves in the vertebral column. The cervical curve only shows itself at the time when the child begins to “hold up its head,” in the sitting posture to which it gradually becomes accustomed—that is to say about the third month. On the other hand, as soon as the child begins to walk (the second year), the prevertebral muscles and those of the loins act upon the lower regions of the spine and produce the lumbar curve.
Thus, perhaps, the chief fact which determines the erect attitude so characteristic of man is the excessive development of his brain, and the consequent development of the brain-case.