Though we necessarily compare man with the highest existing apes, we must not suppose that the man-like ape from which the earliest ape-like men developed was in fact a gorilla or a chimpanzee. The survival of the gorilla and chimpanzee at this day necessarily implies that they were not the actual ancestral forms which became modified and superseded in the course of man's development. Very probably the ape (the creature more ape-like than man-like, of which more anon) from which man took his direct descent had already developed a plantigrade foot--that is a foot of which the sole is placed on the ground for support, as it is in gibbons, baboons, and bears, but not in most apes, nor in cats, dogs, sheep, and horses! And probably the hands of that ancestral ape were already used more dexterously in consequence, and the dog teeth were less needed either in fighting or in breaking up food and so had become smaller.
This reflection brings us to the differences between the teeth of a man and those of apes. The face of apes is drawn forward so as to approach in form the "muzzle" of a dog. It is far less muzzle-like in the more man-like apes than in the dog-faced baboons, and in the least civilized living races of man is much less prominent--what is called "prognathous"--than in the highest existing apes. In civilized living races of man it is markedly reduced, so that in the habitual carriage of the head, with the eyes looking forward over a horizontal plane at right angles to the vertical or upright body, the front border of the jaws, in which the chisel-like incisor teeth are set, usually projects but very little beyond the brow or forehead. In Greek sculpture and other examples regarded by us as types of "beauty," the jaws do not project at all. Such a face is called "orthognathous." This modification of the shape of the face is due to the progressive dwindling in the size of the front part of the jaw and its teeth in the series dog, ape, less-civilized man, highly-civilized man, and is accompanied by an increase in the size of the front part of the brain. The number of the teeth and their arrangement in groups are identical in man and the apes. The most important difference is in the size of the front teeth, and especially in the size of the "corner" teeth (one on each side above and below), also called eye-teeth, dog-teeth, or "canines." In the highest apes, as in all monkeys, the canine teeth are very large, and even tusk-like in the males, projecting above the horizonal line formed by the crowns of the other teeth. This projecting of the canine teeth results in their not meeting one another point to point when the jaws are closed, but necessitates one, the lower, shutting in front of the other, and a space is left in the row of teeth, both in the upper and the lower jaw, for this interlocking of the great canines. It is called a "diastema." Man stands in strong contrast to the apes in this respect. His canines do not project beyond the level of the neighbouring teeth, and there is no "diastema" or gap in either the upper or lower row of his teeth.[9] There is no trace of such a gap nor any excess of size of the canines in any living race of men, and what is more remarkable, the jaws of very ancient prehistoric men which have been found in the Middle Pleistocene--the Neander or Moustierian men as well as the more ancient jaw from Heidelberg (see p. 286)--do not show any difference in this respect from the most advanced European race. On the other hand, it is one of the most remarkable features presented by the recently discovered "Piltdown" lower jaw that it had a larger canine tooth than that of any recent or fossil man, and consequently a gap or "diastema" in the row of teeth (see Chap. XXX). This difference between men and apes is all the more marked since the grinders or cheek teeth (called also molars) of man and the higher apes agree very closely, each to each in order of their position, in the pattern formed by the irregular surface of the crown. There are some slight differences in relative size and in the order of their "cutting" or growth, but these are trivial. The jaws of man show their derivation by gradual dwindling from the larger projecting jaws seen in apes and monkeys, in the close setting (that is to say, "crowding") of the teeth, and also in the dwindling and late "cutting" of the last tooth, in each jaw above and below, which we call the wisdom tooth. The "wisdom teeth" are in the higher races of men on their way to total disappearance. In lower races of men they are larger than in the higher, and in the man-like apes are of full size, and there is plenty of room in the jaw for them.