SECTION II. From London Lancet, July 18, 1903
Changes which the Jaws and Teeth of Man have undergone during Man’s Evolution from his Anthropoid Ancestors
During man’s progress upwards from the anthropoid his diet has undergone a progressive change, and a parallel adaptation has taken place in his jaws and teeth. Dietetically considered, we may divide his evolutionary career into the following epochs[8]: (1) the anthropoid stage; (2) the pre-cooking human stage; (3) the cooking pre-agricultural stage; (4) the early agricultural stage; and (5) the late agricultural stage.
1. The anthropoid stage.—The diet of man’s anthropoid ancestors was probably much the same as is that of existing anthropoid apes; it consisted, namely, of raw vegetable and animal food, necessitating a vigorous use of the maxillary apparatus. This latter, we may assume, was of the type belonging to the anthropoids—i. e., the jaws were massive and markedly prognathic; the denture was the same as it is in existing man, but the teeth were larger, especially the upper canines, which served as weapons of offence and defence; the third molars (the wisdom teeth) were as large as the other molars and were provided with three fangs, and there was an ample portion of alveolar ridge behind them; there was no chin. No doubt the massiveness and the marked prognathism, which characterised the jaws at this stage, served other ends than that of mastication; it is obvious that projecting jaws and teeth are much more effectual for seizing and lacerating prey than are the orthognathic jaws of modern man.
2. The pre-cooking human stage extends from the time man’s ancestors first assumed the human form till they learned to apply fire in the preparation of their food. During all this period the jaws and teeth were probably used as much, or almost as much, for mastication as during the anthropoid stage; raw animal food had to be torn from the bones, the latter had to be crunched, while the bulk of the raw vegetable food needed then no less than it needs now prolonged and vigorous mastication in order to liberate the starch and other nutritive ingredients from their undigestible cellulose envelopes.[9] Nevertheless, the jaws and teeth underwent considerable change during this period, for not only were they with every advance in intelligence called less and less into requisition for purposes of offence and defence, but the jaws, at least, became materially modified in correlation with the expanding cranial cavity and in connection with the assumption of the erect posture. It is, I think, rather for these reasons than in consequence of alterations in the nature of the food that the masticatory apparatus now gradually lost its more bestial aspect and assumed an essentially human type, becoming towards the close of the period much the same as may be observed among the most primitive peoples now living.
3. The pre-agricultural cooking period.—The characters of the maxillary apparatus belonging to this period are still available for study, the aboriginal Australians, the Bushmen, Negritos, and many Esquimaux not having yet emerged from it. So far as mastication is concerned, cooking influences vegetable far more than animal food, for it not only softens it but by rupturing the undigestible cellulose chambers and liberating their contents relieves mastication of one of its essential functions. Wherefore, with the advent of cooking, man’s jaws and teeth began to get smaller, and they have continued to diminish in size up to the present time. No great diminution, however, took place at first, inasmuch as the diet still continued to be largely animal (and prior to the use of knives and forks such food had to be torn by the teeth), while the coarse vegetable food of this date, even when cooked, still needed laborious mastication. The chief differences between the maxillary apparatus of this early cooking age as compared with that of the present day are as follows: the jaws of the earlier period—e. g., in the aboriginal Australian—are more massive, and their sagittal diameter is greater, giving rise to decided prognathism, the teeth for the most part are larger and stronger, the third molars being nearly, if not quite, as big as the other molars, and provided with three fangs, while there is a considerable portion of alveolar ridge behind them. The third molars, however, show a decided tendency to be smaller than the rest, and the alveolar ridge behind them is less marked than in the previous period, features, I doubt not, attributable to the influence of cooking in diminishing mastication. Dental caries is rare and is chiefly met with in the third molars.
4. The early agricultural age.—All the existing primitive races which have attained to the cultivation of the soil may be regarded as belonging to this period. Previously to it man was mainly carnivorous, owing to the comparatively limited quantity of vegetable food available, so long as the supply was left to nature alone; but when by cultivation this supply was increased and, at the same time, rendered more constant and certain, he gradually became less carnivorous and more vegetarian in his diet. The result of agriculture, however, is not only to increase the supply of vegetable food, but to diminish its fibrous, cellulosic ingredients, and thus to render it more easily masticated. Hence at this stage we find the maxillary apparatus becoming smaller than in the previous period, although the difference as shown—e. g., by the examinations of the skulls of the African negroes and the Melanesians—is less pronounced than we might perhaps have anticipated; prognathism is not so decided, the jaws are smaller, also the teeth, especially the third molars, which now for the first time show a tendency to be furnished with two instead of three fangs, while the alveolar ridge behind them is distinctly shorter than in the preceding period. Dental caries, hitherto rare, now becomes more frequent.
5. The late agricultural period.—A mid-agricultural period might be described, but I shall take no account of it here, but pass on to a consideration of the late agricultural period—that, namely, in which we ourselves live. The chief characteristic of the food of this period is its softness. Cooked animal food requires, indeed, more mastication than raw, but the vegetable food of to-day, owing to the combined effects of improved agriculture, and skilful milling and cooking, is so soft as to excite comparatively little mastication. The present may, in fact, be described as the age of pap. Hence the jaws and teeth are now called upon to perform far less work than in any earlier stage of our evolution, and there has taken place in consequence a great diminution in their size, more especially in the size of the jaws, so that there is now often no room for the teeth to take up their normal positions, and there is generally a complete absence of alveolar ridge behind the last molars. The latter are, moreover, apt to be very small or even absent, while dental caries is alarmingly frequent.
It will thus be seen that from the period of the anthropoids to the present time, a progressive change in the size and shape of the jaws and teeth has been taking place, a change which is to be explained by (1) the cessation of the need for using them for offensive and defensive purposes; (2) the growing capacity of the cranium and the assumption of the erect position; (3) the progressive alteration in man’s diet; and probably also (4) considerations of beauty. The first three factors have operated through natural selection, the last through sexual selection, which has come into play, I would suggest, chiefly within recent times. Probably the most pronounced change which has taken place in the jaws during the agricultural periods has been the suppression of prognathism which, in the woman especially, is very unsightly, and tends to diminish the likelihood of marriage.