C. Loring Brace

Structural Reduction in Evolution

1. While there is no doubt that currently accepted factors (mutation, selection, genetic drift, and migration) (Neel and Schull, 1954; Dobzhansky, 1955a) play a major role in determining the direction of evolution, it is suggested that a process of considerable importance has been overlooked. The probable effect of mutations will be towards structural reduction. If the structure controlled by the locus in question has no adaptive significance, then it will be reduced in the course of time. 2. It is suggested that some of the major and formerly unexplained changes which have occurred in human evolution are the results of probable mutation effect. Reduction in the size of the teeth and face and of the skeletal and muscular systems may have been brought about by such a mechanism, as a result of changes in the principal human adaptive mechanism, culture. The rise and distribution of depigmentation is treated in like manner. 3. Finally it is urged that changes in all outstanding morphological characteristics be reviewed, and considered in the light of changes in the major selective factors relating to the survival of the organisms under consideration.

Tooth Wear and Culture: A Survey of Tooth Functions Among Some Prehistoric Populations [and Comments and Reply]

Studies of hominid fossils have frequently reported that one of their outstanding characteristics is their heavily worn teeth. Many skeletal remains of modern man also show this condition of dental attrition, which is probably related to certain cultural activities. The varieties of foods consumed by primitive man and the specialized tool functions of the teeth have left significant marks in the form of worn occlusal surfaces over the dental arches. This paper discusses some of the functions of the teeth indicated by these marks and suggests that tooth wear should be studied carefully in order to gain significant information about the activities of past populations.

Gradual Change In Human Tooth Size In The Late Pleistocene And Post‐Pleistocene

Starting with the onset of the last glaciation approximately 100,000 years ago and continuing to the end of the Late Pleistocene approximately 10,000 years ago, human tooth size began to reduce at a rate of 1% every 2,000 years. Both the mesial‐distal and the buccal‐lingual dimensions of mandibular and maxillary teeth were undergoing the same rate of reduction. From the beginning of the Post‐Pleistocene until the present, the overall rate of dental reduction doubled, becoming approximately 1% per thousand years. Buccal‐lingual dimensions are now reducing twice as fast as mesial‐distal dimensions, and maxillary teeth are reducing at an even more rapid rate than mandibular teeth. Late Pleistocene rates are comparable in Europe and the Middle East. The Post‐Pleistocene rates are also the same for Europe, the Middle East, China, Japan, and Southeast Asia. It is suggested that the cookery at the beginning of the Late Pleistocene allowed the earlier changes to occur. The use of pottery within the last 10,000 years further reduced the amount of selection that had previously maintained usable tooth substance. Reduction then occurred as a consequence of the Probable Mutation Effect (Brace, 1963; McKee, 1984).

Krapina, “Classic” Neanderthals, and the evolution of the European face☆☆☆

Abstract Except for the front end of the dental arch, tooth size remained at approximately the same level throughout the Middle Pleistocene. The Krapina Neanderthals at the end of the last interglacial differed from Homo erectus only in having larger front teeth. From that time on, tooth size in populations at the northern edge of the area of human occupation in the Old World has reduced approximately in proportion to the time elapsed. The “Classic” Neanderthals of western Europe, in fact, have teeth that are 15% smaller than those of the earlier Krapina Neanderthals and only 5% larger than the early Upper Palaeolithic. Reduction since the early Upper Palaeolithic has proceeded another full 20%. It is suggested that the development of heated stone cooking in the Mousterian, originally for the purpose of thawing frozen food, reduced the forces of selection that had previously maintained tooth size during the Middle Pleistocene. The operation of the Probable Mutation Effect, then produced the observed reductions.

The Potassium-Argon Dating of Late Cenozoic Rocks in East Africa and Italy [and Comments and Reply]

A technique for the potassium-argon dating of high potassium feldspars of less than 50,000 years age is described. The technique is applied to the obtaining of high precision ages in the time-range 60,000-2,000,000 years. Sufficient data are presented to show that the time-scale of Plio-Pleistocene glaciations is greater than 10 years and that the time-scale of hominoids capable of fashioning tools by the working of stone is at least 1.75 10 years. Several other points on the time-scale of human evolution are presented. The time-scale of rift faulting in Kenya is established and the ages of several Italian volcanoes are presented.

Old World sources of the first New World human inhabitants: A comparative craniofacial view

Human craniofacial data were used to assess the similarities and differences between recent and prehistoric Old World samples, and between these samples and a similar representation of samples from the New World. The data were analyzed by the neighbor-joining clustering procedure, assisted by bootstrapping and by canonical discriminant analysis score plots. The first entrants to the Western Hemisphere of maybe 15,000 years ago gave rise to the continuing native inhabitants south of the U.S.–Canadian border. These show no close association with any known mainland Asian population. Instead they show ties to the Ainu of Hokkaido and their Jomon predecessors in prehistoric Japan and to the Polynesians of remote Oceania. All of these also have ties to the Pleistocene and recent inhabitants of Europe and may represent an extension from a Late Pleistocene continuum of people across the northern fringe of the Old World. With roots in both the northwest and the northeast, these people can be described as Eurasian. The route of entry to the New World was at the northwestern edge. In contrast, the Inuit (Eskimo), the Aleut, and the Na-Dene speakers who had penetrated as far as the American Southwest within the last 1,000 years show more similarities to the mainland populations of East Asia. Although both the earlier and later arrivals in the New World show a mixture of traits characteristic of the northern edge of Old World occupation and the Chinese core of mainland Asia, the proportion of the latter is greater for the more recent entrants.

Australian Tooth-Size Clines and the Death of a Stereotype [and Comments and Reply]

Tooth size in Australia ran from a minimum in the Cape York Peninsula of northern Queensland to a maximum in the Murray Basin. The available data suggest that the earliest Australians possessed large jaws and teeth and that subsequently genes for smaller tooth size entered Australia from the northeast corner a model which is consistent with the evidence for the spread of a variety of cultural and technological items. While the evidence is tentative at best, it is consistent with the view that more developed food-preparation techniques had ocurred outside of Australia, allowing dental reduction to occur. The spread of these elements into Australia may be symbolized by the influx of the small-tool tradition early in the Holocene, and it may have been made possible by associated resource-utilization techniques that promote survival in areas previously sparsely utilized, such as the central desert and the coastal margins. This would account for the tooth-size gradient visible down the east coast and from Cape York to the western desert. The largest teeth in Australia survived in just those areas most favorable to human habitation where one would expect the genetic contribution of the earliest inhabitants to be most prominently represented. Tasmanian affinities are clearly with southeastern Australia. After initial occupation, Australia was subject to a continuous trickle of cultural-biological influence from the north rather than having been the receptacle for specific waves, migrations, or invasions.

Sexual dimorphism and human tooth size differences

Abstract Current male/female differences in tooth size are due to the male/female differences in body bulk that exist in any given human population. These differences are residues of the sexual dimorphism that was maintained for adaptive reasons during the Middle Pleistocene. Late in the Pleistocene the development of food processing techniques led to the reduction of both male and female dental dimensions. Dental sexual dimorphism, however, was maintained until the very end of the Pleistocene when the hunting of large game animals by crude techniques was replaced by a focus on great numbers of small game caught by more sophisticated means and by an increasing utilization of plant foods. The subsequent reduction in dimorphism represents the actions of the Probable Mutation Effect operating under conditions of relaxed selection. The conclusion offered is that the smallest degree of sexual dimorphism visible in the modern world is to be found among those populations that are separated by the greatest interval of time from precursors who depended for their survival on a Pleistocene big game hunting mode of subsistence.

Oceanic Tooth-Size Variation as a Reflection of Biological and Cultural Mixing [and Comments and Reply]

Tooth size in Oceania varies from a minimum equivalent to the figure for the pre-Chinese inhabitants of Taiwan to a maximum equivalent to the figure for large-toothed Australian Aborigines. The minimum figure is found among the easternmost and weternmost inhabitants, and the maximum figure occurs in the highlands of New Guinea. Elsewhere, intermediate figures are evident, and it is apparent that the populations in which they can be observed display phenotypes that are intermediate in pigmentation and hair form between those on the Asian mainland and those whose identification with an equatorial habitat can be traced back into the Pleistocene. In addition, it is evident that the small-toothed populations speak languages that are most closely related to hypothetical Proto-Austronesian While the largest-toothed populations speak languages that are not related to Austronesian at all. To the extent that tooth size rises above the level of that found in the most typical Autronesian-speakers, the language deviates from hypothetical Proto-Austronesian. This suggests that the original population of New Guinea and some adjacent islands continued in situ from well back into the Pleistocene. Within the last 4,000 years, populations which had been shaped by long-term residence on the Asian mainland moved out into the Pacific via Taiwan and the Philippines. Superior navigation and resource utilization capabilities allowed them to colonize previously uninhabited islands maintaining much of their original phenotype, but where they encountered the earlier inhabitants on the larger Melanesian landmasses they display the effects of cultural and phenotypic mixing in proportion to the contribution of the two main parent populations.

Did La Ferrassie I Use His Teeth as a Tool? [and Comments and Reply]

Ten years ago, in CURRENT ANTHROPOLOGY, Brace advanced the hypothesis that the classic Neandertals were not evolutionary dead ends, but rather represented populations that evolved into modern man. Braces selective mechanism to account for the supposed reduction of the classic Neandertal dentition and face was founded on the assumption that the rounding wear on Neandertal incisors resulted from extensive use of these teeth as tools. In this paper, observations on incisor wear and occlusion in Bushmen, a fossil cercopithecoid, and La Ferrassie I are presented as evidence against the hypothesis that rounded wear on front teeth indicates habitual use of the teeth as tools. It is suggested that the rounded wear more likely results from contact with abrasives in the diet. The observations on tooth wear are interpreted to controvert Braces selective mechanism for the supposed reduction of the classic Neandertal dentition and face.