This article examines the changes in the dentition of modern humans. The changes will be reviewed and the theories about the causes will be discussed. It will be argued that food preparations with cooking have minimized the importance of teeth for survival and this has contributed to the reduction in the size of the dentition.
II. TIME PERIODS IN ARCHAEOLOGY
Anthropologists and others have divided the elapse time since the advent of stone tools 2.5 million years ago into three 'lithic' (Gr. = of stone) time frames. Read this next statement carefully: these are time periods defined by material culture.
The Paleolithic (the 'old stone age') encompass the late Pliocene and all of the Pleistocene up to the end of the last Ice Ages about 12,000 years ago. The Mesolithic (the 'middle stone age') which began at the end of the Pleistocene, is seen as a time of transition leading to the Neolithic (the 'new stone age). The terms used here is based largely on archaeology in Europe.
(My note: the time frames for these vary in other regions on Earth. The terms are different also. The Mesolithic in European prehistory corresponds to the Archaic in North America and the Jomon in Japan. These different terms reflect independent traditions of scholarship.)
(1) Upper Paleolithic. This marks the appearance of fully modern humans in Europe about 35,000 years ago. The hallmark of this time is big game hunting, often with the atlatl. It is the last of the Ice Age, a time of cave art, figurines, and a wide variety of finely made tools. Some authors will refer to this time period as 'Late Pleistocene'.
(2) Mesolithic. In Europe, this begins about 11,000 years ago. It is marked by the end of the Ice Age, the extinction of large game, an adaptation to a more varied food base and the earliest pre adaptations to farming which is soon to come.
(3) Neolithic. This time period began about 9,000 years ago in Europe. Plants and animals are domesticated, finely polished stone tools are made, pottery and village life appear. The Mesolithic, Neolithic, and the time until today is sometimes called the 'recent' or the Holocene (Gr kainos= recent).
II. THE STUDY OF CHANGES IN TOOTH SIZE
There is a widespread assumption that human evolution effectively ceased with the appearance of anatomically modern Homo sapiens about 35,000 years ago. Usually, this assumption is based on theoretical expectations rather than empirical analysis of actual data distributed through time. Changes in the dentition since humans first appeared in Europe are well documented.
Teeth are an excellent opportunity for study. Teeth are preserved in greater numbers than are other parts of the skeleton. They are a close reflection of the genotype, they are affected by the forces of natural selection, and they are easily treated by quantitative methods.
These remarks about genotype require clarification now. Some human traits, such as ABO blood type are controlled by single genes and are Mendelian in heritability. They are not affected by environment. Others, such as achieved adult standing height are polygenic. These traits can be modified by environment. (My note: we discuss simple and polygenic inheritance in unit 5.1 The Two Worlds of Genetics. To my knowledge, there are no dental traits other than agenesis that are controlled by single gene. Dental traits, like nearly all morphological traits, are polygenic.)
Teeth, since they develop with the jaws are pretty much unaffected by environment (with some exceptions, such as trauma or severe nutritional stress). Teeth are thus nice models for study. They have a further advantage: they are easy to measure, both in living and in fossil forms.
The measurements taken for the studies cited in this article were mesiodistal diameter and labiolingual diameter. Crown heights were not measured since those would be variable due to occlusal tooth wear. The authors recognize that proximal wear does reduce tooth diameter. They therefore excluded teeth that showed significant proximal wear from their studies. Both right and left sides were measured. Groups from various archaeological time periods were measured and compared.
III. DIFFERENCES IN TOOTH SIZE
There are major differences in tooth size between the living populations of the world. The bar graph in figure one illustrates the variations seen in living populations.
There are a number of problems in comparing different populations and drawing conclusions from those differences. First, is the issue of sexual dimorphism. It is virtually impossible to sort loose teeth by sex. With living populations, the observer can make a judgement, but this becomes much more difficult with incomplete skeletons--especially when postcranial remains are absent.
Why does it matter? When teeth are lumped together in a unisex fashion, that embedded variable makes comparison for other variables less distinct.
Second is the relationship of tooth size to body size. It is widely accepted that there is a low but positive correlation between tooth size and body size within any given populations. Some authors argue that tooth size has become 'decoupled' from body size during the recent evolutionary past as the rate of tooth size decline has accelerated.
Human tooth size has undergone a clear-cut reduction during the Upper Paleolithic and the rate of that reduction has accelerated since the end of the last Ice Age. With the onset of the last glaciation in Europe, dental reduction began to occur among the northernmost inhabitants of the Old World for which we have evidence.
Tooth size began a gradual reduction during the Upper Paleolithic at the rate of roughly 1% per 2,000 years until the end of the Pleistocene about 10,000 years ago. Beginning about 10,000 years ago, the rate of reduction seems to have doubled to about 1% every 1,000 years. See Figure 3 .
In Europe, both tooth size and body size have decreased over the last 50,000 years. When tooth size is considered in relation to body size, the reduction in the dentition is even more dramatic than the reduction in body bulk.
Australian aborigines have not followed an identical path. Even though Australian tooth size has been decreasing since the late Pleistocene, that tooth size has remained larger in proportion to body size than was true even for 'classic' Neandertals of Western Europe.
IV. WHY HAS REDUCTION OCCURRED?
There is no single explanation of the mechanisms in dental reduction. Since human teeth have become smaller, it raises a question: what advantage is there to having smaller, weaker, and more friable teeth? Several hypotheses are presented here.
(1) Dental reduction is a result of facial reduction. This in effect says that forces of selection don't effect teeth directly. Also, there is a logical dilemma if we insist that teeth are directly affected by forces of natural selection.
(2) Smaller teeth conserve precious biological resources; therefore, smaller teeth are advantageous. A similar argument is made for why we can't synthesize vitamin C: in our ancestral past it was readily available. Early primates 'deleted' the ability to manufacture vitamin C as a biological economy measure.
(3) As early humans acquired culinary skills, teeth ceased to have survival value and they are just dwindling away due to the Probable Mutation Effect (PME). This hypothesis attributes the reduction of teeth in size and number to cultural factors.
The PME was proposed by Brace in 1963. It has provoked plenty of discussion and criticism. I cite it here because it is interesting for the light it sheds on some of our basic beliefs in biology.
Does every structure have a function? What about the appendix? What is the advantage of having the Carabelli trait? Are they there for survival value, or are they . . . just there? Ernst Mayer has pointed out that it is very difficult to prove the selective value of many characters.
Evolution is about loss of structures as well as acquiring them. Besides teeth, humans have lost tails, body hair, vitamin C synthesis, and heavy brow ridges above the eyes. Snakes have lost legs and some birds have become flightless. Many subterranean animals have lost their eyesight. What forces drive their loss? There are two general ideas. Both date from the time of Darwin.
One says that periodic mutations, like 'wear and tear', will gradually reduce an unneeded structure, until it simply fades away. The PME falls into this camp.
The other general idea says that loss of structures is a positive economic benefit to an organism. The loss of vitamin C synthesis capability previously mentioned is an example.
For a lively and creative article on eyes, mole rats and the subject of loss, see the Jared Diamond article cited below.
What the Probable Mutation Effect says is this: the most likely result of mutation will be a reduction of phenotypic manifestation of the trait under the control of one or more (molecular DNA) loci. When selection is suspended, the structure simply deteriorates to a lesser version of its original condition.
Although an interesting model, the PME has been theoreticallyh and empirically rejected as a valid mechanism by many authorities. For a thoughtful review, see Calagno (1989).
(My note: if every structure had a purpose, you'd expect the human genome to be particularly clean and efficient. It isn't. Apparently the human genome is a genetic junkyard. More than 90% of it isn't even transcribed. It may be just genetic debris left over from structures no longer utilized.)
V. THE IMPACT OF TECHNOLOGY ON TOOTH SIZE
Food preparation technology may have eliminated the forces that previously maintained tooth size. It is true that teeth have had paramasticatory functions--such as processing animal skins. While teeth have a primary use in feeding, the important issue is not so much the food itself but what was done to it before it was eaten.
We are used to cartoon images of cave men cooking whole animals over an open air camp fire. Historically, however, more sophisticated techniques have been used. One is the earth oven--a technique observable today in a Polynesian luau where a pig is baked. If you have ever baked a potato in the hot earth and coals under a campfire, you understand the technique. Meat cooked in such a fashion becomes quite tender.
A variant of heated stone cookery is hot stone boiling, a technique used by many Native Americans at the time of contact.
Once pottery was developed, food preparation probably accelerated the reduction of tooth size. Along with pottery as part of the 'neolithic package' was efficient stone pounding and grinding. The beneficiaries of these techniques could survive without any teeth at all. In the Upper Paleolithic and before, edentulous human remains are uncommon. After the onset of the Neolithic, burial of edentulous persons is usual. Survival without teeth had become easier.
Over the last 7,000 years in Europe, the Middle East, China, Japan and Southeast Asia--the rate of dental reduction is effectively the same. One intriguing issue remains: why have Australian aborigines lagged behind in molar tooth size reduction? The arrival of technology may be the answer. The earth oven may have arrived there last, and from that it could be predicted that the Australian aborigines were the last to begin the Late Pleistocene trajectory of human reduction. In China, it the greatest reduction in tooth size is seen in those peoples who had the earliest Neolithic cultural phase.
If you've kept your sense of humor while reading this article, you might enjoy this poetry from Brace et al, 1991.
Now dental reduction is fast,
And Man shall be toothless at last;
He eschews his chews
And will choose to lose
The teeth that he had in the past.
VI. OTHER CHANGES IN TEETH
Associated with the overall dental reduction is a trend for substantial decrease in sexual dimorphism in tooth size. The evidence suggests that reduction in sexual dimorphism is more related to changes in male dentitions than to changes in female tooth size. During the Upper Paleolithic and Mesolithic, males become more gracile (slender, slim, or slight) at a faster rate than females. This trend in males may be related to changes in hunting patterns and the types of animals exploited.
..... CJ '99
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