These notes are a reading guide for chapter 3. This section is about how the size and shape of teeth are recorded, symmetry and asymmetry, heritability, sexual dimorphism, correlations, and dental reduction in humans over time. These notes will stress important concepts and make correlations that are not readily evident in your reading.
Getting into dental anthropology isn't a neat and tidy process. These notes are intended to ease that process. In this chapter, two types of variation are discussed.
Metric variation is evaluated by making measurements of teeth. It isn't as easy as you may think.
Non-metric variation is the subjective judgement of certain features. One feature well known to clinicians is the Carabelli trait. Another non-metric feature is the shovel-shaped incisor. There are many others of interest to the dental anthropologist. Several are discussed in this unit.
I. Introduction, pp 68-70
Teeth are uniquely suited for study. Tooth crowns erupt full sized into the mouth child early in life. They are easily accessible by just looking in the mouth! Impressions can be made and these teeth can be easily compared with teeth from antiquity. Finally, teeth survive well in the fossil record.
There are limitations in the study of teeth.
(1) Determining the boundaries between species by teeth alone is very difficult. Missing is important data about behavior, breeding, and overall appearances are absent.
(2) Most living mammals show overall differences between the sexes. Male and female gibbons are about the same size, while male gorillas are twice as big as females.
(3) Even when large numbers of teeth are available from a cemetery collection, it may not be a fair sample of the larger population.
II. Overall Tooth Dimensions, Metric Variation, pp 70-85
The familiar maximum measurements of the teeth are the ones most commonly used. Mesiodistal measurement, called crown diameter, is often used. Proximal wear reduces this dimension, and introduces statistical error. Buccolingual measurements are unaffected by proximal wear, but become inaccurate in excessively worn teeth. Crown height measurements are most affected by wear and are therefore the least useful in population studies.
The section on dental indices touches on several important issues.
(1) Metric measurements have normal (Gaussian) distributions. Nearly every trait measured in dental anthropology is polygenic, distributes in a normal 'bell curve,' and must be treated statistically. We will expand greatly on this issue when we get to our units on genetics and human variation.
(2) Garn and others have studied correlations of various dental measurements. Read pages 72-73 carefully. One, on page 73 is especially intriguing. In modern populations, it has been shown that anterior tooth diameters vary inversely with posterior teeth diameters. This may have implications for paleoanthropology. In the week one Notes, observe that Paranthropus has large cheek teeth compared to the anteriors. Somewhat the reverse condition is true for Neandertal which has relatively large anterior teeth compared to posterior teeth.
(3) Now, you clinicians, look at page 73 where only low correlation is shown between deciduous teeth and permanent teeth diameters. Question: what does this say about the accuracy of the 'leeway space' measurement so carefully taught to dental students?
The Butler Field Theory and the Clone Theory are two time-honored theories to explain variation in tooth diameters and/or heterodonty. We cover the Field Theory in unit 2.1.
Take a moment and examine the crown diameter plots on pages 74, 76, and 77. This is a way of presenting crown diameters that is usually unfamiliar to clinical dentists. The plots are a way of visually comparing the relative sizes of different teeth numbered one to eight from the midline in the maxilla or the mandible. Tooth sizes for an individual or composites for a population can be presented-always read carefully for what the author is attempting to illustrate.
Bilateral symmetry of the dentition is taken for granted, but asymmetry brings up several intriguing ideas. We begin with two definitions.
Directional asymmetry is where one side is consistently larger in a population.
Fluctuating (random) asymmetry is more what you would expect; it is where the largest side varies between individuals. It tends to be greater than directional asymmetry. Fluctuating asymmetry may be increased by Selyian stress during development.
Later, in the section on human growth and variation we will revisit the effect of disease and stress on human development. For now we can say that during growth enamel hypoplasias, Harris lines, and fluctuating asymmetry of teeth are indicators of disease and/or nutritional stress. Many animal studies have suggested that bilateral symmetry is a key index of health in choosing a biologically fit mate.
Many studies have been made of inheritance of crown diameters. In humans, crown diameters tend to be highly heritable, but different teeth have different heritabilities. The genetic control of crown size is apparently spread over several chromosomes.
In living humans, body size sexual dimorphism is about 10%. Estimates for Homo erectus are about 30%. In Australopithecines, males were about twice the size of females.
What about teeth in modern humans? The greatest sexual dimorphism is in lower canines (7.3%) followed by upper canines. Premolars are the least dimorphic. Sexing loose human teeth is very difficult. The average size in individual teeth is very small, on the order of 0.4-0.5 mm.
Canine crown diameters for early Australopithecine males and females plot out as two distinct populations. For modern humans, canine mesiodistal diameters plot out as a single, asymmetrical mode.
In living humans, the correlation between crown diameters and body size is low. The largest teeth today are found amongst Australian Aborigines. Europeans and Asians are at the smaller end of the scale. For an interesting current article on tooth size and its implications for ethnic origins, see Schnutenhaus, S. and Rosing, F. "World Variation of Tooth Size" in Alt, K., Rosing, F., and Teschler-Nicola, M. Dental Anthropology Fundamentals, Limits, and Prospects. New York SpringerWien, 1998
The section 'Crown diameters and hominid evolution' pp 83-85 is a topic we will revisit later in this course. This topic touches on important questions relevant to clinical practice. Why are third molars so variable, often impacted, and frequently missing? Why are permanent upper lateral incisors so variable, or on occasion missing. More broadly, what changes are taking place in the human evolution.
Like other sections in this book, the author is often hard to pin down, but these two general trends seem to be taking place in human evolution.
(1) Cheek tooth diameters progressively decrease from early Homo, Homo erectus, Neandertals, and Homo sapiens.
(2) Crown diameters in modern humans appear to have been steadily decreasing over the last 40,000 years. It seems to be part of a general reduction in robustness. You might call it the 'couch potato trend.' It has affected males more than females.
Why has this occurred? It seems to be related to culture. The trend has apparently increased in the last 10,000 years with the introduction of domestication, food preparation, urbanization, and all the benefits (or afflictions) of modern life. Anthropologists call biological evolution coordinated with culture biocultural evolution.
How is dental reduction mediated at the genetic level? The probable mutation effect has been suggested. It is Murphy's law applied to genes. In brief, it says that the genes for traits no longer needed for survival will go awry.
III. Non-metrical Variation in Tooth Form, pp 85-105
The most famous of these features is the Carabelli cusp on the mesiolingual cusp of upper first permanent molars or upper second deciduous molars. This trait illustrates the special problems of dealing with non-metric traits. Metric traits vary in their expression, so standardization is necessary. Albert Dahlberg at the University of Chicago was the first to offer casts called 'plaques' which provided consistent standards for independent researchers. Dahlberg passed the responsibility for these onto Christy Turner II a number of years ago; others are also now in circulation. Non-metric traits have been especially useful in tracking human migrations. Hrdlicka and Turner have used incisor shoveling to trace American Indian origins from Siberia.
Metric traits and non-metric traits are polygenic. Several genes appear to be responsible their production.
We cover genetics in a future unit, but this is an appropriate time to introduce you to the concept of penetrance. The genes for a trait may vary in their phenotypic expression. When a trait is only weakly expressed, this condition is called partial penetrance.