I was recently working on the methods chapter of my dissertation (I’ll pause so that you may lever your jaw up off of the floor – “JB? Working? On her dissertation?”) when I stumbled upon a fantastic old school anthropology quote that struck my fancy. I’m currently mired in the midst of my section on adult aging, and given the fragmentary nature of many of the Marroquíes Bajos remains, traditional techniques that rely on cranial sutural closure, the auricular surface of the os coxa, or the pubic symphyseal face are rarely useful. However, I do have a lot of teeth, over 750 in the two mortuary locales I’ve analyzed so far. Accordingly, I’ve been carefully scoring dental attrition (a more fanciful term for the enamel wear that gradually exposes dentin over the course of an individual’s lifetime) for all teeth, in the hope that this will prove informative about the age distribution of my mortuary population. However, as AEW Miles pointed out over 50 years ago, there are other things besides age that can affect the severity of dental attrition:
“It is usual to attribute the greater degrees of tooth wear to the nature of the food; namely to food of fibrous or tough character requiring much chewing, or to contamination of food with abrasive substances such as sand, soil or ashes. Quite apart from these considerations, however, chewing habits and conventions in early times are likely to have been quite different from those of the present day. Those living less sophisticated lives, with few man-made pleasures, both in the past and the present time, would be inclined to spend more time savouring the act of mastication. This could lead to increased wear in the same way that habitual tooth grinding, as a nervous habit, leads to marked wear of the teeth” (17).
The reason I am so enamoured of this quote is because as a graduate student, I devote an inordinate amount of my time to “savouring the act of mastication”, both because I need some form of ready bribery to motivate myself to write my dissertation, and because I am in a part of the world where the food is abundant and delicious. Since I’m also as unsophisticated as all get out, I really do meet all of the requirements necessary to be true “savourer of mastication”.
Despite Miles’ slightly patronizing stance when it comes to the joys of chewing, he made significant contributions to the development of standards for aging individuals based on dental attrition. Miles came up with his strategy in response to a thorny bioarchaeological problem. He had access to around 190 individuals from an Anglo-Saxon burial site at Breedon-on-the-Hill in Leicestershire, a sizeable sample for an interested osteologist. The catch was that these individuals were represented solely as skulls or fragments of skulls, and “owing to circumstances associated with advancing quarrying operations it was not possible to recover the post-cranial skeletons”(1962:881)….Which I take is archaeological parlance for “the skeletons themselves have also been quarried out”.
In order to investigate any aspects of demography from this 7th- 9th century A.D. population, Miles had to be clever, and invent a way to assess age without having any access to post-crania. Working from a simple observation about dental eruption schedules, Miles was able to come up with a defensible strategy. His logic was simple, but elegant, and can effectively be broken down as follows:
1. The timing of mandibular molar eruption is relatively predictable, with M1 erupting at around 6 years of age, M2 erupting at around 12 years of age, and M3 erupting at around 18 years of age;
2. It follows that if an individual has their M1, M2 or M3 erupting, you will be able to produce a fairly narrow and accurate estimate of age for them. Miles was also able to rely on Schour and Massler’s (1941) standards for dental development, which provided tight estimates of age for subadult individuals using the development and eruption schedules of all the teeth, not only the molars;
3. Accordingly, the degree of wear on the dentition of subadult individuals can be compared to the individual’s estimated age to produce an approximate rate of wear. For example, if the M1 erupts at 6 years of age, and the M2 erupts at 12 years of age, the M1 in an individual whose M2s are just erupted will have wear showing six years of functional age.
4. The functional age of a specific tooth can then be added to the age of eruption for that tooth in order to produce an age estimate for the individual in question. So in the case above, with an M1 showing six years of functional age, you would add: 6 (functional age) + 6 (age of eruption of tooth class) to produce an age estimate of 12 years.
5. The rates of wear calculated from subadults of known age-at-death can then be used to extrapolate the functional (and then chronological) age of older individuals from the same population, assuming that rates of wear are consistent within the population.
Importantly, during his data collection Miles observed that not all classes of molars wear at the same rate. In particular,“…it takes only six years for M1 to reach a state of wear that it takes M2 and M3 respectively six and a half and seven years to reach…[so]…if a third molar is found which matches a first molar which shows 18 years of functional age, by a simple calculation it is possible to say that the third molar is 21 years of functional age”(1962:20). By relying on the wear gradient 6:6.5:7 (M1:M2:M3), starting with the wear-scores of sub-adults with known ages-at-death, Miles was able to estimate the age of older individuals based on the relative amount of wear on each category of tooth.
The basic principles underlying the Miles method continue to be used to this day by bioarchaeologists. Wear is assessed visually,and the the amount of visible dentin is scored qualitatively for the teeth of the individual you are examining. Osteologists often use visual depictions like the Brothwell chart shown below as a benchmark against which to compare the dentition they are evaluating. (I prefer to use the modification of the Scott system outlined in Standards, because I always have a copy of Standards on hand).
Finally, while the Miles method is a useful strategy for estimating adult age using dental attrition, it has two significant difficulties. First, it relies on samples which contain a large number of subadults with relatively complete dentition. If your population stems from the 7th-9th century AD in England, you’re all set, but if you’re working on a more fragmentary Copper Age sample in, say, Iberia, this will prove a little bit more problematic. Second, Miles based his eruption times on a combination of data derived from a slightly modified Schour and Massler chart (1941), which was collected from a modern reference population, and padded it out with additional data collected from contemporary subjects. This reference data-set is, as a result, not entirely commensurate with prehistoric, pre-industrial populations who would have consumed different types of foods and led different types of lives than modern populations.
Fortunately, Gilmore and Grote have created a modification of the method to address these problems; one that applies specifically to samples “where juveniles and post-cranial elements are under-represented” (2012:182), with estimates of molar eruption times deliberately derived from non-industrial populations.
The Gilmore and Grote modification of the Miles method calculates the average difference in wear between an individual’s M1s and M2, then sums these values for the sample population to establish a mean and standard deviation for the sample M1-M2 difference. The wear score for each class of tooth is produced by averaging all scores available for both maxillary and mandibular categories of the same tooth (e.g., for a given individual i, the M1 wear score is the average score of all quadrants for the LM1, RM1, LM1 and RM1). All individuals with extremely low M1 wear scores are excluded from calculating the population wear rate, to avoid producing artificially constrained values based on the small M1-M2 differences that would result from very recently erupted M1s. After winnowing the sample, average wear per year is calculated for the sample population by dividing the population average difference by the number of years between the eruption of the first and second molars. Once the population wear rate has been calculated, Gilmore and Grote follow Miles’ basic strategy of adding the age of eruption to functional age to estimate age at death.
You need wear scores for all the individuals in your population in order to use the Gilmore and Grote modification of the Miles method (so as to be able to calculate the average wear per year for each tooth category). Accordingly, I won’t be able to use this to estimate the age of the adults in the Marroquíes Bajos mortuary population until the end of my dissertation research, but I will provide a brief update and, I hope, a discussion of the basic demographic structure of my population.
Until then, I’ll have to figure out something else to sink my teeth into….see what I did there?
Buikstra, J. E. and D.J. Ubelaker. 1994 Standards for Data Collection from Human Skeletal Remains, Edited by J.E. Buikstra and D.J. Ubelaker. Arkansas Archaeological Survey Research Series No.44, Fayetteville.
Gilmore CC, & Grote MN (2012). Estimating age from adult occlusal wear: a modification of the miles method. American journal of physical anthropology, 149 (2), 181-92 PMID: 22763560
Miles AE (1962). Assessment of the Ages of a Population of Anglo-Saxons from Their Dentitions. Proceedings of the Royal Society of Medicine, 55 (10), 881-6 PMID: 19994189
As a note, Schour and Massler’s seminal article on the development of the human dentition is available open-access online, here, as part of the Journal of the American Dental Association Centennial series. Download it and add it to your library!
Image Credits: Image of Brothwell’s scoring system found here.
Photo Credits: All photos were taken at the Museo de Jaén in summer 2014 and are used with the permission of the museum.