I. Introduction pp 148-153
Teeth are often the only part of the body that survives to be excavated in a cemetery. Enamel is of special interest since it carries with it information on diet, oral hygiene, dentistry, stress, occupation, cultural behavior, and subsistence (food-getting).
Enamel forms in a sequential manner beginning with the future cusp tips/incisal edges and proceeds apically. Any injury or severe disease will leave a permanent record in the enamel. The resulting defects remain visual artifacts are a record of the formative process.
II. Incremental Structures in Enamel pp 153-164
Human enamel forms at the rate of approximately 4 microns per day. The formative periodicity has been confirmed by periodic marker injections, but it is questioned whether they exhibit a circadian (L = about + day) rhythm.
Of special interest are the brown striae of Retzius which appear 30-40 microns apart and correspond to intervals of approximately seven days. In humans, that rhythm is called circaseptan rhythm (L = around + seven). The average human incisor contains about 150 brown striae. Human canines contain about 180, while molars contain 120-150.
The first 30-40 brown striae are obscured under the incisal edge. The remainder terminate on the surface of the teeth. Unworn incisor labial surfaces often reveal the brown striae as the familiar perikymata.
Why do the brown striae appear in intervals of approximately seven days? There is no clear answer, although some underlying biological regularity is suspected. Body rhythms have become a serious study in human biology well beyond the popular conception of 'biorhythms.' When we get to human variation, we'll see documentation of daily, seasonal, and annual cycles in human growth and development.
III. Ageing with Perikymata
An interesting line of research has emerged using the brown striae of Retzius and perikymata in determining the age at death of contemporary humans and fossil hominids. In this discussion I will follow the logical thread of argument. A lively debate about ageing with perikymata can be found in the literature-if you are interested.
(1) Each spacing of the brown striae of Retzius is about 7 days.
(2) The total time of enamel calcification for any tooth can be calculated using a ground section of a tooth to count the perikymata, thus:
(3a) A less time-consuming and destructive technique is to count perikymata on the unsectioned tooth.. The brown striae that DO terminate on the tooth surface appear as perikymata. They can be visually counted. Each perikymata represents a time cycle of about 7 days between brown striae.
(3b) The cuspal brown striae that DON'T terminate on the tooth surface as perikymata can be estimated if you know the thickness of cuspal enamel. Thus:
(4) Now, another idea: IF you know the age of enamel calcification onset, THEN you can calculate the age of death for an individual who died when enamel formation was incomplete. Let us illustrate with a hypothetical example:
IF an incisor tooth begins enamel calcification at the day of birth, THEN you can calculate the age of a deceased infant by counting the brown striae in that incompletely formed tooth.
60 brown striae x 7 = 420 days of age
zero days + 420 days of age is 1 year 2 months
Keep in mind folks that the truth of any conclusion rests on the validity of its underlying assumptions AND moving to the conclusion in a logical manner.
The age of initial calcification of teeth histologically in humans is well documented, so the 'start date' for any given tooth is known.
Is it sequence and timing of dental development the same in the great apes? No, it is not. Why does it matter? It does in determining the age of death in youthful fossil hominids and in determining the rate at which they complete their life histories. The dental age at death is important in two famous fossils: the Australopithecine 'Taung Baby' found by Dart in 1924, and the Homo erectus 'Turkana Boy.' Initially, human standards were applied-incorrectly as we now know.
IV. Correlating the Eruption of M1 (First Permanent Molar) in Hominoids
A. Life history
The dentition is important in growth and development. If we accept that teeth are essential for life after weaning, then we can see that teeth must integrate into the broad scheme of growth and development. Deciduous teeth become important after weaning, permanent teeth replace deciduous ones in serial fashion to maintain the biting surface, and the last of the molars can't emerge until facial growth provides room for them.
In primates, is there a correlation between the eruption age for M1 and other developmental stages? Indeed there is. Plotted graphically for several species, the age of M1 eruption correlates quite well with weaning. In a more general way, the 'life history periods' of gestation, age at weaning, birth space, and longevity are reflected in M1age of eruption.
In living species, the age of M1 eruption correlates very well with brain size. The larger the brain, the later the eruption of M1.
The increase in brain size is one of the important trends in hominid evolution. IF the evidence of living primates can apply to hominids, THEN as hominids got brainier, their life histories were stretched out (and M1 erupted later and later).
B. Incremental Lines and 'Setting the Clock' for fossil hominids.
Are you following all of this? Let us summarize what we've discussed and put this all together for you by following the argument step by step.
1. Enamel forms in an incremental fashion.
2. The brown striae of Retzius seen in enamel seem to reflect time intervals of about seven days.
3. IF the brown striae are counted in a ground section and the total of them multiplied by 7 (days), THEN the total time for enamel formation until completion or death can then be determined.
4. Brown striae terminate on the surface as perikymata. IF the perikymata are totaled and to that added an estimate of the hidden cuspal brown striae, THEN total time for enamel formation can be determined. (See Mann et al below for discussion of the problems and limits of these calculations. They say that the counts grossly underestimate the actual formation time.
5. Onset time for enamel formation in individual teeth has been established in humans. This age can be added to calculated enamel formation times to determine the age of death of an individual who dies while crown enamel is forming.
6. Dental eruption time for M1 in apes is earlier than it is in humans.
7. M1 eruption age in primates correlates well with brain size. There is a well-established increase in brain size from Australopithecines on down to us (about 4x).
8. Brain size is known for earlier hominids. IF brain size is smaller, then M1 should be at a younger age in such primates. The eruption time for the 'Taung baby' M1 and its age of death should be at 3-4 years, not at the human six years.
9. The 'clock' for onset of enamel formation and eruption must be 'speeded up' in smaller hominids because they go through their life histories more quickly than do we.
..... CJ '99
The following resources were used for this article
Mann, A., Monge, J., and Lampl, M. "Investigation Into the Relationship Between Perikymata Counts and Crown Formation Times" AJPA (86) October, 1991 pp 175-188.
Smith, B. Holly "Dental Development and the Evolution of Life History in Hominidae" AJPA (86) October, 1991 pp 157-174.
AJPA Vol 86 No 2 contains several additional articles about ageing specimens using perikymata counts.