I. Jaws and Teeth
Jaws--particularly lower jaws--and teeth are by far the most available fossil evidence for hominid evolution. Since teeth are used for food processing, they reveal much about subsistence and behavior. The dental apparatus is very conservative; dramatic changes are rare. For instance, human and ape dentitions retain more or less the same pattern established more than 20 million years ago. These patterns are helpful in tracing the course of hominid evolution.
II. Dental Differences Between Modern Humans and Apes
Let us summarize some basic differences between modern humans and the other great apes.
(1) Apes, particularly males, have large canines that essentially prevent side-to-side jaw movement. In contrast, humans have small canines that do not project beyond the occlusal plane, permitting a side-to-side 'milling' motion.
(My comment:: dentists take this side-to-side mobility in humans for granted. In elephants, the lower jaw moves anteroposteriorly in relation to the upper jaw instead of side-to-side as in humans. The lamellae on the molar occlusal surface are arranged transversely for maximum effectiveness as the opposing molars move against each other.)
(2) Sexual dimorphism of canine teeth is typical of apes where there is significant body size difference between males and females. This is true of chimpanzee, gorilla, and orangutan. The size difference between human male and female canine teeth, though measurable, is rather small. Human males do not have large canine teeth that project beyond the occlusal plane as they do in the apes.
(3) Humans shed their deciduous teeth and receive their permanent teeth at a significantly older age (six years vs 3 years for M1) than do large apes. You will recall that humans have long, drawn-out childhoods. The time for dental development has similarly been extended.
This was significant for Raymond Dart's 'Taung Child.' A more ape-like time schedule of development is more appropriate for Taung. That creature is now estimated to have been three to four years of age at the time of its death.
(4) The sequence of tooth eruption is different in humans than it is in the apes. In humans, permanent canines erupt before permanent second molars. In apes it is the reverse: ape permanent canines erupt after the permanent second molars.
The ape and human eruption patterns can be summarized as shown below:
Ape.........M1, I1, I2, M2, P3, P4, C , M3
Human.....M1, I1, I2, P3, C , P4, M2, M3
(5) The dental arch in humans forms a catenary arch. In apes, the cheek teeth are in 'boxlike' parallel rows. Also, humans have an uninterrupted tooth row while apes have primate spaces to accommodate the articulation of the large canine teeth.
(My note: humans have a continuous tooth row ((no spaces)) in the adult dentition. In about half of children, however, there are diastemata (plural of diastema) in the deciduous dentition. The correspond in location to the ape diastemata. The spaces in children are known in dental literature as 'primate spaces.')
(6) Humans (and their hominid ancestors) have thick enamel while apes tend to have thin enamel. One exception is the orangutan which has enamel of intermediate thickness.
III. Applications in Study of Hominid Evolution
Recent reevaluation of the Taung child suggests that it was probably about three to four years old instead of six. It seems very likely to have followed an ape developmental age pattern instead of a modern human one.
The eruption patterns in Homo erectus which lived from 1.5 million to somewhere about 400,000 years ago are equivocal but seem rather ape-like.
A Neandertal child from 60,000 years ago suggest a human pattern. Just as this article is being prepared, news reports tell of DNA studies that show significant differences between Neandertal mitochondrial DNA and our own.
Thin enamel has been described as an adaptation for fruit eating. Thick enamel is a response to processing tougher plant foods. The dichotomy may be misleading because the thickness of the enamel layer depends on two factors: the speed at which it is deposited and the length of time for enamel formation.
Enamel in teeth forms in layers. Evidence of that development is the Striae of Retzius. They are more prominent after birth beginning with the neonatal line.
To grasp the next idea, think of the annual rings seen in a freshly cut tree trunk. When they are widely spaced there is rapid growth, when they are narrow, there was slow growth. In tooth enamel, the pace of deposition at the "slow" daily secretion is <2.5 microns per day in contrast to the "fast" pattern rate of 5-7 microns per day.
In general, humans have fast growing enamel and apes have slow growing enamel--but apes seem to have fast growing enamel for part of the enamel forming cycle. It seems that hominids followed a distinctly apelike pattern of dental development until relatively recently in evolutionary history.
These variations in growth rate of enamel are useful for classifying hominid ancestors. Since enamel forming patterns--like the dentition in general seem to be conservative, their distinctive features are useful in classification. Think of these features as like the stuff in your attic: the stuff we keep tells about our past. The same applies to the formation of enamel.
We end this brief article with a commentary on tooth enamel wear patterns. Those patterns are a signature of diet that show up nicely on scanning electron microscope studies. The general implications are these:
(1) All early hominids appear to fit into the fruit-eating frugivore category, along with modern chimpanzees and orangutans. The pattern seen is a rather smooth enamel surface into which are etched a few pits and scratches.
(2) A major shift occurs, however with Homo erectus, whose enamel is heavily pitted and scratched. It is significant that tooth wear pattens indicate somewhat of an abrupt change in hominid activities. You will recall that Homo erectus was the first hominid to move out of Africa, use fire, showed systematic tool making, and a significant brain expansion. The change in wear pattern is believed to reflect the adoption of a meat eating diet. It is believed that the early meat eaters were scavengers
..... CJ '98
Sources and further reading
Berkovitz et al Color Atlas and Textbook of Oral Anatomy. St. Louis: Mosby Year Book, 1992.
Beynon, A. et al "On Thick and Thin Enamel in Hominoids" Amer. J. Phys. Anthrop. 86: 295-309 (1991)
Hillson, S. Dental Anthropology New York: Cambridge University Press, 1996.
Lewin, R. Human Evolution an Illustrated Introduction, 3rd ed. Cambridge: Blackwell Scientific Publications, Inc., 1993.