Deciduous Teeth Development/Morphology
Primates, like most eutherian mammals are diphyodont (have two generations of teeth) although some mammals have deciduous teeth that are not replaced (usually the first post-canine tooth), some have lost a few or all of their deciduous teeth, and others shed them all before birth. All deciduous teeth are small enough to begin to form before birth but small immature jaws cannot accommodate large permanent teeth (one reason for diphyodonty). In all Hominids there are 20 deciduous teeth (di2: dc1: dm2 in each quadrant of the mouth). Deciduous molars are, perhaps more correctly, sometimes referred to as deciduous premolars. A successional permanent tooth develops beneath and lingual to each one. Deciduous tooth crowns are whiter, have larger pulp chambers, have thinner enamel, are more bulbous than permanent tooth crowns and lack a sinuous cervical margin. Deciduous tooth roots are proportionately longer than permanent tooth roots relative to their crown height and deciduous molars have more widely splayed roots with little common root trunk. Great ape deciduous incisors are, like their permanent counterparts, larger than human deciduous incisors. Deciduous ape canines are much taller and have a concave posterior border that extends onto a distal talon or talonid, whereas human deciduous canines are narrowest at the cervix and broaden out to become widest in the mid-crown. Deciduous ape canine crowns project beyond the occlusal plane and create a space (diastema pl. diastemata) in the opposing tooth row. This diastema is pre-canine in the upper tooth arch and post-canine in the lower tooth arch. Deciduous molar teeth tend to have more pointed and, in the case of dm2, more equally sized cusps than permanent molars. Their more equal size may reflect the fact that each cusp begins its initial mineralization closer together in time than permanent molar tooth cusps do.
Deciduous teeth are common to mammalian species, but differences in dental development provide significant information about the natural life history and evolutionary strategies of those species.
The presence of deciduous teeth has allowed anthropologists to determine the approximate age at death for a number of early hominin fossils. The ability to distinguish between a juvenile and an entirely different species is critical in assembling the fossil record. Based on the skull of the Nariokotome youth, an example of a juvenile homo erectus, it appears that the first molar eruption occurred later in chronologic age compared to australopithicus, which featured a growth history similar to that of the great apes. In contrast, humans are likely to have acquired our early weaning and extended childhood more recently in our evolutionary history. This early weaning may have allowed for a shorter inter-birth interval in our long-lived, slow growing species (Dean 2009). Tooth wear characteristics are also informative, with greater wear in earlier weaning great apes (gorilla) vs. later weaning species (chimpanzee and orangutans). Paranthropus and Australopithecus africanus juveniles seem to exhibit wearing patterns consistent with early weaning, lending evidence that our life history and shortened interbirth interval may extend farther back than molar eruption suggests (Aiello 1991)
Using data about dental eruption, studies by Shigehara reveal that shorter-lived mammals complete somatic development prior to sexual maturity, while longer-lived primates tend to reach sexual maturity before transition to adult dentition is completed. Life history theorists theorize that long-lived, slow growing mammals offset the risk of dying before somatic maturation by reaching reproductive maturity first. The order of tooth eruption is also significant, with modern humans developing permanent anterior teeth significantly sooner than great apes relative to eruption of the first molar. Strikingly, the Taung child fossil of Australopithecus africanus demonstrates an eruption pattern more consistent with the great apes. Evidence from Taung and other australopithecine fossils indicates that the life history pattern of modern humans may have developed as recently as tens of thousands of years ago rather than millions (Smith 1992).
All humans undergo deciduous teeth shedding. The process provides common ground for comparison between different human communities. Differences in eruption, wear, and defects provide insight into community nutrition, health, and environmental conditions.
Debate continues about why human anterior teeth develop early relative to first molar eruption. The development may be recent in human evolution, and correlates with the extended life span and altered life history that humans experience. Relatively earlier loss of milk teeth in humans may have allowed for shorter interbirth intervals to compensate for our slow growth and extended juvenile stage of development.
Increased risk of mortality during our extended juvenile stage may have selected for rapid progression to fertility and early weaning. The resulting reproductive fitness advantage may have covered the disadvantage of slow growth and accelerated deciduous tooth loss in humans.
Enamel defects in deciduous teeth correlate with physiologic stress during development. Enamel defects are most common on canine teeth, and occur in approximately 90% of gorillas and orangutans compared to 45% of chimpanzees. These defects may correlate with prenatal stress to the mother (malnutrition) or perinatal stress of birth. In addition to interspecies differences, enamel can be tracked longitudinally to evaluate the health of populations (Lukacs 2001).
John Lukacs uses enamel hypoplasia in skeletons from prehistoric Indian Jorwe societies to refute the hypothesis that the Jorwe suffered a significant decline in economic status from 1400-700 BCE. He demonstrates that the early Jorwe deciduous teeth exhibit enamel hypoplasia at double the rate of later Jorwe samples. Strikingly, early Jorwe experienced deciduous canine hypoplasia at rates comparable to modern chimpanzees and impoverished populations (40-45%). Correlations that address environmental conditions, species-specific susceptibility, and maternal health may be informative. The dental record provides a robust and lasting source of information about primate development and human origins (Lukacs 1998).
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Enamel hypoplasia in the deciduous teeth of great apes: variation in prevalence and timing of defects., , Am J Phys Anthropol, 2001 Nov, Volume 116, Issue 3, p.199-208, (2001)
Physiological Stress in Prehistoric India: New Data on Localized Hypoplasia of Primary Canines Linked to Climate and Subsistence Change, , Journal of Archaeological Science, Volume 25, p.571 - 585, (1998)
Life History and the Evolution of Human Maturation, , Evolutionary Anthropology, p.134-142, (1992)
The natural history of deciduous tooth attrition in hominoids, , Journal of Human Evolution, Volume 21, p.397 - 412, (1991)