Estimating age at death
Development and Eruption Sequence
The regular pattern, sequence, and timing of development and eruption of the dentition provides arguably the most accurate method of estimating age at death of children and adolescents. From the time that the first deciduous teeth begin to emerge at approximately 6 months, to the eruption of the M3 at around age 18 years, several patterns emerge than can easily categorize a child or adolescent into general age groups.18, 32 The eruption sequence of the molars from M1 to M3 occurs typically at 6, 12, and 18 years, respectively, which can allow one to easily categorize subadults through visual examination. This method is particularly useful for subadults when estimated dental age can be checked against the associated skeleton’s symphyseal age (i.e., age determination based upon the fusing of the ends of bones).33
A seminal chart of dental development was published by Schour and Massler in JADA in 1941, which categorized eruption patterns into 22 stages starting from 5 months in utero to 35 years.34 A number of other charts followed in years to come using data from more populations, including Ubelaker’s 1989 method and the more recent London Atlas of Human Tooth Development and Eruption.35, 36
Tooth Wear and Attrition
“Our ancestors wore the enamel off of their teeth,”6 so levels of occlusal wear have been used to estimate age in adult skeletons from pre-medieval and non-Western populations.18, 37 A number of studies have shown that premodern skeletons can be aged reliably by wear patterns,16, 18 but these studies likely underestimate individuals over 50 years of age, due to the nature of attrition and tooth loss over the lifespan.18
Population histories and affinities
Teeth have been used as a proxy for genetic studies, due to their high heritability, when estimating population relationships and affinities.9 There is geographic and historical variation in tooth dimensions, overall size, number of teeth (hypo- and hyperdontia) and crown and root morphology.38 Thus, the distribution frequencies of these metric and non-metric traits can contribute to the determination of an individual’s ancestry.
Metric Dental Variation
Crown dimensions (particularly mesiodistal and buccolingual) vary among human populations, but cannot be explained by a simple geographic or environmental explanation. There has been a distinct reduction in crown sizes throughout human evolution,11 but there is wide intraregional variation in size among modern populations: Australian natives have the largest total crown size among modern humans, along with Neanderthals and Homo erectus,11, 38-40 while the smallest crowns are distributed as widely as the Lapps of Scandinavia and the San Bushmen of southern Africa.11, 38 One study has found that on a global level, overall tooth size indicates large geographic population distinctions (but with wide intra-regional variation), with native populations of Australia having the largest teeth, followed by native North American and Sub-Saharan Africans; East Asians, Indians, and Europeans were found to have the smallest teeth.40 Despite the high heritability of tooth dimensions, overall tooth size may be affected by prenatal and developmental stress.41
Non-Metric Dental Traits
Non-metric traits, particularly accessory cusps, fissure patterns and incisor shoveling have been shown to be more indicative of population history and relationships, and frequencies of such traits vary according to major racial groupings.11, 39 The tubercle of Carabelli, or Carabelli’s cusp, is a supplemental tubercle, or fifth cusp, along the mesiolingual cusp of maxillary molars ranging in expression from a small furrow to a full cusp.11, 19 It is common among Europeans (up to 85%) and least common among Pacific Islanders.
Seminal research on nonmetric dental trait complexes have distinguished between Asian and Caucasian dental complexes and have further divided Asian traits into a Sinodont pattern of “trait intensification” among Southeast Asians, Pacific Islanders, and the Jomon.39 Additionally, three major groups of the native peoples of the Americas can be distinguished by their expression of a three-rooted mandibular first molar: Eskimo-Aleut, Na-Dene, and “others.”39
Incisor shoveling, described as a measurably deep lingual fossa, as well as double-shoveling, are found more frequently in East/Northeast Asia and the Americas, and least frequently in North Africa and Sub-Saharan Africa.42 As predicted by a number of anthropological studies, Sub-Saharan Africans show the highest level of intraregional variation in non-metric dental traits,42 which is consistent with the Out-of-Africa theory of modern human origins.41
Diets and health
Teeth are particularly vulnerable to physical and biological trauma, unlike other skeletal elements, because they come into direct contact with the external environment. Investigating pathological or deliberate alterations to human teeth can tell us not only about the individual being examined, but the customs, diets, and health of a population. Modifications to teeth, particularly dental work, are tremendously valuable in forensic identification.
Occupational, cultural, and lifestyle habits can be determined from physical modifications to tooth structure, from intentional evulsion to observable grooves from tooth-picking.11, 18 Grooves appearing on posterior teeth have also been attributed to the removal of sinewy material from animal prey.2, 11 Intentional mutilation of teeth—commonly the anterior teeth—for cosmetic or cultural purposes has been practiced for several thousands of years in the Americas, Africa, and parts of Asia.2
Diets can be inferred by patterns of tooth wear.18, 38 Generally, hunter-gatherers exhibit more occlusal wear than agriculturalists, particularly in incisors which were often used to process hides. Agriculturalists that used stone grinding tools to process grains show increased pitting43 and heavily abraded posterior teeth.38, 44 While tooth wear (attrition and abrasion) has drastically decreased since the Middle Ages, there has been a concomitant increase in caries, non-carious cervical lesions, and erosion,43 resulting from dietary as well as food-processing changes.
The advent of agriculture and reliance on fermentable carbohydrates has led to a drastic increase in dental caries,18 which became a “major health scourge” after 1500.38 In the past, social status and sex may have affected the distribution of caries because of an unequal distribution of high-sugar foods18; in modern times, females consistently exhibit higher caries rates than males,38, 45 with explanatory hypotheses ranging from more frequent snacking to hormonal changes during pregnancy.45, 46
Calculus, or calcified plaque, is commonly found in archaeological remains in both supragingival and subgingival forms.11, 20, 44 Calculus is easily observable and is indicative of periodontal disease, while periodontitis can be observed by porosity or pitting of the alveolar bone or the presence of abscesses.43, 44 Periodontal disease has been found throughout antiquity and as far back as 3 mya in a specimen of Australopithecus africanus.44 Presence of nutritional deficiencies such as that indicated by scurvy has been associated with periodontal disease in historical studies, as have cultural practices such as cocoa-leaf and betel-nut chewing.43, 44
The enamel matrix of teeth is secreted by ameloblasts in a circadian rhythm, leaving microscopic lines in enamel structure called cross striations, which form circaseptan bands referred to as brown striae of Retzius.8, 41 The rhythmic slowing-down of the production of ameloblasts evident on the surface of the tooth produces perikymata; periods of pathology or other nutritional disturbances or periods of stress produce noticeable striae often referred to as Wilson bands8, 11 or linear enamel hyposplasia (LEH).39, 41, 43 These hypoplastic lines can tell us that a period of environmental stress, illness, or nutritional deficiency occurred at a certain point in an individual’s life and can indicate a marked period of stress or famine for a community.39, 41, 43, 47 LEH may be difficult to assess in modern populations, in which teeth are “heavily abraded by toothbrushing.”48
An additional result of the incremental deposition of the enamel matrix during tooth development is that components of hydroxyapatite crystals are subject to replacement by elements to which they are exposed during mineralization; the resulting striations can be analyzed histologically to determine presence of trace elements or isotopes, that can be used to reconstruct dietary lifestyle changes, geographic movements, stressful environmental periods, or exposure to toxicity in an individual’s life.49, 50 In particular, lead and strontium isotopes carry heavy geographic signals, which can be used to determine an individual’s geographic origin or migrations.7, 51
Barium/calcium (Ba/Ca) ratios have been used to investigate weaning times among primates. Barium is highly concentrated in mother’s milk and, like lead, follows calcium pathways; nursing infants absorb barium from mother’s milk much more easily than from other dietary sources.52, 53 Research on Ba/Ca ratios from the shed teeth of orangutans found that after an initial drop in barium levels between 16 and 18 months, the ratio varied cyclically with seasonal availability of fruits and other foods. This suggests that during lean times, the growing orangutans supplement their diet with mother’s milk, extending the weaning period well into childhood, sometimes as long as into the 9th year.52 This finding corresponds to other research on primates in marginal environments, and sheds light on the causes of the shortened weaning time of modern and archaic humans. A 2013 study of a juvenile Neanderthal tooth showed a drop in barium after 7.5 months, falling to prenatal levels at 1.2 years, which is well within the range of weaning practices of modern humans.53