Dental wear refers to the gradual loss of dental hard tissues, and its patterns are greatly affected by dietary habits. This study examined dental wear in the Jomon people, prehistoric hunter-gatherers of the Japanese archipelago, to determine whether molar wear rate was relatively fast or slow compared with that of other hunter-gatherer populations with different dietary habits. To evaluate the rate of dental wear, we used average score differences between adjacent molars. Considering that this parameter is independent of age, we compared the Jomon wear rate value with those of the other hunter-gatherers and pastoralist populations available in the literature. Results indicated that the Jomon people had a significantly lower rate of wear compared to populations in arid regions with a higher reliance on abrasive plant-based diets. Furthermore, the Jomon wear rate was comparable to or lower than that of populations in cold climate with less dependency on plant foods. We suggest that the low rate of wear seen in the Jomon people stems from the use of pottery, accompanied by an increased reliance on carbohydrates, possibly associated with changes in cooking methods.

Dental wear refers to the gradual loss of dental hard tissues. It is primarily caused by a combination of tooth-on-tooth contact (attrition), interaction between the tooth and food or other exogenous substances (abrasion), and chemical dissolution (erosion). The mechanical causes of wear, attrition and abrasion, have been widely discussed (Kaidonis et al., 1993; Larsen, 2015), with the latter frequently considered the predominant cause of tissue loss (Romero et al., 2019). The rate of enamel abrasion is influenced by the type and amount of abrasives in food and the frequency of chewing. Therefore, dental wear patterns reflect differences in diets and cultural practices, and anthropologists have investigated past dietary habits across populations and time through comparisons of its patterns (Molnar, 1971, 1972; Smith, 1972, 1984; Hinton, 1982; Powell, 1985; Rose and Ungar, 1998; Kaifu, 1999; Bernal et al., 2007; Deter, 2009; Larsen, 2015; Godinho et al., 2023).

When comparing patterns of tooth wear across populations, it is crucial to consider the difference of age profile among populations. This is because tooth wear progresses with age, resulting in a significant correlation between one’s age and the absolute degree of tooth wear (Mays, 2015). Therefore, it is not appropriate to directly compare the average degrees of tooth wear among skeletal populations because these populations typically vary in their age distributions. On the other hand, the rate of wear has been shown to be independent of age (Smith, 1972; Miles, 2001). This leads to the assumption that the rate at which teeth wear relative to each other is independent of the absolute amount of wear, and hence the rate of wear can be used as an indicator for comparing groups. By examining the differences in degrees of wear between adjacent teeth of an individual (wear gradient), it is possible to estimate the rate of wear. Researchers have analyzed wear rate by taking advantage of the natural eruption schedule with an interval of about 6 years between the first, second, and third molars (M1, M2, and M3) (Smith, 1972; Lunt, 1978; Scott and Turner, 1988; Benfer and Edwards, 1991; Miles, 2001; Gilmore and Grote, 2012). Consider M1 and M2, for instance: because M1 erupts about 6 years earlier than M2, M1 functions for 6 years longer than M2, the difference in wear between M1 and M2 will correspond to 6 years of wear, and this relationship is maintained until the end of the functional life of the teeth. Therefore, assuming that the wear rates of M1 and M2 are equivalent, the difference in degree of wear between M1 and M2 remains constant. The difference is expected to be larger in populations with higher overall wear rates, making it a useful indicator of the rate of wear. Although some studies found no differences in wear rates between molars (Nowell, 1978; Dreier, 1994; Mays, 2002), others identified small to substantial differences (Miles, 1962; Akpata, 1975; Santini et al., 1990). In this context, although further research is needed to validate this assumption, the wear gradient between molars, in combination with ordinal scoring methods, has been used to estimate the rate of wear (Lunt, 1978; Benfer and Edwards, 1991; Constandse-Westermann, 1997; Gilmore and Grote, 2012). Notably, Scott’s scoring system (Scott, 1979) has been supported by several studies (Benfer and Edwards, 1991; Gilmore and Grote, 2012).

The links between diet, masticatory behavior, and the degree of occlusal surface wear have been demonstrated through population comparative studies with different subsistence patterns (Larsen, 2015). Particularly in the context of transition from hunting-gathering to farming, researchers have extensively investigated temporal changes in patterns of tooth wear. Consistently, these studies have shown that the absolute degree of wear tends to decrease with the transition from hunting-gathering to farming (Hinton, 1981; Smith, 1984; Inoue et al., 1986; Kaifu, 1999; Bernal et al., 2007; Deter, 2009; Larsen, 2015; Godinho et al., 2023). The primary explanation for this shift lies in the transition from a diet predominantly consisting of hard-textured and potentially highly abrasive foods to softer-textured foods. Moreover, it has been suggested that changes in diet from those relying on non-domesticated plants to an intensified use of domesticated foods, along with associated changes in food preparation techniques, also contributed to the decrease in tooth wear (Smith, 1984; Powell, 1985; Larsen, 2015).

In contrast to this commonly accepted notion of hunter-gatherers with higher rates of tooth wear, the values of variations in tooth wear among different hunter-gatherer populations are reported by considering their distinct subsistence practices (Molnar, 1971, 1972; Tomenchuk and Mayhall, 1979; Hinton, 1981; Sciulli, 1997; Bernal et al., 2007; Lieverse et al., 2007; Littleton, 2017). Gilmore and Grote calculated rates of wear for hunter-gatherers and pastoralist populations from a variety of latitudes with diverse population histories and diets in their study of age estimation using molar wear (Gilmore and Grote, 2012). The populations from dry and temperate climates, including the Afalou, Native Californian, Khoi-San, and Australian Aborigine, have the highest rates of wear, while those from high latitudes and very cold climates, including Alaskan, Buriats, Chukchi, and Sami, have relatively lower rates of wear. It has been noted that the higher rates of wear in populations such as the former groups are related to the tendency to incorporate a higher proportion of abrasive plant foods, as well as to the fact that gritty contaminants are more likely to be introduced in dry environments (Brothwell, 1963; Molnar, 1972; Scott and Turner, 1988; Gilmore and Grote, 2012). On the other hand, the latter groups tend to incorporate more meat and maritime resources. Frozen and dried meats are prone to grit contamination during the cooking process and, because of their toughness, require more frequent chewing, causing severe dental wear (Molnar, 1972; Scott and Turner, 1988). Nevertheless, their rates of wear still tend to be relatively lower than in those populations with greater reliance on plant foods. An increased reliance on pastoralism in the Eurasian groups and contact with industrialized societies can be other factors that reduce wear rates (Molnar, 1972; Gilmore and Grote, 2012).

In this study we examine the rate of molar wear of the Jomon people and compared it with the wear rates of other hunter-gatherers with different dietary habits. The Jomon people are Holocene prehistoric hunter-gatherers of the Japanese archipelago and are characterized by their having one of the earliest pottery-making traditions in the world (Imamura, 1996). The dentition of the Jomon has been shown to exhibit the heaviest wear compared with subsequent populations in the Japanese archipelago, and in some cases the crowns may be completely worn away (Inoue et al., 1986; Kaifu, 1999; Fujita and Ogura, 2009). However, it is not well known whether the rate of dental wear of the Jomon is relatively high or low compared with that of other hunter-gatherers. The diet of the Jomon consisted of a variety of wild seasonal marine and/or terrestrial food resources (Habu, 2004; Kobayashi et al., 2004). Considering the use of pottery for cooking and boiling by the Jomon, as well as their high caries rate (Turner, 1979; Fujita, 1995), we can assume that their dietary and/or cooking strategy affected their rate of dental wear. Differences in patterns of dental wear can help understand masticatory behavior in human populations, and assessing the wear rate of the Jomon will help to understand the nature of dental wear variation in hunter-gatherer societies.

We employed a total of 180 mandibular molar rows obtained from adult Jomon individuals, sourced from the archaeological collections curated by the University Museum, The University of Tokyo. The Jomon people inhabited the entire Japanese archipelago for nearly 10000 years. However, the materials used in this study exclusively consist of those excavated from shell-mound sites in the Tohoku, Kanto, and Tokai regions during the Middle to Final Jomon periods (c. 5400–2300 BP) (Figure 1, Table 1).

Figure 1. Locations of the Jomon sites used in this study. All specimens were excavated from shell-mound sites in either the Tohoku, Kanto, or Tokai regions. Numbers in parentheses indicate the number of specimens.

To assess the rate of occlusal wear, we used the gradient of the occlusal wear between M1 and M2. We specifically selected specimens in which both M1 and M2 were preserved on either the left or right sides, aiming to secure a representative sample that captures wear patterns characteristic of the Jomon people of the region/time period (e.g. Figure 2B–E). Certain exclusion criteria were applied to ensure the reliability of our analysis. This process resulted in the removal of 41 out of 221 samples, leaving us with sample size of 180 for estimating the rate of occlusal wear. First, caution should be given in the presence of caries or crown chipping. As caries and crown chipping are common, their presence alone was not a reason for exclusion; but when the damage of caries or chipping was significant enough to occupy more than half of a quadrant of the occlusal surface, making accurate scoring difficult and potentially deviating from the normal wear pattern, such cases (18 individuals) were excluded from the analysis. Additionally, we considered the impact of tooth loss, particularly the absence of third molars and opposing maxillary molars. Individuals with missing third molars or maxillary molars due to antemortem tooth loss were excluded (18 individuals), even if the M1 and M2 were well preserved, as this could disrupt the overall wear pattern of the remaining dentition. Subadult individuals, whose third molars had not yet erupted and/or been in function, were also excluded from the analysis beforehand (e.g. Figure 2A). In cases where an individual lacked a third molar due to postmortem tooth loss, we relied on the condition of the first and second molars to determine whether the individual had reached the age at which the wear gradient would be established. Finally, individuals with completely worn enamel rims were excluded from the study (8 individuals). This exclusion was necessary because their teeth had reached the maximum possible wear, and their condition would not accurately represent the wear gradient prior to extreme wear (e.g. Figure 2F).

Figure 2. The progress of occlusal wear of mandibular molar row observed in the Jomon people. From left to right: first, second, and third molar (M1, M2, and M3). Mesial: left; lingual: top. Specimens B–E are included, while A and F are excluded from the calculations because they are not considered to represent the natural wear pattern (see text for details).

Occlusal wear of the mandibular molars was assessed using Scott’s scoring technique (Scott, 1979). When both the left and right sides were well preserved, the average score of them was used. If only one side was preserved or another side was poorly preserved, only the better-preserved side was used for analysis. This scoring method visually divides the occlusal surface into four quadrants, assigning a score of 1–10 for each quadrant. The overall tooth score is the sum of the scores of the four quadrants, ranging from 4 to 40. This scoring technique is straightforward, easy to use, and one of the most widely cited methods, and is recommended by an osteological textbook (Buikstra and Ubelaker, 1994). Overall, it has been shown to be highly repeatable with low inter-observer error, but there are some considerations to be made. First, inter-observer error is high in the early stages of wear before dentine exposure (up to a score of 4 per quadrant) (Lagan and Ehrlich, 2021). Second, as described in Scott’s original paper, there could be a consistent tendency that one observer would score teeth slightly lower or higher than another observer (Scott, 1979). In this study, the former reservation can be minimal, as we excluded subadult individuals in the very early stages of wear (wear gradient not yet established). Regarding the latter, scoring was done by a single author (K.N.) for consistency.

To determine the age-independent rate of wear from adult individuals, we calculated the gradient of wear scores between the first and second molars as a proxy. The rate of wear for the Jomon as a population was estimated as the average of the score differences calculated for each individual. Previous researchers have also utilized wear gradients between molars to estimate rates of wear in conjunction with ordinal scoring methods (Smith, 1972; Lunt, 1978; Benfer and Edwards, 1991; Constandse-Westermann, 1997; Gilmore and Grote, 2012). Meta-analyses combining data from multiple different studies is difficult because different researchers use different and incompatible dental wear scoring methods (Tochihara, 1957; Molnar, 1971; Scott, 1979; Smith, 1984; Lovejoy, 1985). However, among those scoring methods, Scott’s scoring technique is deemed suitable when estimating rates of wear by calculating the differences of scores between molars (Benfer and Edwards, 1991; Gilmore and Grote, 2012).

After calculating the average and variance of the score differences for the Jomon, we compared them with the data of hunter-gatherers and pastoralist summarized in Gilmore and Grote (2012). In the process of modifying an age estimation method, they calculated the average score difference between molars using Scott’s system for nine different hunter-gatherer and pastoralist populations. The materials consisted of individuals from a variety of latitudes, from dry and/or temperate to high latitudes and cold regions, with diverse population histories; each population conformed to a general geographic area with a broadly consistent diet and lifestyle (Gilmore and Grote, 2012). Supplementary Table 1 shows population summary statistics derived from Gilmore and Grote (2012). Note that we mostly followed their sample selection and removal criteria, although they used the average of the scores for each quadrant, so their published values were multiplied by four to be the sum of the scores for each quadrant when compared with our data.

As the data used for comparison were limited to summary statistics (average and variance of score differences), differences among populations were compared visually, and parametric one-way analysis of variance was performed assuming score differences as a proportional scale. Care must be taken in statistical analyses when using ordinal scales to assess the degree of wear because the intervals between sequential scores are not necessarily equal (Lunt, 1978). However, the Scott’s system is elaborate with a large number of gradations (represented by scores of 4–40), and it has been pointed out that the score per tooth approaches a continuous scale (Scott, 1979; Benfer and Edwards, 1991; Rose and Ungar, 1998).

Figure 3 shows the wear score of the molars for each individual of the Jomon in this study. The relationship between M1 and M2 and between M1 and M3 are plotted against each other. M1 and M2 wear scores are closely correlated with each other (r_s = 0.94). The wear scores are less closely correlated between M1 and M3 (r_s = 0.60), probably due to the variable timing of eruption and morphological variation of the M3. Therefore, only the differences between M1 and M2 were considered in this study.

Figure 3. Molar wear scores plotted against the other molar types for every individual of the Jomon specimens in this study. There is a correlation between the scores, and the score difference is approximately constant regardless of M1 wear score. M3 wear is less closely correlated with M1 than M2.

The average difference between M1 and M2 scores for the Jomon people was 3.27. Table 2 shows the averages and variances for each region (Tohoku, Kanto, and Tokai), and for the combined Jomon sample as a whole. Analysis of variance for the three regions (Figure 1, Table 1) showed no significant differences (P = 0.404).

Figure 4 shows the average differences between M1 and M2 scores for the Jomon people and comparative hunter-gatherers and pastoralist populations. The Jomon (3.27) has the second lowest rate of wear, it is clearly lower than in populations from dry to temperate climates, such as the Afalou (7.88), Native Californians (6.92), Khoi-san (7.04), and Australian Aborigines (6.08). On the other hand, when compared to the high-latitude and cold-climate populations, such as the Alaskans (4.47), Buriats (5.20), Chukchi (3.36), Sami (2.92), and Fuegians (4.92), the rate of wear for the Jomon is similar or mostly lower.

Figure 4. Average score differences in M1 and M2 for the Jomon and comparison populations. Afalou, Native Californian, Khoi-San, and Australian Aborigine are from dry to temperate climates, while the others are from high-latitude and cold-climate regions. The black lines for each bar show ± 1 standard deviations. Populations whose Bonferroni corrected P values are <0.05 as a result of comparisons with the Jomon are marked: *P < 0.05, ***P < 0.001.

Analysis of variance of average score differences among the combined Jomon sample and the comparative populations rejected the null hypothesis of no differences among populations (P = 1.11 × 10^–16, Table 3). Therefore, we further tested whether there was a significant difference in average score differences between the Jomon and each of the comparative populations. The result shows that the Jomon had a smaller average score difference than the Afalou, Native Californian, Khoi-san, Australian Aborigine, and Buriat samples (the null hypotheses of no difference in average score differences between the Jomon and each population were rejected with Bonferroni-corrected P values of 1.1 × 10^–16 < 0.001, 1.7 × 10^–14 < 0.001, 1.1 × 10^–16 < 0.001, 1.9 × 10^–12 < 0.001, and 1.7 × 10^–2 < 0.05, respectively).

Overall, the results indicate that the Jomon people had a relatively low rate of molar wear compared to other hunter-gatherer populations. It is intriguing to note that the rate of molar wear in the Jomon people is not only lower than in world populations living in arid and temperate regions, considered heavily reliant on an abrasive plant-based diet (the Afalou, Native Californian, Khoi-San, and Australian Aborigine), but also comparable to or lower than those populations with potential contact with industrialized societies (Khoi-San, Australian Aborigine, Fuegian, Buriats, Chukchi, and Sami) or having some reliance on dairy pastoralism products (Buriats, Chukchi, and Sami).

Dietary habits have a crucial effect on rates of wear. The abrasiveness of consumed materials and the frequency of chewing are the primary factors. The abrasiveness of ingested materials can be affected by the content of the diet itself, as well as by contaminants such as sand and dust introduced during food preprocessing. Meanwhile, the frequency of chewing is affected by the toughness of the dietary items and the cooking techniques employed. In addition to dietary factors, tooth wear can be influenced by non-dietary practices. For instance, certain hunter-gatherer populations often engage in habitual non-dietary chewing practices that involve putting objects such as bulbs, grass stems, or tough skin into their mouths (Molnar, 1972). Other habits such as chewing tobacco or coca leaves are also known to significantly contribute to tooth wear (Molnar, 1972). Moreover, using teeth as tools for tasks such as tanning tree bark and animal hides can lead to abnormal or highly distinctive wear patterns due to the considerable mechanical load (Larsen, 2015). Some individuals from the Jomon period also displayed abnormal (or atypical) wear patterns, likely caused by non-dietary uses of their teeth. However, these exceptional cases were not included in calculating the overall rate of wear that represents the Jomon people of the region/time period examined in this study. Thus, we focus on dietary and related habits in exploring the factors that explain why the Jomon people had a relatively lower rate of molar wear compared with other hunter-gatherer populations.

The diets for the Jomon people have been assumed to be well-balanced, consisting of a variety of wild seasonal marine and/or terrestrial food resources (Habu, 2004; Kobayashi et al., 2004). Analysis of carbon and nitrogen isotopes in human bone collagen also suggests that the coastal Jomon population diet primarily consisted of a combination of marine products, C3 plants, and terrestrial mammals (Minagawa, 2001; Kusaka et al., 2010). However, regional and temporal variations are evident (Yoneda, 2008; Kusaka et al., 2010). For instance, at the Sanganji shell-mound site in the Tohoku region, there was a smaller proportion of marine product use and potential utilization of C4 plants (Minagawa, 2001). In Chiba Prefecture, Kanto region, an increase in dietary diversity is observed from the Middle to Late Jomon period (Yoneda, 2008). The variable food sources of the Jomon may have caused a significant effect on the dental wear rates of the populations.

In general, plant resources are known to cause more severe dental abrasion than marine products and meat. However, as summarized above, the diet of the Jomon people of the regions and time period studied here did not exhibit a particular bias toward marine products or meat. Additionally, rice, which significantly softens in texture when cooked, had not yet been introduced to the Japanese archipelago. Therefore, available food types alone cannot explain the observed reduction in dental wear rate of the Jomon people. Moreover, the fact that they used stone mortars and pestles for preprocessing plant resources, possibly introducing abrasive exogenous substances, would suggest that their dental wear rate should be higher than that of groups that primarily rely on a meat-based diet, such as the Eskimo. However, we found that the wear rate of the Jomon was unexpectedly low.

Given these findings, we consider another factor in their food preparation methods that plausibly contributed to the relatively low rate of tooth wear among the Jomon people: their pottery-making culture. Tooth wear is greatly affected not only by the material properties of food but also by extra-oral food preprocessing methods (Leigh, 1925; Molnar, 1972; Smith, 1984; Scott and Turner, 1988; Godinho et al., 2023). Boiling and cooking can reduce the toughness of food. This, in turn, lessens the need for prolonged mastication and contributes to lower rates of wear. Studies analyzing microwear patterns in populations that adopted boiling foods in ceramic vessels have demonstrated that such innovations in food preparation techniques effectively reduce dietary texture and abrasiveness (Molleson and Jones, 1991; Molleson et al., 1993; Larsen, 2015).

While we propose that the use of pottery and associated preprocessing of food might be the primary factor contributing to the low molar wear rate among the Jomon people that we examined, it has been observed that other Jomon populations of the Initial Jomon period experienced more pronounced dental wear despite employing pottery for boiling and cooking (Kaifu et al., 2017). This observation suggests that the introduction of pottery and associated cultural practices did not immediately lead to a reduction in molar wear. It is essential to recognize that the specimens analyzed in the present study date from the Middle to Final Jomon periods, implying that there could have been changes in the rate of dental wear, and plausibly also in dietary habits, compared with the earlier periods. Additionally, the Jomon people were not the sole practitioners of boiling and cooking among hunter-gatherer societies. Therefore, the use of pottery alone cannot independently account for the observed reduction in dental wear.

Considering studies on oral hygiene in the Jomon period, we can propose a scenario that is consistent with the unexpectedly low rate of wear after the Middle Jomon period. It has been indicated that the Jomon people had a higher dental caries rate compared with other-hunter-gatherer societies (Turner, 1979; Fujita, 1995; Saso, 2020). Caries formation is known to increase with greater carbohydrate consumption, as seen in societies transitioning to agricultural economies (Leigh, 1925; Powell, 1985; Larsen, 2015). It is plausible that the Jomon people used pottery to extract starch from plant resources, resulting in a cooking method more conducive to caries formation. Consequently, the softer texture of their prepared food might have contributed to a decrease in chewing frequency, ultimately leading to a decline in the rate of dental wear. This higher caries rate appears to be prominent since the Middle Jomon periods (Fujita, 1995; Saso, 2020). To summarize, the observation of a relatively low rate of dental wear among the Jomon people compared with other hunter-gatherer populations can be attributed to the use of pottery and the accompanying increased reliance on carbohydrates, possibly accompanied by changes in their cooking methods.

In this study, we estimated the average score differences between adjacent molars for the Jomon people and compared these with existing literature values for other hunter-gatherers populations. The results suggest that the rate of wear for the Jomon people is relatively low compared with the wear rates of other hunter-gatherers. However, it is crucial to acknowledge that the comparative data are based on summarized values of ordinal scale data, and in some populations, the sample size is limited. When considering diet and wear patterns, it is also necessary to have a more detailed understanding that includes factors beyond wear rates, such as dental caries, periodontal disease, and tooth crown chipping. As such, further research is essential to gain a more comprehensive understanding of tooth wear patterns in relation to dietary habits. Moreover, there is a need for more detailed studies within the Japanese archipelago to explore the relationship between dental wear patterns and dietary habits, particularly among Jomon populations from periods preceding the Middle Jomon period. The current investigation provides new insights into the variation of dietary practices and their potential impact on tooth wear patterns.

We would like to thank Dr Gen Suwa for his advice and reading of the manuscript. This work was supported by JSPS KAKENHI grant numbers JP20KK0162 and JP21H04983.

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