No abstract

Across mammalian species, the periodicity with which enamel layers form (Retzius periodicity) in permanent teeth corresponds with average body mass and the pace of life history. According to the Havers-Halberg Oscillation hypothesis (HHO), Retzius periodicity (RP) is a manifestation of a biorhythm that is also expressed in lamellar bone. Potentially, these links provide a basis for investigating aspects of a species' biology from fossilized teeth. Here, we tested intra-specific predictions of this hypothesis on skeletal samples of human juveniles. We measured daily enamel growth increments to calculate RP in deciduous molars (n = 25). Correlations were sought between RP, molar average and relative enamel thickness (AET, RET), and the average amount of primary bone growth (n = 7) in humeri of age-matched juveniles. Results show a previously undescribed relationship between RP and enamel thickness. Reduced major axis regression reveals RP is significantly and positively correlated with AET and RET, and scales isometrically. The direction of the correlation was opposite to HHO predictions as currently understood for human adults. Juveniles with higher RPs and thicker enamel had increased primary bone formation, which suggests a coordinating biorhythm. However, the direction of the correspondence was, again, opposite to predictions. Next, we compared RP from deciduous molars with new data for permanent molars, and with previously published values. The lowermost RP of 4 and 5 days in deciduous enamel extends below the lowermost RP of 6 days in permanent enamel. A lowered range of RP values in deciduous enamel implies that the underlying biorhythm might change with age. Our results develop the intra-specific HHO hypothesis.

Objectives Human tooth enamel retains evidence of growth in the form of Retzius lines. The number of daily growth increments between the regularly occurring lines defines their repeat interval, or periodicity. Retzius periodicity is often incorporated into enamel formation times, age‐at‐death reconstructions, or used to provide a basis from which to explore an underlying biorhythm. Biological anthropologists typically assume that RP remains constant within an individual and does not vary along the tooth‐row. Here, we test that assumption. Materials and methods RP was calculated from n = 223 thin sections of human permanent teeth from individuals of British and southern African origin. Forty individuals provided multiple teeth (n = 102 teeth) and a further 121 individuals each provided a single tooth. Results We report first evidence that RP of permanent teeth does not always remain constant within an individual. Of those individuals that provided multiple teeth, 42% (n = 17/40) demonstrated a decrease in RP along the tooth row, with most shifting by two or more days (n = 11). Across the entire sample, mean RP of anterior teeth was significantly higher than molars. Mean premolar RP tended to be intermediate between anterior teeth and molars. Discussion Our data do not support the assumption that RP invariably remains constant within the permanent teeth of an individual. Transferring RP from molars to incisors within an individual can result in a miscalculation of formation time and age‐at‐death by up to 1 year. Implications for biological anthropologists and the source of the underlying long period biorhythm are discussed.

The Havers-Halberg Oscillation (HHO) hypothesis links evidence for the timing of a 28 biorhythm retained in permanent tooth enamel (Retzius periodicity) to adult body mass and 29 life history traits across mammals. Potentially, these links provide a way to access life 30 history of fossil species from teeth. Recently we assessed intra-specific predictions of the 31 HHO on human children. We reported Retzius periodicity (RP) corresponded with enamel 32 thickness, and cusp formation time, when calculated from isolated deciduous teeth. We 33 proposed the biorhythm might not remain constant within an individual. Here, we test our 34 findings. RP is compared between deciduous second and permanent first molars within the 35 maxillae of four human children. Following this, we report the first RP's for deciduous teeth 36 from modern great apes (n=4), and compare these to new data for permanent teeth (n=18) 37 from these species, as well as to previously published values. We also explore RP in teeth 38 that retain hypoplastic defects. 39 R e s u l t s s h o w R P c h a n g e d w i t h i n t h e m a x i l l a o f e a c h child, from thinner to thicker 40 enameled molars, and from one side of a hypoplastic defect to the other. When considered 41 alongside correlations between RP and cusp formation time, these observations provide 42 further evidence that RP is associated with enamel growth processes, and does not always 43 remain constant within an individual. RP of five days for great ape deciduous teeth lay below 44 the lowermost range of those from permanent teeth of modern orangutan and gorilla, and 45 within the lowermost range of RP's from chimpanzee permanent teeth. Our data suggest 46 associations between RP and enamel growth processes of humans might extend to great apes. 47These findings provide a new framework from which to develop the HHO hypothesis, which 48 can incorporate enamel growth along with other physiological systems. Applications of the 49 HHO to fossil teeth should avoid transferring RP between deciduous and permanent enamel, 50 or including hypoplastic teeth. 51

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