Allometric analysis of dental variation in a human population

There exists an extensive literature that deals with interspecific allometry, eg, brain size-body size relationships among species. Yet comparatively little attention has been paid to intraspecific or static adult allometry. An intraspecfic allometric analysis was conducted on the complete permanent dentition of a prehistoric American Indian population (N = 156). Mesiodistal and buccolingual measurements were logarithmically transformed and regressed on log transformations of femur length, an estimate of body size. When measurements of antimeric teeth were introduced together into common regressions on femur length, 20 of the 32 slopes were significantly different from zero. Thirty-one of the slopes ranged between zero and one and clustered between 0.2 and 0.4. Hence, negative allometry describes the tooth size-body size association, ie, taller individuals in general possess absolutely but not relatively larger teeth than shorter individuals. In addition, no significant sex differences for the regression slopes were observed. Though significantly correlated, tooth size and body size variables are too weakly associated to permit accurate predictons from regression equations. Evolutionary implications of intraspecfic dental allometry are discussed.     

Allometric relations in plant growth

Allometric relations Y = aX^b are shown to exist in phyllotaxis, under the form r = k log R = p ()(m + n) ^–2 , where r = Y is the normalized internode distance in the cylindrical representation of phyllotaxis and R is the plastochrone ratio in the centric representation, p()=a is a constant for every angle of intersection of the opposed parastichies of the visible pair (m, n), for every m and n, and for all possible limit divergence angles corresponding to the Fibonacci-type sequences..., m–n, n, m, m + n=X, 2m + n, ..., and where b=–2. Richards' phyllotaxis index will be deduced.This work was supported by the Natural Science and Engineering research Council Canada, grant A6240     

Variation in the developing root cone angle of the permanent mandibular teeth of modern man and certain fossil hominids

The radiographic appearance of the developing root apex of 461 mandibular teeth was studied using ellipsopantomographs of 77 individuals. The angle formed by the mesial and distal aspects of the developing root cone was measured for each quarter of root development in the permanent mandibular teeth of each subject. Large values for this angle in the more coronal part of the root gradually decreased towards the root apex where values were small in all teeth studied. This gradual decrease in the angle of the developing root cone is concordant with the faster increase in root length but slower rates of dentinogenesis reported for the apical region of human teeth in the literature, as the decrease is likely to reflect the increasing numbers of odontoblasts that become involved in root formation as it proceeds apically. The developing root cone angles of one of the five fossil hominid specimens studied (KMN-ER 820) fall outside the range for the developing root cone angle of the developing premolar and permanent second molar roots in modern Homo sapiens and is evidence of a faster rate of root elongation than is typical for modern man.     

Crown dimensions of deciduous teeth from prehistoric India

Mean mesiodistal and buccolingual diameters are presented for deciduous teeth from the important Chalcolithic site of Inamgaon (1400–700 B.C.), a prehistoric farming community on the Deccan Plateau of western India. The deciduous teeth from Inamgaon are consistently larger than deciduous teeth of modern populations of European descent and smaller than the deciduous teeth of modern Australian aboriginals. Comparative data for prehistoric deciduous teeth are rare, especially for populations of southern Asia. The deciduous teeth of Mesolithic Europeans are comparable in size to certain dimensions of the Inamgaon teeth, and a small sample of deciduous teeth from the Iron Age site of Pomparippu (Sri Lanka) exhibits larger anterior teeth and smaller molar teeth than does the sample from Inamgaon.     

The evolution of brain size and intelligence in man

The brain size of hominids has increased approximately threefold during the evolution of the hominids fromAustralopithecus toHomo sapiens. It is proposed that the principal reason for this increase is that larger brains conferred greater intelligence, and greater intelligence conferred a selection advantage. A number of anthropologists have difficulty accepting this thesis because they believe that brain size is not associated with intelligence in man. Evidence is reviewed, and new evidence from two studies is presented, to show that brain size as measured by head size is positively correlated with intelligence as measured by intelligence tests. On two recent samples statistically significant correlations of .21 and .30 were obtained between estimates of brain size and IQ. It is considered that brain size is positively associated with intelligence in man and that this is the major reason for the increase in brain size of the hominids during the last 3.2 million years.     

Third molar agenesis in human prehistoric populations of the Canary Islands

The occurrence of third molar agenesis was recorded in a sample of 1,492 maxillary and 1,718 mandibular arches belonging to the prehistoric settlers of the Gan Canaria, Tenerife, and La Gomera Islands (Canary Islands). There were significant sex differences only in the Tenerife sample for the maxilla, the incidence in females being higher than in males. In the Gran Canaria sample, the total frequency (male and female combined) of third molar agenesis (individual count method) was 8.7% for the maxilla and 9.3% for the mandible. In the Tenerife and La Gomera samples, the frequencies were 11.1% and 10.7% for the maxilla and 14.6% and 13.3% for the mandible. In the Tenerife sample, the differences between both jaws were statistically significant. The incidence of missing third molars in the mandible was significantly higher in Tenerife than in Gran Canaria, but the other sample differences were statistically nonsignificant. Bilateral absence of third molars was observed in about two-thirds of the specimens examined. Some correlation between both jaws for the occurrence of third molar agenesis was found. The hypotheses that have been proposed in order to explain third molar agenesis in man are discussed. It is suggested that the loss of the third molar in Homo sapiens could be produced by a heterochronic phenomenon of postdisplacement, as a consequence of the phylogenetic tendency toward the delay of the onset of the third molar formation, and that the genetic factors responsible for the absence of these teeth could be related to the general process of delay in tooth formation.     

Anatomo-anthropological typology of the jaws in man

Basing on the morphological signs selected in the morphologically contrast samples of the facial skeleton, 27 variants of the upper and lower jaws have been obtained for each, characterizing interconnection and interdependence of several morphological signs. Into each of these variants people of various races and sex, with massive or gracile masticatory apparatus can get. However, the reduction degree, directness, temps and successiveness of reorganizations together with exogenic influence within the limits of the normal reaction in various parts of the masticatory apparatus of the individuals, that belong to the same variant, is identical. Thus, an anatomo-anthropological typology of the dento-maxillary apparatus is obtained, owing to which it is possible to study and compare: 1--individual morphological peculiarities, 2--regional and ethnic norms in morphology of the jaws, 3--within the 27 variants of the structure selected it is possible to study functional, biochemical and other factors. The results demonstrate, that 27 morphological variants of the jaw structure can serve as anatomical marker for distinguishing 27 risk groups for caries.     

Eruption of permanent teeth in Brazilian Whites and Blacks

A total of 302 White and 904 Black children six, nine and 12 years of age were studied for the eruption of their permanent teeth. Differences between the two ethnic groups were not large, but the Black subjects were generally more precocious at the beginning of the process (especially the girls, both in the upper and lower incisors). At nine and 12 years of age, however, these dissimilarities disappear. The latter do not seem to be related to distinct rates of extraction or agenesis of deciduous teeth. No significant differences were observed in the degree of asymmetry present in the permanent dentition of the White and Black individuals studied.     

Size and morphology of the permanent dentition in prehistoric Ohio Valley Amerindians.

Metric and morphological characterizations of the permanent teeth from a total of 155 prehistoric Amerindians are presented. The individuals represent samples from three Ohio Valley burial complexes (considered together as the Late Diffuse group): Glacial Kame, Adena and Ohio Hopewell. Metric data include common measures of central tendency and dispersion. From these measures estimates and analyses of the magnitude of sexual dimorphism and relative variability are presented as well as analyses of the patterns of these estimates. Forty morphological characters are also tabulated. The results indicate a number of provisional hypotheses: the generally larger tooth size of the Late Archaic Indian Knoll when compared to the Late Diffuse groups is consistent with the hypothesis of mitigated selective pressures in more technologically advanced groups; although tooth size is smaller in the Late Diffuse groups, dental morphology is as complex, or more so when compared to the Indian Knoll group. Since morphology and size do not covary exactly the biocultural forces resulting in smaller tooth size do not seem to act as strongly on dental morphology; odontological differences within the Late Diffuse arise primarily between the Glacial Kame-Adena and the Ohio Hopewell. These differences correspond to major biocultural changes in this area; although provisional hypotheses concerning odontological variability are erected, hypotheses concerning evolutionary trends must await the discovery of evolving lineages within these groups; similarities are noted among all compared groups including the pattern and magnitude of sexual dimorphism and relative variability. These parameters may be similar for all eastern Amerindians during this period; finally, the morphology of the deciduous dentition, which generally predicts that of the permanent teeth, is found to be less complex than the permanent teeth. This may be the result of a selective disadvantage for the individuals in the deciduous dentition sample which is reflected in the dentition.     

中国古人类牙齿尺寸演化特点及东亚直立人的系统地位

对中国境内不同时代人类牙齿测量数据的发析显示,中国古人类牙齿尺寸的总体演化趋势与世界其它地区人类一致,呈缩小变化,其中一个表现特点是中国直立人与早期智人在牙齿尺寸上不差别不大,但作者根据对中国古人类化石形态特征,生存年代等方面的综合分析认为中国直立人在化石形态,生存年代等方面均与智人有明显的不同,取消直立人,将其并入智人意见的证据还是不够充分的,目前仍宜将直立人与智人作为人属内两个不同的种来看待,     

Allometric disparity in rodent evolution

In this study, allometric trajectories for 51 rodent species, comprising equal representatives from each of the major clades (Ctenohystrica, Muroidea, Sciuridae), are compared in a multivariate morphospace (=allometric space) to quantify magnitudes of disparity in cranial growth. Variability in allometric trajectory patterns was compared to measures of adult disparity in each clade, and dietary habit among the examined species, which together encapsulated an ecomorphological breadth. Results indicate that the evolution of allometric trajectories in rodents is characterized by different features in sciurids compared with muroids and Ctenohystrica. Sciuridae was found to have a reduced magnitude of inter‐trajectory change and growth patterns with less variation in allometric coefficient values among members. In contrast, a greater magnitude of difference between trajectories and an increased variation in allometric coefficient values was evident for both Ctenohystrica and muroids. Ctenohystrica and muroids achieved considerably higher adult disparities than sciurids, suggesting that conservatism in allometric trajectory modification may constrain morphological diversity in rodents. The results provide support for a role of ecology (dietary habit) in the evolution of allometric trajectories in rodents.     

Primary teeth in Anura are nonpedicellate and bladed

1. Scanning electron microscop of the dentition of four anuran species (Discoglossus pictus, Bombina orientalis, Rana cyanophlyctis, Rana temporaria) revealed that at least the primary teeth, which are established during metamorphosis, are more or less bicuspid, bladed and nonpedicel-late. Juveniles and adults, however, have pedicellate teeth. 2. Thus, regardin the pedicellate condition, the sequence of stages in tooth development of Anura und Urodela show some similarities, although the primary teeth of urodeles develop already before metamorphosis.     

Covariation of Mitochondrial Genome Size with Gene Lengths: Evidence for Gene Length Reduction During Mitochondrial Evolution

Reduction of genome size and gene shortening have been observed in a number of parasitic and mutualistic intracellular symbionts. Reduction of coding capacity is also a unifying principle in the evolutionary history of mitochondria, but little is known about the evolution of gene length in mitochondria. The genes for cytochrome c oxidase subunits I–III, cytochrome b, and the large and small subunit rRNAs are, with very few exceptions, always found on the mitochondrial genome. These resident mitochondrial genes can therefore be used to test whether the reduction in gene lengths observed in a number of intracellular symbionts is also seen in mitochondria. Here we show that resident mitochondrial gene products are shorter than their corresponding counterparts in -proteobacteria and, furthermore, that the reduction of mitochondrial genome size is correlated with a reduction in the length of the corresponding resident gene products. We show that relative genomic AT content, which has been identified as a factor influencing gene lengths in other systems, cannot explain gene length/genome size covariance observed in mitochondria. Our data are therefore in agreement with the idea that gene length evolves as a consequence of selection for smaller genomes, either to avoid accumulation of deleterious mutations or triggered by selection for a replication advantage.     

Patterning the size and number of tooth and its cusps

Mice and rats, two species of rodents, show some dental similarities such as tooth number and cusp number, and differences such as tooth size and cusp size. In this study, the tooth size, tooth number, cusp size and cusp number, which are four major factors of the tooth patterning, were investigated by the heterospecific recombinations of tissues from the molar tooth germs of mice and rats. Our results suggest that the dental epithelium and mesenchyme determine the cusp size and tooth size respectively and the cusp number is co-regulated by the tooth size and cusp size. It is also suggested that the mesenchymal cell number regulates not the tooth size but the tooth number. The relationships among these factors in tooth patterning including micropatterning (cusp size and cusp number) and macropatterning (tooth size and tooth number) were analyzed in a reaction diffusion mechanism. Key molecules determining the patterning of teeth remains to be elucidated for controlling the tooth size and cusp size of bioengineered tooth.