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Department of Human Evolution - Max Planck Institute for Evolutionary Anthropology
 
   
  Abstracts
 
 
 
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The evolution of premolar and molar crown morphology within the hominin clade
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Comparative analysis of the Sima de los Huesos and Gran Dolina-TD6 dental samples. Phylogenetic and evolutionary implications.
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Handling variability in timing and patterns of macrostructural dental development. New concepts and applications to fossil hominids
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Imaging dental microanatomy using innovative Portable Confocal Microscopy
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Neural network analysis by using the Self-Organizing Maps (SOMs) applied to human fossil dental morphology: a new methodology
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Correlation between white-light scanning interferometry and scanning electron microscopy applied to buccal, dental microwear of extant Hominoidea and Plio-Pleistocene fossil Homininae
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Perikymata counts and imbricational enamel formation times in Neandertals and recent modern humans
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Genetic evolutionary history of the primate dentition
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Reconstructing infant diet from tooth enamel
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Of mice and many dental characters
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A critical re-evaluation of the development and eruption of the dentition in living human populations and its paleoanthropological implications
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Analysis of cusp morphology of upper molars of South African Australopithecines
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µCT of the post-canine dentition: methodological aspects of three-dimensional data collection.
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The subspecies category in early hominin taxonomy: lessons from the study of dental variation in extant apes
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Perikymata packing pattern and the extension rate of the enamel
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Dental development in Pan paniscus from enamel microanatomy analysis
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Inferring primate growth, development and life history profiles from dental microstructure.
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A computerized model for reconstruction of dental ontogeny: A new tool for studying evolutionary trends in the dentition
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New Perspectives on Chimpanzee and Human Molar Development.
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Dental microwear and paleoanthropology: cautions and possibilities
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Tooth wear diversity in early hominid molars - a case study
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Dental topographic analysis and diets of early hominins
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HRXCT analysis of hominoid molars: quantitative volumetric analysis and 3D reconstruction of enamel and dentin volumes.
   
 
 
 
The evolution of premolar and molar crown morphology within the hominin clade

S Bailey1 & B Wood2

1Departmant of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig (Germany)
2Department of Anthropology, George Washington University, Washington D.C. (USA)

In order to investigate the evolutionary history of the hominin dentition it is important to determine which dental characters are primitive and which are derived. While attempts have been made to use the dentition to determine the pattern of evolution within the hominin clade, few studies have used dental non-metric traits as the primary focus of cladistic analyses, nor have many tried to determine the polarity of such traits. As a result many important questions remain unanswered. For example, can the molecular phylogeny be recovered from higher primate dental morphological data? What is the nature of inter- and intraspecific non-metric dental variation? While we have a good understanding of modern human dental variation, we know much less about non-metric variation in living hominoids, and of course even less about such variation in fossil hominins. In order to answer these and other questions it is necessary to devise an objective system for scoring non-metric morphological variation in tooth crowns.

Because much of the study of dental morphology has focused on recent living or archeologically-derived human samples, it is not surprising that the standard used to assess non-metric tooth crown variation is based on the variation observed in contemporary modern humans. Recent application of the modern human scoring system (the Arizona State University dental anthropology system or ASUDAS) to Middle-Late Pleistocene fossil hominins revealed that there are several morphological traits present in these hominins that are not accounted for by the system devised for modern humans. This is because the traits are either absent or do not vary in modern humans, or because they do not distinguish among geographic populations. The use of a modern human scoring system will, therefore, minimize differences between samples of fossil hominins (especially those in which the modern human traits are fixed) and also between fossil hominin and modern human samples. The bias is towards making samples appear to be more modern human-like than they are because only modern human traits are used in the analysis. In a recent study of morphological variation among Pliocene hominins and contemporary humans the results showed a surprisingly low morphological divergence between Paranthropus and australopiths – lower than that obtained between Southeast Asian and Polynesian human populations (Irish and Guatelli-Steinberg, 2003).

The primary problem is that morphological traits that differentiate the Pliocene hominins were not included in the analysis. A similar study of phenetic divergence based on non-metric dental traits revealed a low level of divergence among chimpanzee subspecies and species when compared to divergence among some modern human populations (Bailey, 2005). It was suggested that this is at least partially the result of the choice of dental traits, which had to be shared among modern humans and chimpanzees in order to make the resulting divergence values comparable. As is the case with Pliocene hominins (and probably other fossil hominins as well) there are a number of dental traits that differentiate among chimpanzee species/subspecies that are not part of the modern human scoring system. Without including such traits in any analysis an accurate assessment of morphological variation cannot be made.

We have examined the crowns of chimpanzee species and subspecies, australopiths, Paranthropus and Homo in order to investigate two different dental trends widely recognized in Plio-Pleistocene hominin evolution. They are a reduction in crown size and morphological complexity in Homo, and an increase in crown size and morphological complexity in Paranthropus. A phenetic assessment of maxillary and mandibular molar crown non-metric traits revealed that two australopith species (A. africanus and A. afarensis) are much more similar to each other than either is to Paranthropus, and together they are all distinctively different from chimpanzee molars (P. troglodytes and P. paniscus). The difference between Paranthropus and australopith postcanine teeth was 12 times greater than that between the australopith species and the divergence between the two australopith species was about twice that of the two extant chimpanzee species. The characters that contribute to the increase in crown complexity seen in Paranthropus do not appear to be primitive retentions from a great ape ancestor, and the trend for trait intensification appears to be already present in A. afarensis. These traits primarily include supplementary cusps in the maxillary and mandibular molars, but also the expanded talonid of the mandibular P4. Homo exhibits the primitive condition for many of the molar traits, but has lost many other primitive traits (upper molar anterior and posterior foveae, for example) that are present in the australopiths. Like australopiths, relative to Pan early Homo possesses a larger mandibular P4, on average, with a somewhat expanded talonid. But this trend is subsequently reversed in later Homo. Our study reveals that some of the trends said to be characteristic of Homo, actually only begin with later Homo (e.g., H. erectus).

Our investigation also revealed that many characters, some symplesiomorphic and shared with chimpanzees, are present and variable in Pliocene hominin taxa. Moreover, we conclude that the scoring criteria for certain traits (e.g., Carabelli’s cusp, protostylid) must be modified in order to encompass the variation observed in extinct hominin taxa. We will present an assessment of the utility of these characters, and will propose new criteria for scoring others.

   
 
 
 
Comparative analysis of the Sima de los Huesos and Gran Dolina-TD6 dental samples. Phylogenetic and evolutionary implications.

M. Martinón-Torres1* J. M. Bermúdez de Castro2 , M. Bastir13, A. Gómez1, S. Sarmiento1, A. Muela1, M. Lozano4, J. L. Arsuaga5

1 Department of Palaeobiology, Museo Nacional de Ciencias Naturales, CSIC, 28006 Madrid, (Spain)
2 Centro Nacional de Investigación sobre Evolución Humana (CENIEH), Burgos (Spain)
3 Hull York Medical School; The University of York; Heslington; York YO10 5DD, (UK)
4Department of Prehistory, Universitat Rovira i Virgili. Plaza Imperial Tarraco 1, 43005 Tarragona (Spain)
5Centro Mixto UCM-ISCIII de Evolución y Comportamiento Humanos, C/ Sinesio Delgado 4-6 (Pab.14) 28029 Madrid (Spain)

*mariamtmncn.csic.es

The Sima de los Huesos (SH) and Gran Dolina (TD) sites in Sierra de Atapuerca, Spain, have each one yielded an impressive fossil hominin sample representing a Middle Pleistocene and a Late Lower Pleistocene European populations, respectively. Paleontological evidence, paleomagnetic analyses, and radiometric dates (U/Th) suggest an interval of 400 to 500 ky for the SH hominins. Concerning Gran Dolina, radiometric dates (ESR and U-series) combined with paleomagnetic analyses and fossil evidence indicate an age range between 780 to 860 ky for the Aurora Stratum of the TD6 level, where the fossil hominins were found. We have assigned SH hominins to the Homo heidelbergensis species, whereas the TD6 hominins are representative of Homo antecessor, the species named in 1997 to accommodate the variability observed in the TD6 fossil human assemblage.
   Dental collections of the SH and TD6 sites include, so far, more than five hundred deciduous and permanent teeth. We have selected a set of dental traits potentially useful for the cladistic analysis and studied their polarity across the hominin fossil record. Through the comparative morphometric analysis of SH and TD6 dental samples we will explore the relationship between the Lower and Middle Pleistocene European populations as well as the possible evolutionary scenarios of the first human settlement in Europe.

       
 
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Handling variability in timing and patterns of macrostructural dental development. New concepts and applications to fossil hominids

J Braga

Laboratoire d'Anthropologie, Université Bordeaux 1 (France)

Developing teeth do not mineralize randomly with respect to one another because they are developmentally associated to each other and essentially grow as a unit, independent of chronological age. The resulting dental mineralization sequence is a well delimited and internally coherent morphological structure, assembled through ordered and interactive processes between its growing component parts (teeth), embedded within the jaws, and undergoing transformations on both developmental and evolutionary time scales. It becomes essential to determine the variability of these developmental processes and how they possibly conferred evolvability. However, there is a lack of any kind of unifying analytical approach providing a qualitative and comprehensive description of the macrostructural dynamics of dental development. Studies on dental development during human evolution also lack any conceptual framework. We hypothesize that growing teeth may well represent evolutionary and developmental modularity. From this fundamental concept, and in order to provide a picture of the variability of the dental macrostructural developmental processes as a possible substrate for morphological changes during human evolution, we describe and test the first approach quantifying all possible connections between developing permanent teeth. Our results indicate that, in extant humans, the relationships between, on the one hand, incisors and, on the other hand, all other permanent teeth (canine, premolars and molars), represent the most significantly plastic patterns of development. We conclude by shedding some light on our approach to new levels of understanding of the nature and variability of dental developmental sequences in fossil hominids and our closest living relatives, the chimpanzees.

   
 
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Imaging dental microanatomy using innovative Portable Confocal Microscopy

T Bromage1, R Lacruz2, A Perez-Ochoa3 and A Boyde4

1Deparments of Biomaterials & Basic Science, College of Dentistry, New York University (USA)
2Institute for Human Evolution, University of the Witwatersrand, Johannesburg (South Africa)
3Department of Paleontology, Universidad Complutense de Madrid (Spain)
4Department of Anatomy and Developmental Biology, University College London (UK)

The study of hominid enamel microanatomical features is usually restricted to the examination of fortuitous enamel fractures by low magnification stereo-zoom microscopy or, rarely, because of its intrusive nature, by high magnification compound microscopy of ground thin sections. To contend with limitations of magnification and specimen preparation, a Portable Confocal Microscope (PCM) has been specifically developed for the non-contact and non-destructive imaging of early hominid hard tissue microanatomy. This unique instrument can be used for high resolution imaging of both the external features of enamel, such as perikymata and microwear, as well as internal structures, such as cross striations and, commonly, the cuspal striae, from naturally fractured or worn enamel surfaces. Because there is veritably no specimen size or shape that cannot be imaged (e.g. fractured enamel surfaces on intact cranial remains), study samples may also be increased over what would have been possible before. We have recently applied this innovative technology to the study of enamel microanatomical features from naturally occurring occluso-cervical fractures of the South African hominid, Australopithecus africanus representing different tooth types. We present for the first time detailed information regarding cross striation periodicity for this species and, in addition, we present data on striae-EDJ angles in a large sample of teeth. Our results characterize a pattern of enamel development for A. africanus, which is different to that reported for the genus Paranthropus, as previously observed.

   
 
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Neural network analysis by using the Self-Organizing Maps (SOMs) applied to human fossil dental morphology: A new methodology.

A Coppa1, R Vargiu1, and F Manni2

1Department of Animal and Human Biology, Section of Anthropology, University of Rome "La Sapienza" (Italy)
2UMR 5145 - Eco-Anthropology Group, National Museum of Natural History ,Musée de l'Homme, Paris (France)

In the last ten years the use of Self Organizing Maps (SOMs), a specific application of artificial neural networks, has become widespread in a number of different disciplines (economics, ecological and environmental studies, biology, medicine, etc.) but, to our knowledge, SOMs were never applied to anthropological data before.

Usually, SOMs have been used to solve problems of classification, prediction, categorization, optimization and data-mining, since they represent a method to approximate systems that cannot effectively be modeled by classic statistical methods. Their use seems more successful where there is no linear relation between the predicted variable and the data used for the prediction itself.

In anthropological sciences predictions are commonplace, but the relations between the different variables are seldom linear. This is the case for bone and dental variables that change through time and space under the influence of genetic and environmental factors interacting in a complex frame. As a consequence, we see in SOMs a suitable tool to address the classification of dental samples from palaeontological excavations.

SOMs are intended for a non-analytic exploration of large groups of vectors (inputs) that are mapped on a lattice according to their similarity. As an example, vectors can be the frequencies of different skull measures or, as in this case, discrete traits of dental morphology. In this process a) identical vectors will be mapped at the same position of the map b) slightly different ones close to each other, while c) very different vectors will be mapped far from each other. The visual aspect of data representation obtained by SOMs is somewhat similar to a classical Multidimensional Scaling (MDS) or to a Principal Component Analysis (PCA) plot but, in contrast to these techniques, SOMs can handle up to several thousands of inputs on a standard computer.

The method is topology-oriented: distances between mapped inputs do not correspond to an MDS representation (MDS takes a set of dissimilarities -as in a distance matrix - and returns a set of points such that the distances between the points tend to be as close as possible to the dissimilarities), but describes more accurately the neighborhood of items. For this reason, SOMs might be preferred to MDS or PCA when all the different inputs (vectors) slightly differ one from another, as is often the case with palaeontological data.

SOMs are based on “competitive learning”, an adaptive process in which the cells in a neural network gradually become sensitive to different input categories. SOMs can handle vectors with missing components (measures) without interpolating missing data. This is probably one of the most important advantages of Self Organizing Maps.

To validate the use of Self-Organizing Maps (SOMs) we have applied them to a large dataset concerning the human dental morphology of the euroasiatic and northern african Middle and Upper Pleistocene specimens.

Acknowledgements
This research was supported in part by MURST COFIN03 and “Progetti di Ateneo” University of Rome “La Sapienza”.

   
 
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Correlation between white-light scanning interferometry and scanning electron microscopy applied to buccal, dental microwear of extant Hominoidea and Plio-Pleistocene fossil Homininae

F Estebaranz & A Pérez-Pérez

Secc. Antropologia, Dept. Biologia Animal. Fac. Biologia, Universitat de Barcelona.
Av. Diagonal 645, 08028 Barcelona (Spain)

Dental microwear analysis is based on the assumption that there exists a correlation between ingested diet and microwear patterns on the enamel surface of teeth, so diet can be reconstructed by quantifying enamel microwear. Abrasive particles, such as plant phytoliths or silica based sands incorporated into food items, along with food processing techniques and tooth morphology, are responsible for the microwear features observed. Dental microwear has been largely studied in both extant and extinct primates, including human populations. The dietary and ecological information that can be derived from dental microwear analyses makes it a recurrent technique also for non-primate species, such as muskrats, sheep, bats, moles, antelopes, pigs and even dinosaurs. In the attempt to reconstruct species’ ecology and diet, microwear research has become a successful procedure. It was tentatively applied during 1950’s, though without quantitative results, and definitively developed in the 80’s and well through the 90’s, when researchers proposed new techniques and methods to standardize microwear analysis and quantify microwear variables. The proliferation and persistence of different methods for quantifying microwear patterns obligate to be very accurate and conservative in defining microwear variables, though inter-observer error rates cannot be neglected. The use of semiautomatic methods to quantify microwear features does not guarantee inter-observer reproducibility and homogeneity of results, with unexpected causes of error affecting dental microwear results, such as taphonomy, microscopy drawbacks of back-scattered electrons, or differences in SEM reproducibility depending on sample shape and orientation.
Thus, a shift to fully automatic microwear quantification techniques has been pointed out as a priority in order to avoid subjectivity and inter-observer error. The aim of our presentation is to contribute to the standardization of automatic procedures to quantify microwear. Despite microscopy technologies for 3D topographic surface analyses are currently available, research on enamel surface has mainly focused on tooth morphology and wear, or on surface modifications of fossil mammalian bones, rather than on clear applications to dietary analysis of enamel microwear. Significant steps are now taken for the first time in several of many possible right directions. Our contribution to this yet long methodological standardization centers on the analysis of the correlation between SEM microwear variables, derived from semi-automatic quantification, and automatic measures of surface roughness and shape derived from interferometric microscopy.
The sample studied comes from our Hominidae tooth mould collection hosted at the University of Barcelona. One M2 teeth was selected from each specimen studied, since it was the most represented tooth in the sample and it has been largely used for microwear research. If the lower, left M2 was missing, the lower, right one was chosen instead, and lower M1 teeth were preferred rather than upper teeth in case of absence of M2. Only teeth that showed clear microwear features on the buccal enamel tooth surface were selected. The preliminary sample obtained (N=48) includes 2 teeth belonging to A. anamensis, 13 to A. africanus and 19 to A. afarensis specimens, as well as 10 of Pan troglodytes troglodytes, Gorilla gorilla gorilla and Pongo pygmaeus pygmaeus individuals from different museum collections. Negative replicas were obtained with polyvinylsiloxane President MicroSystem Regular Body (ColtèneTM) and positive tooth moulds were obtained with epoxy resin Epo-Tek #301 (ColtèneTM) and Ferropur PR-55 (QdATM). Moulds were mounted on aluminium stubs with term fusible gum and an argent belt (Electrodag 1415M-Acheson) was applied in order to prevent accumulation of electrostatic charges. Finally the casts were sputter coated with a 40A gold layer to allow for SEM observations. Micrographs of the buccal enamel surface of tooth moulds were obtained using a Cambridge Stereoscan-120 SEM microscope; acceleration voltage was set at 15KV and working distance was 18-25 mm. SEM micrographs were taken at 100× magnification on the intermediate third on the buccal surface, avoiding both the cervical and occlusal thirds of tooth crown, and enhanced with Adobe Photoshop v. 7.0. Microwear features were measured using SigmaScan Pro 5.0 (SPSSTM) in a 0.56 mm2 square surface area of well preserved enamel. Measures of striation density, length and orientation were derived for all the teeth studied.
At the same time, 3D topographic analyses were made using a WYKO NT1100 Optical Profiling System (VeecoTM) interferometric microscope hosted at the Plataforma de Nanotecnologia, Parc Científic de Barcelona (PCB, University of Barcelona). The buccal tooth surface was placed perpendicular to the objective major axis, and 3D images were captured with a 50× objective, covering an area of 124.4×94.6 ?m. Image quality was set to maximum and 736×480 pixel resolution digital images were obtained. Before surface roughness was measured background noise was eliminated from the image so normal-like frequency distribution of point depths was obtained. A median pass filter was applied to filter-out noisy and spiky data, and overall surface curvature was corrected to discriminate roughness due to microwear feature density from that caused by enamel surface morphology. Measures of surface roughness and topography were derived over the entire measured array (Figure 3). Roughness measures included Ra (average roughness), Rq (root-mean-squared roughness), and Rt (peak-to-valley difference).
In a preliminary analysis the correlation between Rt (3D roughness) and Nt (SEM total number of scratches) has been considered. A highly significant negative correlation between the two variables (Rt, NT) was obtained (Pearson correlation coefficient r=-0,412 P=0,004 N=48) suggesting that the number of scratches observed with SEM decreases with overall image roughness. By species, the correlation between Rt an NT remains negative for A. afarensis, the best represented sample, Pongo and Gorilla (Table 1). However, when the sample is split down into species categories only A. afarensis shows a significant correlation (r=-0,52; Table 1). Despite the sample studied is small, tooth enamel roughness seems to be the main factor affecting SEM microwear density, and post-mortem damage might be responsible for the higher values of enamel roughness observed. If so, Rt measures of roughness could be used as indicative of enamel preservation in ancient specimens. In fact, the fossil Hominin specimens studied show higher regression slopes (negative correlations) than the extant Hominoidea, since the later consist in museum specimens not affected by post-mortem damage. The higher roughness values of some fossil specimens seem to be indicative of a significant post mortem deterioration of these specimens.
If a significant pattern of correlation between striation density and enamel surface roughness emerges from the analysis of a larger sample, non-damage enamel surfaces might be characterized by the absence of correlation between the studied variables. Well preserved enamel surfaces need to be studied to discriminate dietary related from post-mortem induced roughness. The automatic characterization of microwear still requires detail research. The attribution of automatically measured variables to dietary behavior of human populations requires that a direct relationship between ecological conditions, diet, microwear and roughness be ascertained, discarding post-mortem damage, tooth morphology and wear as possible causes to enamel roughness. Great precautions should be taken when implementing automatic procedures of enamel surface analyses as indicative of diet.

   
 
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Perikymata counts and imbricational enamel formation times in Neandertals and recent modern humans

D Guatelli-Steinberg1, DJ Reid2, T Bishop3, CS Larsen1

1Dept. of Anthropology/Dept. of Evolution, Ecology and Organismal Biology, The Ohio State University (USA)
2Dept. of Oral Biology, University of Newcastle-upon-Tyne (UK)
3Dept. of Statistics, The Ohio State University (USA)

The strong relationship between dental development and the timing of life history stages in extant primate species makes possible the use of fossil teeth to gain insights into the life-history strategies of extinct hominin species (Smith, 1991; Smith and Tompkins, 1995). The timing of tooth eruption, especially the age at which the first molar erupts, is highly correlated with brain size and the length of growth periods across the primate order (Smith, 1991; Smith and Tompkins, 1995). Significantly, small brained Plio-Pleistocene hominin species have been shown to have had abbreviated crown formation times (Dean et al., 2001) and early M1 eruption (Bromage and Dean, 1985) in relation to modern humans.

The question of where Neandertals fall on the continuum of accelerated versus protracted development remains unsettled. The study of dental eruption indicates that the prolonged developmental period of modern humans was likely to have also been present in Neandertals. (Tompkins, 1996). However, recently, Ramirez-Rozzi and Bermudez de Castro (2004) demonstrated that Neandertal imbricational enamel formation is accelerated with respect to Upper Paleolithic modern humans. The present investigation was undertaken to ascertain if the imbricational enamel formation times of Neandertal anterior teeth are truly uniquely accelerated with respect to those of modern humans.

Perikymata and striae of Retzius were counted on a total of 370 unworn (or minimally worn) anterior teeth of Neandertals and three comparative modern human groups. Sample sizes ranged from 56 teeth in Neandertals to 134 teeth in one of the comparative samples. Perikymata are wave-like enamel surface manifestations of underlying striae of Retzius, incremental lines in enamel which are visible in transmitted light microscopy that have a modal periodicity of 8-10 days in hominins (Dean et al., 2001). Crown height was divided into deciles and perikymata (on whole teeth) or striae of Retzius (in thin sections) counted within each. Thus, growth trajectories as well as total perikymata/striae counts were analyzed. Statistical analysis included computing the 95% confidence intervals per decile and per tooth type (UI1, UI2, UC, LI1, LI2, and LC) for each population sample. In addition, a generalized linear model was used to model imbricational enamel growth and an ANOVA was conducted. Both the 95% confidence intervals and the ANOVA revealed significant differences in growth across the four populations. Importantly, perikymata counts in Neandertals were significantly higher than those of one of the modern human populations. If a modal periodicity of nine days is assumed, then these results indicate that imbricational enamel formation spans in Neandertal anterior teeth are indeed encompassed within the modern human range of inter-population variation.

Although this investigation employed perikymata/striae counts to assess imbricational enamel formation times, recent research by the second author and MC Dean indicates that perikymata/striae counts vary among individuals in large part because their cross-striation periodicities vary. In this paper, we also consider how this fact may affect the interpretation of perikymata/striae count variation across populations.

   
 
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Genetic evolutionary history of the primate dentition

LJ Hlusko1 & MC Mahaney2

1Department of Integrative Biology, University of California, Berkeley, CA (USA)
2Department of Genetics, Southwest Foundation for Biomedical Research, San Antonio, TX (USA)

The vertebrate fossil record documents numerous cases of an apparent inverse relationship between the size of the anterior and posterior dentitions as they evolve over time. For example, the Suidae Nyanzachoerus lineage is characterized by a decrease in the size of the premolars coupled with an increase in the size of the molars. The Pliocene Hominidae robust Australopithecus lineage has an increase in the size of the postcanine dentition coupled with a decrease in the size of the anterior dentition. Papionins document the same phenomenon during the Pliocene evolution of Theropithecus. In order to understand what these evolutionary trends mean in terms of adaptation and speciation, we first need to decipher the genetic mechanisms that may underlie such morphological trends. When combined with recent advances in developmental biology and genetics, statistical genetics can provide us with the means to more thoroughly interrogate the genetic architecture of such dental variation.

We will present results from an ongoing study of the genetics of dental variation in >600 captive, pedigreed baboons (Papio hamadryas). Our quantitative genetic analyses provide estimates of the additive genetic (heritability) and non-genetic contributions to the overall morphological variance in traits such as size, enamel thickness, cusp positioning, and cingular remnants. Additionally, we are identifying pleiotropic affects between dental traits on the same tooth crown, between teeth in the same tooth row, and across the dental arches. Specifically, we will focus our talk on our analyses of pleiotropy between the various regions in the dental arcade as a means through which to elucidate the genetic architecture that underlies population level variation in the relative sizes of the anterior and posterior dentition.

Identifying these mechanisms in the baboons provides a starting point for understanding the genetic influences on similar trends in other taxa, for example, hominids and suids. As we expand our genetic analyses taxonomically, we will be able to compare and contrast the genetic architecture of these phenomena across mammals.

   
 
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Reconstructing infant diet from tooth enamel

L Humphrey1, C Dean2, T Jeffries3

1Department of Palaeontology, The Natural History Museum, London (UK)
2Department of Anatomy and Developmental Biology, The University College, London (UK)
3Department of Mineralogy, The Natural History Museum, London (UK)

The weaning process is determined by the needs and constraints of both mother and infant and represents an important life history transition. The timing of the first intake of complementary (non-milk) foods is a specific and recognizable life history event. For the mother, the age at which complementary foods are introduced and rate at which breast milk is replaced are major determinants of fertility due to their effect on duration of lactational amenorrhea and their relationship to inter-birth interval. For the infant, weaning involves the gradual withdrawal of a secure and balanced source of nutrition and loss of immunological support provided by breast milk. Dietary supplementation exposes the infant to new sources of infection associated with contaminated foods and increases the risk of illness and malnutrition.

Strontium calcium ratios (Sr/Ca) are an effective means of examining dietary change during infancy since human milk has a very low Sr/Ca compared to most solid foods. Strontium and calcium are incorporated into developing teeth in a manner that reflects changing physiological concentrations in the body. Physiological levels change significantly at birth, during breast-feeding and with the first intake of solids, primarily because of differential discrimination against strontium by the placenta, mammary gland and gut. Studies of the weaning process using bone or tooth chemistry have typically adopted a cross sectional approach, in which a single sample from each individual is analysed and changes in diet are inferred from sample trends. Systematic micro sampling of tooth enamel offers the potential for individual profiles of dietary change to be reconstructed in the first few years of life. This type of longitudinal approach has several advantages compared to more traditional methods.

We use laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to record Sr/Ca ratios in 30 micron diameter samples taken at regular intervals along a series of trajectories running from the enamel dentine junction (EDJ) to the enamel surface. Using this technique, we can identify mode of feeding at birth and the age of introduction of complementary foods for each individual studied. This technique allows us to directly access behavioral evidence of life history from the dentition and can be used on modern, archaeological and fossil teeth.

   
 
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Of mice and many dental characters

J Jernvall

Evolution and Development Unit, Institute of Biotechnology, University of Helsinki (Finland)

For over a century dental variation found within species has been used to project how dental diversity could evolve. Recently, to help identify species in the fossil record, the typical range of dental variation found in populations has attracted renewed research interest. While developmental biology studies of mice are uncovering details about molecular signaling and morphogenesis, present day knowledge about development is often too crude to address question about population level variation. New avenues to study detailed changes in morphology include 'modulatory' signaling molecules which seem to fine tune the overall effects of signaling molecule networks. Mice with dental phenotypes that lack functional copies of these modulatory molecules still have teeth but with altered cusp morphologies. One example of the effects of modulatory signaling on tooth morphology is the ectodysplasin gene. Total inactivation of, or increase in, ectodysplasin production changes several dental characters without the loss of occlusion. The effects of ectodysplasin and other modulatory genes indicate that even large morphological changes in dental form could have simple genetic underpinnings. Compared to normal within species variation, mice with experimentally changed modulatory gene expression show larger range of variation in dental characters. Thus, while modulatory molecules may play a role in evolutionary change, there is likely to be another layer of genetic regulation, as a set of promoter modules cis-regulating the transcription of signaling molecules. This second regulatory layer would further fine tune the effects of signaling molecules and limit variation found within species. However, both character analysis of transgenic mice and mathematical modeling simulating tooth development suggest that normal variation within species may increase when certain morphological thresholds are reached during evolution. In hominoids, these kinds of thresholds with increased variation may be hypothesized to involve the addition of small conules or partial molarization of the premolars.

   
 
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A critical re-evaluation of the development and eruption of the dentition in living human populations and its paleoanthropological implications

A Mann1 & J Monge2

1Department of Anthropology, Princeton University, Princeton, New Jersey (USA)
2Department of Anthropology, University of Pennsylvania, Philadelphia, PA (USA)

Patterns of dental calcification and eruption in modern humans have been employed as standards for comparisons with earlier members of the human lineage. The goals of these studies have been to interpret life history variables in the past as well as in the construction of phylogenetic relationships.

Recent research in dental maturation in both chimpanzees and modern humans suggests that a critical re-evaluation of the standards in current use may be necessary. Zihlman and colleagues (2004) have shown that the patterns of dental development in a series of known age free ranging chimpanzees from the Gombe Stream National Park (Tanzania) are significantly different from the timing of patterns observed in samples of captive animals. Nadler (1998) has reported recent shifts in the timing of dental maturation in a sample of 150 children of European background.

In a study in progress of a large sample of panoramic X-rays of inner city youth aged 5½ to 13 years, we have found similar significant shifts from published standards from the 1960s in dental maturation.

These data, and others documenting secular trends in a variety of growth and development parameters, suggest that dental development (and growth in general) in these hominoids can be markedly influenced by currently unknown environmental factors. Moreover, these factors can apparently bring about these changes in comparatively short time frames. The implications of these recent findings for our understanding of the evolution of hominin maturation, as founded on studies of the dentition, require careful examination and possibly some re-calibration.

Nadler, G.L. 1998 Earlier dental maturation: fact or fiction? The Angle Orthodontist 68: 535-538.

Zihlman, A., Bolter, D. Boesch, C. 2004 Wild chimpanzee dentition and its implications for assessing life history in immature hominin fossils. Proc. Natl. Acad. Sci. 101: 10541-10543.

   
 
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Analysis of cusp morphology of upper molars of South African Australopithecines

J Moggi-Cecchi1 & S Boccone1

1Laboratori di Antropologia, Dipartimento di Biologia Animale e Genetica
Università di Firenze (Italy)

The number of fossil hominid specimens recovered from South African sites has dramatically increased in the last 20 years. The sites where large numbers of fossil hominid specimens have been discovered have been Sterkfontein, Swartkrans and, most recently, Drimolen. In fact, the fossil materials recovered from the Sterkfontein Formation represent, with no doubt, the largest collection of early hominid specimens from a single locality.

Hominid specimens recovered from these sites have been usually attributed to A. africanus (from Sterkfontein), A. robustus (Swartkrans, Drimolen and Sterkfontein) and South African early Homo (Swartkrans, Drimolen and Sterkfontein). Hominids from Sterkfontein Member 4 have, with few exceptions, been assigned to Australopithecus africanus. In recent years, several studies have suggested that a small handful of specimens from Sterkfontein Member 4 are difficult to accommodate in that species and may represent another taxon (eg Clarke, 1988, 1994; Lockwood and Moggi-Cecchi, 1998; Lockwood and Tobias, 2002). Other studies could find no evidence against the substantial systematic homogeneity within the Sterkfontein Member 4 hominid dental sample (Calcagno et al., 1999; Suwa, 1990; Wood, 1991a: Moggi-Cecchi, 2003).

In this light, a research project was recently initiated, aiming to provide a detailed characterization of the dental morphology of the Sterkfontein hominid sample.
Two of the specimens suggested to belong to a species other than A. africanus are represented by maxillary fragments (Stw 183 and Stw 252), both bearing cheek teeth. For this reason, the first step was the analysis of upper molar cusp morphology of South African Australopithecines in order to provide a framework within which to test the issue of the possible existence of a hominid species morphologically different from A. africanus within Sterkfontein Member 4.

Occlusal digital photographs of upper molars of South African early hominids were taken. Although 2D digital images are just a crude approximation of the complex shape of the teeth, they are easy to collect at present and are certainly more informative than the traditional dental measurements (MD and BL dimensions). The total number of specimens analyzed was 99 (A. africanus = 41, A. robustus = 51, SA early Homo = 7).

Digital images were analyzed with "Image J" free software. The variables considered were the absolute and relative cusp areas of the four main cusps and the total area (TMA), following Wood and Engleman, 1988. Further, in a sub-sample of relatively unworn teeth (n=75), positions of the cusp tips were identified (following Bailey 2002), thus allowing definition of an occlusal polygon, of which the absolute area (OPA) was measured. A relative area (OPA / TMA) was also calculated. Measurements of the four angles defined between two sides of the occlusal polygon were also taken.

Results showed that broad similarities seem to exist between A. africanus and A. robustus in terms of absolute and relative cusp areas of the upper molars. When specimens attributed to the "second species" (sensu Clarke 1994) were removed from the A. africanus sample, a significant difference existed between the two species (with A. robustus being larger than A. africanus ) in TMA of M3, in the areas of paracone and metacone of M1, and the protocone of M3.

Molar size sequence was M1<M2 <M3 in A. robustus, wheras in A.africanus it was M1<M2>M3. This difference appears to be related mostly to differences in distal cusp size. When angles were examined, a significant difference between A. africanus (the whole sample) and A. robustus was evident in the angle on the paracone, in M1, M2 and M3, and also in the hypocone of M1 and M2. Interestingly, a difference seems to exist between A. africanus and specimens attributed to the "second species" (sensu Clarke 1994) in the protoconal angle of M2. In terms of relative occlusal area, some differences were evident in the M3 of these two groups.

These preliminary results will be presented and discussed in the framework of our available knowledge in terms of cusp areas and proportions of fossil hominids and living great apes.

   
 
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µCT of the post-canine dentition: methodological aspects of three-dimensional data collection.

AJ Olejniczak1 & LB Martin2

1Interdepartmental Doctoral Program in Anthropological Sciences, State University of New York at Stony Brook (USA)
2Departments of Anthropology and of Anatomical Sciences, Stony Brook University (USA).

Phylogenetic, paleodietary, and developmental studies of hominoid primates frequently make use of the post-canine dentition. To study the development, thickness, and shape of the hominoid molar enamel cap, internal dental structures must be revealed, often necessitating the physical sectioning of teeth. The destructive nature of these studies limits sample sizes and access to valuable fossil specimens, and has led scholars to apply several radiographic visualization systems to the study of the dentition. As early as 1918, radiographic techniques were applied to hominid molars in order to visualize the enamel-dentine-junction (EDJ) for the purpose of measuring enamel thickness. Since that time, several non-destructive visualization methods have been attempted, including X-rays, ultrasound, terra-hertz imaging, and computed tomography (CT). Each of these techniques has resolution limitations rendering them inadequate for accurately reconstructing both the EDJ and outer enamel surface, and the majority of studies are still carried out using physical sections.

A comparatively new imaging technique, micro-computed tomography (µCT), has been demonstrated to portray accurately the EDJ and provide reliable measurements of enamel cap thickness and morphology. The research presented here describes applications of µCT to the study of the post-canine dentition. The primary focus is on the quantification of volumes, surface areas, linear thicknesses, and Cartesian coordinates as they relate to µCT parameters (e.g., slice thickness), demonstrating that different types of measurements may require different µCT scanning protocols. A recently constructed database of Hylobates (Symphalangus) syndactylus mandibular molar µCT scans is employed to illustrate the quantification of volumes, surface areas, thicknesses, and shapes using three-dimensional reconstruction techniques. Special attention is given to three long-standing methodological concerns that are now available for study due to the accurate three-dimensional reconstructions: the definition of an “ideal plane of section”, the effects of section obliquity sample variance, and the correspondence between two-dimensional measurements and size scalars and their three-dimensional counterparts.

   
 
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The subspecies category in early hominin taxonomy: lessons from the study of dental variation in extant apes

V Pilbrow

Department of Anthropology, University of Illinois at Urbana-Champaign, Urbana, IL (USA)

In studying patterns of variation and determining the taxonomic composition of a hominin fossil assemblage the phylogenetically closest and thus the most relevant modern comparators are Homo and Pan and following these, Gorilla and Pongo. Except for Pan, however, modern hominids lack taxonomic diversity, since by most accounts each one is represented by a single living species. Pan is exceptional in being the sister taxon to modern humans and represented by two living species. As such the species of Pan have greater relevance for studying the nature of interspecific variation in fossil hominin taxonomy.

Despite their relatively impoverished species representations Pan troglodytes, Gorilla gorilla and Pongo pygmaeus are nevertheless represented by subspecies, which are useful for studying the nature of intraspecific variation, in particular for assessing the utility of the subspecies category in hominin taxonomy. The differences between Neandertals and modern humans are often believed to be indicative of subspecies within a species. It has also been suggested, using baboon analogs, that the interactions between contemporaneous hominin allotaxa could have involved a certain degree of interbreeding, implying that the taxonomic designation of subspecies may be appropriate for describing them.

In this study linear and angular measurements taken on the occlusal surface of molars were used to measure the dental distances separating the traditionally recognized subspecies of the extant apes. The aim of the study was to compare the phenetic distances separating subspecies within and across great ape species so as to determine how variable the subspecies unit is in these model taxa. The ultimate aim was to determine how likely we are to differentiate subspecies in the hominin fossil record and how appropriate our models are for understanding infraspecific patterns of variation. The sample sizes were as follows: P. t. troglodytes 152, P. t. verus 64, P. t. schweinfurthii 79, G. g. gorilla 208, G. g. graueri 61, G. g. beringei 30, P. p. pygmaeus 140, P. p. abelii 25. Measurements were obtained on digital images and were used to calculate squared Mahalanobis distances between subspecies pairs.

Results indicate that P. t. troglodytes and P. t. schweinfurthii are separated by the least dental distance. P. t. verus is separated by a greater distance from these two, but on the whole the distances separating the subspecies of P. troglodytes were less than the distances separating the subspecies of G. gorilla and P. pygmaeus. The dental distances between G. g. gorilla from G. g. beringei were similar to the distances separating P. troglodytes from P. paniscus. This disparity in dental distances, which undoubtedly stem from differing patterns of variation, are nonetheless significant because they suggest that although Pan is our best extant comparator, the species and subspecies of Pan are likely to provide easy falsification of a single species hypothesis, thus increasing the likelihood of overestimating species diversity. The distances between the subspecies of G. gorilla and P. pygmaeus will support the single species hypothesis, but then our choice of model will need to be justified in terms of presumed patterns of variation in the fossil group. The fluidity of subspecies in closely related extant great apes suggests that the dynamic evolutionary unit of subspecies is likely to go unrecognized in the fossil record, labeled instead as populational or species level differences.

   
 
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Perikymata packing pattern and the extension rate of the enamel

F Ramirez Rozzi

UPR 2147 CNRS, Paris (France)
Dept. of Human Evolution, Max-Planck-Institute for Evolutionary Anthropology, Leipzig (Germany)

Perikymata packing pattern (PPP) is considered to reflect variations of the extension rate of the enamel during crown formation. Differences in the PPP between hominid species –i.e. Paranthropus vs. Australopithecus, H. neandertalensis vs. H. sapiens, modern humans vs. great apes, have lead to suggestions that these species present different patterns of variation in the extension rate. However, PPP is not a direct measure of the extension rate. Enamel extension rate indicates the advance of the enamel-matrix front at a given point during crown formation. This takes place by the addition of new ameloblasts which become active at the contact between the advancing matrix front and the dentine, the future enamel-dentine junction (EDJ). PPP corresponds to the variation in the distance between perikymata. PPP and others methods were suggested to estimate the variation of the extension rate of the enamel during crown formation but at the moment not a single work has tested these methods. Our aim is to compare PPP with direct measurements of the extension rate of the enamel in modern humans.

PPP is principally described here for anterior teeth since imbricational enamel corresponds to a high proportion of the crown in incisors. Modern human anterior teeth were sectioned. In each section, the buccal face was divided into ten deciles and the number of striae of Retzius (perikymata) obtained for each decile. In each decile, a group of striae of Retzius was followed to the contact with the EDJ. The distance between these striae was measured from the more cuspal to the more cervical in each group along the EDJ, and this measurement was divided by the number of striae in each group. The obtained value is a direct measure of the extension rate of the enamel. Then, PPP was compared with the variation of the extension rate in each tooth.

In general terms, PPP follows the extension rate of the enamel; perikymata become closer at the cervix and the extension rate decreases from cusp to cervix. However, a detailed analysis of the variation between deciles shows that there is not an exact correspondence between PPP and the extension rate. Except in the most cuspal and cervical deciles, where extreme values in extension rate match those in perikymata number, the extension rate varies between deciles without a corresponding relationship with the increasing number of perikymata cervically.
Other processes certainly influence PPP -i.e. prism with, prism decussation and appositional rate which probably explain the differences observed between modern humans and great apes. It appears that PPP can only be used as a general approach to estimate the extension rate of the enamel but it is not reliable to interpret the variation of extension rate during crown formation.

   
 
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Dental development in Pan paniscus from enamel microanatomy analysis

F Ramirez Rozzi1,2 & R Lacruz3

1UPR 2147 CNRS, Paris (France)
2Dept. of Human Evolution, Max-Planck-Institute for Evolutionary Anthropology, Leipzig (Germany)
3Institute for Human Evolution, University of the Witwatersrand, Johannesburg (South Africa)

Work based on ground sections of teeth have demonstrated that they provide accurate information on dental development in extant and extinct hominoid species. In contrast with radiographic studies, histological analyses are rarely carried out on large sample sizes. However, incremental lines in enamel and dentine allow for the study of the mechanisms underlying crown formation and to establish dental development patterns. Although studies of this type have been conducted in modern humans, chimpanzees, gorillas, orangutans, gibbons as well as some extinct hominoids, almost nothing is known about the Bonobo (Pan paniscus). We present here some aspects of dental development of Bonobo based on the study of a young female individual with the I1 crown just completed.

Sections were obtained for the right I1 and M1. The spacing between successive cross-striations was measured in the outer, middle and inner portions of occlusal, lateral and cervical regions of the enamel. Periodicity of striae of Retzius was obtained and the number of striae/perikymata was used to calculate the imbricational time formation. Length of prism and average distance between cross-striation were used to determine the duration of appositional enamel formation.

Spacing between cross striations, similar to modern humans and great apes, shows a gradual increase from inner to outer portions and a decrease from cuspal to cervical region. It is worth noting that average values in Bonobo are higher than in extant hominid species. Prisms between dentine horn and cusp are longer that 1.15 ET (enamel thickness) and thus the real measure were employed to estimate appositional formation time. Crown formation in Bonobo is shorter than in Chimpanzee probably due to the high appositional rate. However, the relation between I1 and M1 crown formation time does not differ to that reported for chimpanzee. One important result of the analysis concerns the periodicity of striae of Retzius. Contrary to suggestion that periodicity is the same for one individual, as has been observed in other hominid species, teeth in the Bonobo specimen show different periodicity.

   
 
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Inferring primate growth, development and life history profiles from dental microstructure.

GT Schwartz

Dept. of Anthropology & Institute of Human Origins, Arizona State University, Tempe, AZ (USA)

Teeth are a unique biological system in that two of their component hard tissues (enamel and dentine) preserve a permanent record of their growth in the form of daily markings. As a result, direct evidence for the timing of important developmental events during evolution is available from even fragmentary dental remains. Using these incremental markers, it’s possible to reconstruct the life history of extinct primates with unparalleled accuracy. In this talk, I will review current projects aimed at charting the evolution of the uniquely modern human pattern of growth, as well as the unique life history profiles of Malagasy lemurs.

Of all the dental developmental events, age at M1 eruption is most closely tied to the pace of life history. Generally, larger primates exhibit prolonged dental development, later M1 eruption age, and slow life history. However, recent analyses show a much later M1 eruption age in Pongo compared to African apes, suggesting a different scaling relationship among these important biological parameters for large-bodied hominoids. Extrapolating from anthropoids, one might expect the same relationship between the pace of dental development and body size to exist in large-bodied lemurs. However, some of the giant extinct lemurs bear the classic eruption sequence signature of species with rapid crown formation and "fast" life histories, according to "Schultz' Rule," while others exhibit "slow" eruption sequence signatures. Data on the pace of dental development from incremental markings in teeth of some of the largest extinct lemurs (Megaladapis, Palaeopropithecus, and Archaeolemur) in comparison to their smaller-bodied, extant sister-taxa reveal some interesting dissociations between body size and growth rates. In general, there is no simple relationship between crown formation time and either body size or eruption sequence in the lemurs of Madagascar. This finding has important implications for the evolution of primate life histories. Supported by NSF grant BCS-0237126.

   
 
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A computerized model for reconstruction of dental ontogeny: A new tool for studying evolutionary trends in the dentition

P Smith 1, R Müller 2, Y Gabet 3 and G Avishai 1


1 Laboratory of Bio-Anthropology & Ancient-DNA, Institute of Dental Sciences, Faculty of Dental
Medicine, Hebrew University-Hadassah, Jerusalem, (Israel)
2 Institute for Biomedical Engineering, Swiss Federal Institute of Technology (ETH) and University of Zürich, Zürich (Switzerland)
3Bone Laboratory, Institute of Dental Sciences, Faculty of Dental Medicine, Hebrew University
Hadassah, Jerusalem, (Israel)

All hominid molars show the same sequence of cusp initiation, but differ in the extent and spatial orientation of later developmental stages that finalize cusp size, shape, proportions and enamel thickness. Identification of the stage of development that marks the onset of such variation is critical for differentiating between the early and late advent of change in developmental pathways that are associated with inter-species divergence.

Initiation of a cusp tip is denoted by a halt in cell division of a small locus of cells in the inner enamel epithelium and underlying cells of the dental papilla and their transformation into ameloblasts and odontoblasts that produce enamel and dentin. From this initial locus that forms the cusp tip; adjacent cells are recruited, so that a wave of calcification proceeds basally. The final size shape and arrangement of the cusps is proportional to the distribution of (1) mitotic activity, lag in initiation of successive cusps; (2) rate and/or duration of calcification relative to continued mitosis in developing cusps; (3) order of coalescence of advancing fronts of calcification in adjacent cusps; (4) spatial distribution of further growth basal to coalescence of the cusps and (5) amount and distribution of enamel apposition over the surface of the crown of the tooth.

Since cusps grow from tip to base (defined by the plane of coalescence with adjacent cusps), the height and volume of each cusp at the dentin enamel junction (DEJ), reflects the extent of cell division while differences in the order of coalescence are expressed in the relative height of the base of individual cusps. Continued development of the tooth basal to the last coalescence represents the final stage of growth and is complemented by ongoing enamel apposition.

We have developed a 3-dimensional computerized model of a lower molar that enables us to identify and quantify the different stages of tooth development defined above. The model is based on serial micro-computed tomography (micro-CT) images that provide accurate definition of the outer and inner enamel and dentin boundaries of individual cusps. The XY co-ordinates of enamel and dentin are obtained by contouring each micro-CT image, while the Z co-ordinate is calculated from slice location and thickness. From these co-ordinates any desired set of values can be quantified. In the current study we have used them to quantify height and volume of the enamel and dentine of each cusp, measured from tip to coalescence with adjacent cusps, and crown area at the height of maximum contour.

We have tested the validity of this model through its application to a series of recent lower molars and used it (1) to identify the extent of differences in growth between cusps and the base of the crown in order to apportion mesio-distal and bucco-lingual growth; (2) to distinguish between growth defined by cell division as expressed by the DEJ and ameloblast activity as shown by enamel thickness; (3) to identify the order of coalescence of cusps and the extent to which it differs from the known order of cusp initiation.

We propose that this model provides a novel contribution to the identification of ontogenetic trajectories associated with evolutionary trends in tooth size, shape and enamel thickness.

Acknowledgements
This study was supported by Grant No. 032-5302 from the Israeli Science Foundation.

   
 
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New Perspectives on Chimpanzee and Human Molar Development.

TM Smith1, DJ Reid2, MC Dean3, AJ Olejniczak4, R.J. Ferrell5, L.B. Martin6

1Human Evolution Department, Max Planck Institute for Evolutionary Anthropology
2Department of Oral Biology, University of Newcastle upon Tyne
3Department of Anatomy and Developmental Biology, University College London
4Interdepartmental Doctoral Program in Anthropological Sciences, State University of New York at Stony Brook
5Center for Population and Health, Georgetown University
6Departments of Anthropology and of Anatomical Sciences, Stony Brook University.

Previous histological studies of small samples of chimpanzee and humans have suggested similarities in molar crown formation time, which is surprising given life history differences. As part of an on-going study of hominoid molar development, we report on the largest-known sample of chimpanzee and human molar material from several populations, including a re-evaluation of previously examined histological sections. Variation of incremental features within and between genera is examined, including Retzius line periodicity, Retzius line number, and enamel daily secretion rate. Differences due to population-level variation, size variation, and sex are considered.

Retzius line periodicity ranges from 6 - 7 days within chimpanzees and 6 - 12 days within humans. Within upper molars, mesiopalatal cusps show thicker cuspal enamel than mesiobuccal cusps. Within lower molars, mesiobuccal cusps show greater Retzius line numbers and thicker cuspal enamel than mesiolingual cusps, resulting in longer formation times. A negative correlation was found between Retzius line number and periodicity, resulting in similar crown formation times within cusp types, despite the range of individual periodicities. Significant increasing trends in daily secretion rates were found from inner to outer cuspal enamel, ranging from approximately 3-5 microns/day. Humans show slightly lower and higher mean values at the beginning and end of cuspal formation, respectively, but both genera show an overall average of approximately 4 microns/day. This is consistent with recent reports on growth trajectories in hominoids.

Cusp-specific formation time in chimpanzees ranges from approximately two to three years. Within specific cusp types, humans show greater average formation times than chimpanzees, largely due to higher mean periodicity values and/or thicker cuspal enamel. However, formation time within cusp types varies considerably, and the two genera show overlapping ranges, which have implications for the interpretation of small samples.

   
 
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Dental microwear and paleoanthropology: cautions and possibilities

M Teaford

Center for Functional Anatomy and Evolution, John Hopkins University School of Medicine, Baltimore, MD (USA)

Fifty years ago, investigators realized they could gain insights into jaw movement and tooth-use through light-microscope analyses of wear patterns on teeth. Subsequent work rekindled interest in the topic, as many workers shifted to using the scanning electron microscope. Since then, numerous analyses of modern and fossil material have yielded insights into dietary variations within and between species, and new perspectives on the evolution of tooth use and diet in animals ranging from dinosaurs to human ancestors.

However, these analyses are not without their problems. Specifically, sample sizes are small, and SEM images are so complicated that analyses are difficult and time-consuming. Thus, we are only beginning to get a clearer picture of the dental microwear of the early hominids.

Pioneering work by Walker and Puech suggested qualitative differences in dental microwear between early hominids, but it wasn’t until Grine’s analyses of the South African australopithecines that we began to see quantitative, statistical evidence of such differences – with the robust australopithecines showing evidence of a harder, more abrasive diet than the gracile australopithecines. Subsequent work on the anterior teeth, by Ryan and Johanson, and by Ungar and Grine, gave further insights into tooth use in the early hominids, with, for example, the gracile australopithecines showing evidence of heavier incisor use than the robust australopithecines. Recent quantitative analyses have (1) reaffirmed Ryan and Johanson’s suggestions that Australopithecus afarensis shows microwear patterns indistinguishable from those of the modern gorilla, and (2) shown that the earliest members of our genus may also be distinguishable from each other on the basis of their molar microwear patterns.

While this work is promising, and hints at the possibilities of moving beyond standard evolutionary-morphological inferences, into inferences of differences in tooth use between known individuals, there is still a great deal of work to be done. For instance, we still know far too little about the causes of specific microwear patterns, either through laboratory studies or fieldwork. Similarly, we still know surprisingly little about variations in dental microwear patterns (e.g., between sexes, populations, and species). In the face of such challenges, SEM-analyses may be reaching the limits of their usefulness, as researchers strive for quicker, more objective analyses of larger samples. As a result, two methods are beginning to catch attention as possible “next steps” in the evolution of dental microwear analyses.

Solounias and Semprebon have advocated a return to lower magnification analyses, using qualitative assessments of microwear patterns viewed under a light microscope. The advantages of these analyses are that they’re cheap and fast, and may easily distinguish animals with extremely different diets. The disadvantages are that they’re also subjective, they can only detect extreme dietary differences, and they may not be able to detect artifacts on tooth surfaces.

Ungar et al. have begun scale-sensitive fractal analyses of data from a confocal microscope. Advantages include the ability to quickly, objectively and repeatedly characterize wear surfaces in 3D over entire wear facets, at a wide range of magnifications. The main disadvantage lies in the newness of the technique and challenges imposed by developing such cutting edge technology. Also, it remains to be seen how results compare with those from more traditional SEM analyses.With such new approaches, however, we may finally have the tools to take dental microwear analyses to new levels of inference.

   
 
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Tooth wear diversity in early hominid molars - a case study

L Ulhaas1, O Kullmer2, F Schrenk3

1Forschungsinstitut Senckenberg, Paläoanthropologie, 60325 Frankfurt am Main (Germany)
2Forschungsinstitut Senckenberg, Tertiäre Säugetiere, 60325 Frankfurt am Main (Germany)
3Forschungsinstitut Senckenberg, Abt. Quartärpaläontologie und Paläoanthropologie, Frankfurt am Main (Germany)

Functional relationships between diet and tooth morphology form an integral part of paleontological research. The detailed description of occlusal relief and wear patterns of molars provides information about food ingestion and mastication. In early hominids, overall molar morphology is fairly similar. Size measurements, e.g., of Australopithecine molars, show considerable overlap. Shape, as well as wear, are functionally important features that could not be measured previously due to the lack of techniques and methods to quantify the complex wear pattern of hominid teeth.
In a case study, we employ a new approach to compare details on the occlusal surface of worn australopithecine (A. afarensis, A. africanus, P. robustus) molars, in order to highlight differences in wear. High resolution optical topometry enables us to measure 3d parameters on computer models of jaws and teeth.

We compare various occlusal morphologies of worn teeth and attempt to interpret functionality, taking dental and orthodontic principles into account. Therefore, e.g., occlusal relief, cusp geometry and proportions, inclination of cusp slopes and the occlusal surface, and dentine exposition are quantified. Diverse modes of occlusal wear in Autralopithecines reflect differences in mastication behaviour and may indicate dietary diversity in early hominids.

   
 
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Dental topographic analysis and diets of early hominins

P Ungar

Department of Anthropology, University of Arkansas, Fayetteville, AR (USA)

Teeth provide the best available evidence for the diets of early hominins. After all, dental remains dominate fossil assemblages and are durable remnants of the digestive system. Paleoanthropologists have thus developed a number of approaches to teasing information about diet from teeth, drawing on both epigenetic indicators, such as chemical signatures and microwear, and on adaptive signals, such as tooth size and shape. In this presentation I will consider the role of tooth form studies for the reconstruction of feeding adaptations of early hominins, and offer new occlusal shape data for Australopithecus africanus and Paranthropus robustus.

Teeth are adapted to fracture foods with specific mechanical properties. Tough foods (those resistant to crack propagation) are most effectively split with sharp, sloping surfaces, such as blades or shearing crests, whereas brittle foods (those less resistant to fracture) are more efficiently shattered between flatter, less sloping surfaces. Thus, selection should favor occlusal morphologies suited to the fracture properties of critical foods. Indeed, studies of unworn molar teeth of all higher-level primate taxa have shown that species specializing on tough leaves have longer shearing crests than do most frugivores. Further, among frugivores, those that consume harder, more brittle items evince the least occlusal relief. Comparisons of the shapes of tooth surfaces can therefore provide important clues to the diets of early hominins and other fossil primates. Most such studies have been limited to unworn teeth though, because of difficulties with characterizing and comparing occlusal surfaces that change with wear.

What we need is a method to study functional morphology of variably worn teeth. This is where dental topographic analysis comes in. Elevation data representing an occlusal surface are collected using a laser scanner or piezo scanner, and imported into a geographic information systems model. Occlusal surfaces are then interpolated, and summary statistics are calculated for functional attributes including average occlusal slope, relief and surface jaggedness. Because this approach does not involve measurements between landmarks that change or disappear with wear, it is equally useful for characterizing and comparing unworn and worn teeth.

New data collected using this approach are presented for high-resolution replicas of complete M2s of Australopithecus africanus (n = 18) and Paranthropus robustus (n =15). Australopith specimens were sorted into five gross wear categories, and ranked data for all but the most worn molars were analyzed using a two-factor ANOVA with species and wear stage as the factors. Results indicate significant variation between species and among wear stages, but no interaction between the two factors. Australopithecus africanus specimens have more occlusal slope at given stages of wear than do those of Paranthropus robustus. Further, more worn specimens are flatter than less worn teeth for both species. Finally, the degree of difference between the hominin species remains about the same at each wear stage.

These results can be compared with those previously published for other early hominins and extant primates. For example, the differences between Australopithecus africanus and Paranthropus robustus are evidently less than those between A. afarensis and early Homo or between Pan troglodytes troglodytes and Gorilla gorilla gorilla. Further, both of the South African early hominin species have less sloping occlusal surfaces than do either extant African apes or early Homo. These results suggest that neither of the South African australopiths was adapted to consume tough foods, and that P. robustus was better suited to crushing hard, brittle foods than was A. africanus. Differences in occlusal morphology between these early hominins are on the order expected if the two species differed mostly in fallback resources taken during times of resource scarcity.

   
 
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  HRXCT analysis of hominoid molars: quantitative volumetric analysis and 3D reconstruction of enamel and dentin volumes.

David G. Gantt1*, John Kappleman2, Richard A. Ketcham3

1Department of Anatomy, Georgia Campus, Philadelphia College of Osteopathic Medicine, Suwanee, GA, 30024 (USA)
2Department of Anthropology, University of Texas, Austin, TX 78712-1086 (USA)
3High-Resolution X- Ray Computed Tomography Facility, Department of Geological and Earth Sciences, University of Texas, Austin, TX 78712-1101 (USA)

*davidgapcom.edu

Previous investigations have documented that Homo sapiens, Pongo, and Australopithecines have thick enamel, while in Gorilla and Pan the enamel is thin. Over the past several decades I and other colleagues have attempted to describe and understand the range and distribution of enamel thickness in both extinct and extant primates, especially hominoids. These endeavors however, have led to continued controversy and even criticism of the destructive techniques used to produce sectioned teeth, especially of fossil specimens thus limiting our knowledge concerning the significance of enamel thickness in hominoids. An attempt to resolve this issue by nondestructive means began in the 1990’s with the use of Medical CT scanning systems. CT (Computed Axial Tomography) data was obtained on the thickness of enamel; however the results overestimated the real enamel thickness values obtained from section teeth and was unable to resolve the thickness in fossil specimens. Within the pass ten years however, two new CT scanning systems have been developed that allow us to quantitatively study dental hard tissues with a higher degree of accuracy than ever before, that is, the Micro-CT (?CT) and the Industrial CT or HRXCT.
Nearly three decades have passed without a resolution concerning the significance of enamel thickness in hominoids or the meaning of “thick”, “hyper-thick” or “thin” enamel? In order to resolve this issue the most useful measurement is the VOLUME of enamel for it relates directly to the activity of the ameloblasts. The HRXCT is capable of slice thickness of 5 ?m with a focal resolution of 5 ?m. Consecutive serial sections of hominoid molars (Homo, Pan, Gorilla, Sivapithecus, and Proconsul) were obtained by HRXCT. The digital images were then subjected to 3D and volumetric reconstruction software analysis, which allowed us to reconstruct the thickness and volume of both enamel and dentin. Enamel thickness data supports previously published sectioned data for all taxa. Volumetric data also provided support for the general concepts of “thick” and “thin” enamel but significantly demonstrates the differences among hominoids both extant and extinct. Enamel and dentin volumes hold the same ratio within each taxon. Enamel volume in Sivapithecus was 2x that of Pan. Human molars were 2x that of Gorilla, although the molars of Gorilla are significantly larger, and 4x that of Pan. Our work has clearly established the viability of HRXCT as a nondestructive means of analyzing dental hard tissues, of both extant and extinct hominoids. Furthermore, quantitatively data was obtained for the thickness and volume of both enamel and dentin.

   
 
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