Mandibular molar root morphology in Neanderthals and Late Pleistocene and recent Homo sapiens

Neanderthals have a distinctive suite of dental features, including large anterior crown and root dimensions and molars with enlarged pulp cavities. Yet, there is little known about variation in molar root morphology in Neanderthals and other recent and fossil members of Homo. Here, we provide the first comprehensive metric analysis of permanent mandibular molar root morphology in Middle and Late Pleistocene Homo neanderthalensis, and Late Pleistocene (Aterian) and recent Homo sapiens. We specifically address the question of whether root form can be used to distinguish between these groups and assess whether any variation in root form can be related to differences in tooth function. We apply a microtomographic imaging approach to visualise and quantify the external and internal dental morphologies of both isolated molars and molars embedded in the mandible (n = 127). Univariate and multivariate analyses reveal both similarities (root length and pulp volume) and differences (occurrence of pyramidal roots and dental tissue volume proportion) in molar root morphology among penecontemporaneous Neanderthals and Aterian H. sapiens. In contrast, the molars of recent H. sapiens are markedly smaller than both Pleistocene H. sapiens and Neanderthals, but share with the former the dentine volume reduction and a smaller root-to-crown volume compared with Neanderthals. Furthermore, we found the first molar to have the largest average root surface area in recent H. sapiens and Neanderthals, although in the latter the difference between M₁ and M₂ is small. In contrast, Aterian H. sapiens root surface areas peak at M₂. Since root surface area is linked to masticatory function, this suggests a distinct occlusal loading regime in Neanderthals compared with both recent and Pleistocene H. sapiens.     

广西化石猩猩牙齿釉质厚度研究

华南和东南亚发现大量更新世的猩猩牙齿化石。本研究应用CT扫描三维重建的技术方法研究了广西更新世化石猩猩牙齿釉质厚度,并与现生类人猿、现代人、化石类人猿以及早期人类进行比较分析。结果显示:广西猩猩同类牙齿的釉质厚度与牙齿大小相关性很小;臼齿和前臼齿釉质厚度在上下颌之间不存在显著性差异;来自广西不同地区的猩猩化石牙釉质厚度无显著差异。与早期人科成员相比,广西猩猩的牙釉质相对较薄,平均与相对釉质厚度值都明显小于南方古猿、傍人。与早期人属相比,小于直立人、尼人以及非洲和欧洲的早期人属化石。与现代人和现生灵长类相比,广西化石猩猩釉质厚度明显大于大部分猴类和非洲大猿;平均釉质厚度稍大于现生猩猩,而与现代人更为接近;相对釉质厚度小于现代人,而与现生猩猩差异不大,都属于偏厚型釉质。本文讨论了釉质厚度与系统分类演化、食性适应的相关问题,作者推测釉质厚度可能是物种的特征属性,与牙齿功能适应有密切关联。     

New dental remains of Anoiapithecus and the first appearance datum of hominoids in the Iberian Peninsula

New dental remains of the fossil great ape Anoiapithecus brevirostris are described from the Middle Miocene local stratigraphic series of Abocador de Can Mata (ACM) in els Hostalets de Pierola (Vallès-Penedès Basin, NE Iberian Peninsula). These specimens correspond to maxillary fragments with upper teeth from two female individuals from two different localities: left P³–M¹ (IPS41712) from ACM/C3-Aj (type locality; 11.9 Ma [millions of years ago]); and right M¹–M² and left P⁴–M² (IPS35027) from ACM/C1-E* (12.3–12.2 Ma). Relative enamel thickness is also computed in the latter individual and re-evaluated in other Middle Miocene hominoids from ACM, in order to better assess their taxonomic affinities. With regard to maxillary sinus development, occlusal morphology, molar proportions and enamel thickness, the new specimens show greater resemblances with the (male) holotype specimen of A. brevirostris. They differ from Pierolapithecus catalaunicus in displaying less inflated crests, a more lingually-located hypocone, and relatively lower-crowned molars; from Dryopithecus fontani, in the relatively thicker enamel and lower-crowned molars; from Hispanopithecus spp., in the more inflated crown bases, less peripheral cusps and more restricted maxillary sinus; and from Hispanopithecus laietanus also in the thicker crests, more restricted occlusal foveae, and relatively lower-crowned molars. The new specimens of A. brevirostris show some slight differences compared with the holotype of this species: smaller size (presumably due to sexual size dimorphism), and less distally-tapering M² occlusal contour (which is highly variable in both extant and extinct hominoids). The reported remains provide valuable new evidence on dental intraspecific variation and sexual dimorphism in Anoiapithecus. From a taxonomic viewpoint, they support the distinction of this taxon from both Dryopithecus and Pierolapithecus. From a chronostratigraphic perspective, IPS35027 from ACM/C1-E* enlarges the known temporal distribution of Anoiapithecus, further representing the oldest record (first appearance datum) of hominoids in the Iberian Peninsula.     

Brief communication: enamel thickness trends in the dental arcade of humans and chimpanzees

In addition to evidence for bipedality in some fossil taxa, molar enamel thickness is among the few characters distinguishing (thick-enameled) hominins from the (thin-enameled) African apes. Despite the importance of enamel thickness in taxonomic discussions and a long history of scholarship, measurements of enamel thickness are performed almost exclusively on molars, with relatively few studies examining premolars and anterior teeth. This focus on molars has limited the scope of enamel thickness studies (i.e., there exist many fossil hominin incisors, canines, and premolars). Increasing the available sample of teeth from which to compare enamel thickness measurements from the fossil record could substantially increase our understanding of this aspect of dental biology, and perhaps facilitate greater taxonomic resolution of early hominin fossils. In this study, we report absolute and relative (size-scaled) enamel thickness measurements for the complete dentition of modern humans and chimpanzees. In accord with previous studies of molars, chimpanzees show lower relative enamel thickness at each tooth position, with little overlap between the two taxa. A significant trend of increasing enamel thickness from anterior to posterior teeth is apparent in both humans and chimpanzees, indicating that inter-taxon comparisons should be limited to the same tooth position in order to compare homologous structures. As nondestructive imaging techniques become commonplace (facilitating the examination of increasing numbers of fossil specimens), studies may maximize available samples by expanding beyond molars.     

Evidence in hand: recent discoveries and the early evolution of human manual manipulation

For several decades, it was largely assumed that stone tool use and production were abilities limited to the genus Homo. However, growing palaeontological and archaeological evidence, comparative extant primate studies, as well as results from methodological advancements in biomechanics and morphological analyses, have been gradually accumulating and now provide strong support for more advanced manual manipulative abilities and tool-related behaviours in pre-Homo hominins than has been traditionally recognized. Here, I review the fossil evidence related to early hominin dexterity, including the recent discoveries of relatively complete early hominin hand skeletons, and new methodologies that are providing a more holistic interpretation of hand function, and insight into how our early ancestors may have balanced the functional requirements of both arboreal locomotion and tool-related behaviours.     

Talonid crests expression at the enamel–dentine junction of hominin lower permanent and deciduous molars

The application of microtomography (mCT) to dental morphological studies has unveiled a new source of palaeobiological information, particularly in the analysis of the internal structures of teeth. In this study, we assess the expression of talonid crests at the enamel and dentine surfaces in lower permanent and second deciduous molars (M₂ and dm₂) of H. sapiens, H. neanderthalensis and Atapuerca-Sima de los Huesos (SH) hominins. In modern humans, talonid crests are described exclusively in the deciduous teeth (Korenhof, 1982) and interpreted as a primitive mammalian remnant of the talonid attachment to the trigonid. Here we report for the first time the expression of talonid crests of deciduous and permanent molars in H. sapiens, H. neanderthalensis and Middle Pleistocene hominins. We discuss possible evolutionary interpretations and suggest the importance of recording this feature in future studies.     

Dental Ontogeny in Pliocene and Early Pleistocene Hominins

Until recently, our understanding of the evolution of human growth and development derived from studies of fossil juveniles that employed extant populations for both age determination and comparison. This circular approach has led to considerable debate about the human-like and ape-like affinities of fossil hominins. Teeth are invaluable for understanding maturation as age at death can be directly assessed from dental microstructure, and dental development has been shown to correlate with life history across primates broadly. We employ non-destructive synchrotron imaging to characterize incremental development, molar emergence, and age at death in more than 20 Australopithecus anamensis, Australopithecus africanus, Paranthropus robustus and South African early Homo juveniles. Long-period line periodicities range from at least 6–12 days (possibly 5–13 days), and do not support the hypothesis that australopiths have lower mean values than extant or fossil Homo. Crown formation times of australopith and early Homo postcanine teeth fall below or at the low end of extant human values; Paranthropus robustus dentitions have the shortest formation times. Pliocene and early Pleistocene hominins show remarkable variation, and previous reports of age at death that employ a narrow range of estimated long-period line periodicities, cuspal enamel thicknesses, or initiation ages are likely to be in error. New chronological ages for SK 62 and StW 151 are several months younger than previous histological estimates, while Sts 24 is more than one year older. Extant human standards overestimate age at death in hominins predating Homo sapiens, and should not be applied to other fossil taxa. We urge caution when inferring life history as aspects of dental development in Pliocene and early Pleistocene fossils are distinct from modern humans and African apes, and recent work has challenged the predictive power of primate-wide associations between hominoid first molar emergence and certain life history variables.     

Carabelli's trait revisited: An examination of mesiolingual features at the enamel-dentine junction and enamel surface of Pan and Homo sapiens upper molars

Carabelli’s trait is a morphological feature that frequently occurs on the mesiolingual aspect of Homo sapiens upper molars. Similar structures also referred to as Carabelli’s trait have been reported in apes and fossil hominins. However, the morphological development and homology of these mesiolingual structures among hominoids are poorly understood. In this study, we employ micro-computed tomography to image the enamel-dentine junction (EDJ) and outer enamel surface (OES) of Pan (n = 48) and H. sapiens (n = 52) upper molars. We investigate the developmental origin of mesiolingual features in these taxa and establish the relative contribution of the EDJ and enamel cap to feature expression. Results demonstrate that mesiolingual features of H. sapiens molars develop at the EDJ and are similarly expressed at the OES. Morphological variation at both surfaces in this taxon can satisfactorily be assessed using standards for Carabelli’s trait developed by the Arizona State University Dental Anthropology System (ASUDAS). Relative to H. sapiens, Pan has an even greater degree of correspondence in feature expression between the EDJ and OES. Morphological manifestations in Pan molars are not necessarily limited to the protocone and are best characterized by a lingual cingulum that cannot be captured by the ASUDAS. Cusp-like structures, similar to those seen in marked Carabelli’s trait expressions in H. sapiens, were not found in Pan. This study provides a foundation for further analyses on the evolutionary history of mesiolingual dental traits within the hominoid lineage. It also highlights the wealth of morphological data that can be obtained at the EDJ for understanding tooth development and for characterizing tooth crown variation in worn fossil teeth.     

Variation in hominoid molar enamel thickness

Enamel thickness has figured prominently in discussions of hominid origins for nearly a century, although little is known about its intra-taxon variation. It has been suggested that enamel thickness increases from first to third molars, perhaps due to varying functional demands or developmental constraints, but this has not been tested with appropriate statistical methods. We quantified enamel cap area (c), dentine area (b), and enamel-dentine junction length (e) in coronal planes of sections through the mesial and distal cusps in 57 permanent molars of Pan and 59 of Pongo, and calculated average (c/e) and relative enamel thickness (([c/e]/ radicalb) * 100). Posteriorly increasing or decreasing trends in each variable and average (AET) and relative enamel thickness (RET) were tested among molars in the same row. Differences between maxillary and mandibular analogues and between mesial and distal sections of the same tooth were also examined. In mesial sections of both genera, enamel cap area significantly increased posteriorly, except in Pan maxillary sections. In distal sections of maxillary teeth, trends of decreasing dentine area were significant in both taxa, possibly due to hypocone reduction. Significant increases in AET and RET posteriorly were found in all comparisons, except for AET in Pongo distal maxillary sections. Several significant differences were found between maxillary and mandibular analogues in both taxa. Relative to their mesial counterparts, distal sections showed increased enamel cap area and/or decreased dentine area, and thus increased AET and RET. This study indicates that when AET and RET are calculated from samples of mixed molars, variability is exaggerated due to the lumping of tooth types. To maximize taxonomic discrimination using enamel thickness, tooth type and section plane should be taken into account. Nonetheless, previous findings that African apes have relatively thinner enamel than Pongo is supported for certain molar positions.     

Enamel thickness,microstructure and development in Afropithecus turkanensis

Afropithecus turkanensis, a 17-17.5 million year old large-bodied hominoid from Kenya, has previously been reported to be the oldest known thick-enamelled Miocene ape. Most investigations of enamel thickness in Miocene apes have been limited to opportunistic or destructive studies of small samples. Recently, more comprehensive studies of enamel thickness and microstructure in Proconsul, Lufengpithecus, and Dryopithecus, as well as extant apes and fossil humans, have provided information on rates and patterns of dental development, including crown formation time, and have begun to provide a comparative context for interpretation of the evolution of these characters throughout the past 20 million years of hominoid evolution. In this study, enamel thickness and aspects of the enamel microstructure in two A. turkanensis second molars were quantified and provide insight into rates of enamel apposition, numbers of cells actively secreting enamel, and the time required to form regions of the crown. The average value for relative enamel thickness in the two molars is 21.4, which is a lower value than a previous analysis of this species, but which is still relatively thick compared to extant apes. This value is similar to those of several Miocene hominoids, a fossil hominid, and modern humans. Certain aspects of the enamel microstructure are similar to Proconsul nyanzae, Dryopithecus laietanus, Lufengpithecus lufengensis, Graecopithecus freybergi and Pongo pygmaeus, while other features differ from extant and fossil hominoids. Crown formation times for the two teeth are 2.4-2.6 years and 2.9-3.1 years respectively. These times are similar to a number of extant and fossil hominoids, some of which appear to show additional developmental similarities, including thick enamel. Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enamelled extant apes.     

Adaptation to hard-object feeding in sea otters and hominins

The large, bunodont postcanine teeth in living sea otters (Enhydra lutris) have been likened to those of certain fossil hominins, particularly the ’robust’ australopiths (genus Paranthropus). We examine this evolutionary convergence by conducting fracture experiments on extracted molar teeth of sea otters and modern humans (Homo sapiens) to determine how load-bearing capacity relates to tooth morphology and enamel material properties. In situ optical microscopy and x-ray imaging during simulated occlusal loading reveal the nature of the fracture patterns. Explicit fracture relations are used to analyze the data and to extrapolate the results from humans to earlier hominins. It is shown that the molar teeth of sea otters have considerably thinner enamel than those of humans, making sea otter molars more susceptible to certain kinds of fractures. At the same time, the base diameter of sea otter first molars is larger, diminishing the fracture susceptibility in a compensatory manner. We also conduct nanoindentation tests to map out elastic modulus and hardness of sea otter and human molars through a section thickness, and microindentation tests to measure toughness. We find that while sea otter enamel is just as stiff elastically as human enamel, it is a little softer and tougher. The role of these material factors in the capacity of dentition to resist fracture and deformation is considered. From such comparisons, we argue that early hominin species like Paranthropus most likely consumed hard food objects with substantially higher biting forces than those exerted by modern humans.     

Liang Bua Homo floresiensis mandibles and mandibular teeth: a contribution to the comparative morphology of a new hominin species

In 2004, a new hominin species, Homo floresiensis, was described from Late Pleistocene cave deposits at Liang Bua, Flores. H. floresiensis was remarkable for its small body-size, endocranial volume in the chimpanzee range, limb proportions and skeletal robusticity similar to Pliocene Australopithecus, and a skeletal morphology with a distinctive combination of symplesiomorphic, derived, and unique traits. Critics of H. floresiensis as a novel species have argued that the Pleistocene skeletons from Liang Bua either fall within the range of living Australomelanesians, exhibit the attributes of growth disorders found in modern humans, or a combination of both. Here we describe the morphology of the LB1, LB2, and LB6 mandibles and mandibular teeth from Liang Bua. Morphological and metrical comparisons of the mandibles demonstrate that they share a distinctive suite of traits that place them outside both the H. sapiens and H. erectus ranges of variation. While having the derived molar size of later Homo, the symphyseal, corpus, ramus, and premolar morphologies share similarities with both Australopithecus and early Homo. When the mandibles are considered with the existing evidence for cranial and postcranial anatomy, limb proportions, and the functional anatomy of the wrist and shoulder, they are in many respects closer to African early Homo or Australopithecus than to later Homo. Taken together, this evidence suggests that the ancestors of H. floresiensis left Africa before the evolution of H. erectus, as defined by the Dmanisi and East African evidence.     

Variations and asymmetries in regional brain surface in the genus Homo

Paleoneurology is an important field of research within human evolution studies. Variations in size and shape of an endocast help to differentiate among fossil hominin species whereas endocranial asymmetries are related to behavior and cognitive function. Here we analyse variations of the surface of the frontal, parieto-temporal and occipital lobes among different species of Homo, including 39 fossil hominins, ten fossil anatomically modern Homo sapiens and 100 endocasts of extant modern humans. We also test for the possible asymmetries of these features in a large sample of modern humans and observe individual particularities in the fossil specimens.This study contributes important new information about the brain evolution in the genus Homo. Our results show that the general pattern of surface asymmetry for the different regional brain surfaces in fossil species of Homo does not seem to be different from the pattern described in a large sample of anatomically modern H. sapiens, i.e., the right hemisphere has a larger surface than the left, as do the right frontal, the right parieto-temporal and the left occipital lobes compared with the contra-lateral side. It also appears that Asian Homo erectus specimens are discriminated from all other samples of Homo, including African and Georgian specimens that are also sometimes included in that taxon. The Asian fossils show a significantly smaller relative size of the parietal and temporal lobes. Neandertals and anatomically modern H. sapiens, who share the largest endocranial volume of all hominins, show differences when considering the relative contribution of the frontal, parieto-temporal and occipital lobes. These results illustrate an original variation in the pattern of brain organization in hominins independent of variations in total size. The globularization of the brain and the enlargement of the parietal lobes could be considered derived features observed uniquely in anatomically modern H. sapiens.     

New fossils of Australopithecus anamensis from Kanapoi,West Turkana,Kenya (2003–2008)

Renewed fieldwork from 2003 through 2008 at the Australopithecus anamensis type-site of Kanapoi, Kenya, yielded nine new fossils attributable to this species. These fossils all date to between 4.195 and 4.108 million years ago. Most were recovered from the lower fluvial sequence at the site, with one from the lacustrine sequence deltaic sands that overlie the lower fluvial deposits but are still below the Kanapoi Tuff. The new specimens include a partial edentulous mandible, partial maxillary dentition, two partial mandibular dentitions, and five isolated teeth. The new Kanapoi hominin fossils increase the sample known from the earliest Australopithecus, and provide new insights into morphology within this taxon. They support the distinctiveness of the early A. anamensis fossils relative to earlier hominins and to the later Australopithecus afarensis. The new fossils do not appreciably extend the range of observed variation in A. anamensis from Kanapoi, with the exception of some slightly larger molars, and a canine tooth root that is the largest in the hominin fossil record. All of the Kanapoi hominins share a distinctive morphology of the canine–premolar complex, typical early hominin low canine crowns but with mesiodistally longer honing teeth than seen in A. afarensis, and large, probably dimorphic, canine tooth roots. The new Kanapoi specimens support the observation that canine crown height, morphology, root size and dimorphism were not altered from a primitive ape-like condition as part of a single event in human evolution, and that there may have been an adaptive difference in canine function between A. anamensis and A. afarensis.     

人类牙齿齿冠和齿根分离两种技术方法对牙釉质厚度测量的影响

在基于计算机断层扫描技术（CT）和虚拟图像处理技术的灵长类牙齿测量学研究中,经常需要分离三维虚拟模型的齿冠和齿根,再进行后续测量工作,如计算机辅助的生物力学分析、釉质厚度测量等。而分离齿冠和齿根这一步骤,目前有多种方法,如,1)根据齿颈线切分齿冠,或2)人工建立基底平面切分齿冠。为了评估这两种不同的处理方式对后续的牙齿测量学上的影响,本文使用三维方法测量了82例化石和现代人类下颌后部牙齿的釉质厚度,包括南方古猿、早期人属、尼安德特人和现代人。使用配对t检验对比发现,两种方法得到的釉质厚度数值上没有显著差别,但随后进行的种间比较发现,使用基底平面切分齿冠的方法比较费时,更依赖于测量者的人工操作,并且可能弱化了物种间前臼齿绝对釉质厚度的差异,造成系统误差。其原因是对于前臼齿和前部牙齿等齿颈线形状不规则的标本,基底平面难以建立或误差较大。在未来对釉质厚度的种间差异的研究中,特别对齿颈线形状不规则的标本（如人类前部牙齿及猩猩、黑猩猩的牙齿等）,本文推荐使用齿颈线分离齿冠和齿根,测量和计算齿颈线之上的釉质厚度。釉质厚度有一定的分类学、功能形态学和系统发育学意义。本文积累了一批可供未来对比研究的原始数据,并且发现尼安德特人前臼齿的相对釉质厚度显著小于现代人,这与前人利用臼齿、犬齿所做的对比研究结果相同,支持了尼安德特人拥有较薄的相对釉质厚度这一观点。     

Hominin teeth from the early Late Pleistocene site of Xujiayao,Northern China

It is generally accepted that from the late Middle to the early Late Pleistocene (～340–90 ka BP), Neanderthals were occupying Europe and Western Asia, whereas anatomically modern humans were present in the African continent. In contrast, the paucity of hominin fossil evidence from East Asia from this period impedes a complete evolutionary picture of the genus Homo, as well as assessment of the possible contribution of or interaction with Asian hominins in the evolution of Homo sapiens and Homo neanderthalensis. Here we present a comparative study of a hominin dental sample recovered from the Xujiayao site, in Northern China, attributed to the early Late Pleistocene (MIS 5 to 4). Our dental study reveals a mosaic of primitive and derived dental features for the Xujiayao hominins that can be summarized as follows: i) they are different from archaic and recent modern humans, ii) they present some features that are common but not exclusive to the Neanderthal lineage, and iii) they retain some primitive conformations classically found in East Asian Early and Middle Pleistocene hominins despite their young geological age. Thus, our study evinces the existence in China of a population of unclear taxonomic status with regard to other contemporary populations such as H. sapiens and H. neanderthalensis. The morphological and metric studies of the Xujiayao teeth expand the variability known for early Late Pleistocene hominin fossils and suggest the possibility that a primitive hominin lineage may have survived late into the Late Pleistocene in China. Am J Phys Anthropol 156:224–240, 2015. © 2014 Wiley Periodicals, Inc.     

Enamel thickness in the Middle Miocene great apes Anoiapithecus,Pierolapithecus and Dryopithecus

On the basis of industrial computed tomography, relative enamel thickness (RET) is computed in three Middle Miocene (ca 11.9–11.8 Ma) hominoids from Abocador de Can Mata (Vallès-Penedès Basin, Catalonia, Spain): Pierolapithecus catalaunicus from BCV1 and Anoiapithecus brevirostris from C3-Aj, interpreted as stem hominids; and Dryopithecus fontani from C3-Ae of uncertain phylogenetic affinities. Pierolapithecus displays an average RET value of 19.5, Anoiapithecus of 18.6 and Dryopithecus of 10.6. The thick-enamelled condition of Pierolapithecus and Anoiapithecus is also characteristic of afropithecids, including the more derived kenyapithecins from the early Middle Miocene of Eurasia (Griphopithecus and Kenyapithecus). Given the presence of other dentognathic and craniofacial similarities, thick enamel may be interpreted as a symplesiomorphy of the Hominidae (the great ape and human clade), which would have been later independently modified along several lineages. Given the correlation between thick enamel and hard-object feeding, our results suggest that thick enamel might have been the fundamental adaptation that enabled the out-of-Africa dispersal of great-ape ancestors and their subsequent initial radiation throughout Eurasia. The much thinner enamel of Dryopithecus is difficult to interpret given phylogenetic uncertainties, being either a hominine synapomorphy or a convergently developed feature.