Is the genetic structure of Gran Chaco populations unique? Interregional perspectives on native South American mitochondrial DNA variation

This study reevaluates the hypothesis in Demarchi et al. (2001 Am. J. Phys. Anthropol. 115:199-203) that Gran Chaco peoples demonstrate a unique pattern of genetic diversity due to a distinct regional population history. Specifically, they found populations in the central part of the Gran Chaco, or Central Chaco, to have higher within- and lower between-population mitochondrial DNA (mtDNA) haplogroup frequency variation compared to populations in other South American regions. To test this hypothesis of regional uniqueness, we applied analytical and simulation methods to mtDNA first hypervariable (HVI) region sequence data from a broad set of comparative South and Central American population samples. Contrary to the results of Demarchi et al. (2001 Am. J. Phys. Anthropol. 115:199-203), we found that the Gran Chaco's regional within-population diversity is about average among regions, and populations are highly differentiated from each other. When we limited the scale of analysis to the Central Chaco, a more localized subregion of the Gran Chaco, our results fell more in line with the original findings of Demarchi et al. (2001 Am. J. Phys. Anthropol. 115:199-203). Still, we conclude that neither the Gran Chaco regional pattern, nor the Central Chaco subregional pattern, is unique within South America. Nonetheless, the Central Chaco pattern accords well with the area's history, including pre-European contact lifeways and the documented historical use of the area as an interregional crossroads. However, we cannot exclude post-European contact disruption of traditional mating networks as an equally plausible explanation for the observed diversity pattern. Finally, these results additionally inform broader models of South American genetic diversity. While other researchers proposed an east-west continental division in patterns of genetic variation (e.g., Fuselli et al. 2003 Mol. Biol. Evol. 20:1682-1691), we found that in the geographically intermediate Central Chaco, a strict east-west divide in genetic variation breaks down. We suggest that future genetic characterizations of the continent, and subsequent interpretations of evolutionary history, involve a broad regional sampling of South American populations.     

Early Holocene human remains from the Argentinean Pampas: Additional evidence for distinctive cranial morphology of early South Americans

The cranial morphology of Early Holocene American human samples is characterized by a long and narrow cranial vault, whereas more recent samples exhibit a shorter and wider cranial vault. Two hypotheses have been proposed to account for the morphological differences between early and late‐American samples: (a) the migratory hypothesis that suggests that the morphological variation between early and late American samples was the result of a variable number of migratory waves; and (b) the local diversification hypothesis, that is, the morphological differences between early and late American samples were mainly generated by local, random (genetic drift), and nonrandom factors (selection and phenotypic plasticity). We present the first craniometric study of three early skulls from the Argentinean Pampas, dated ～8,000 cal. years BP (Arroyo Seco 2, Chocorí, and La Tigra), and one associated with mega‐faunal remains (Fontezuelas skull). In addition, we studied several Late Holocene samples. We show that the skulls from the Argentinean Pampas are morphologically similar to other Early Holocene American skulls (i.e., Lagoa Santa from Brazil, Tequendama, Checua, and Aguazuque from Colombia, Lauricocha from Peru, and early Mexicans) that exhibit long and narrow cranial vaults. These samples differ from the Late Holocene American samples that exhibit a shorter and wider cranial vault. Our results underscore the important differences in cranial morphology between early and late‐American samples. However, we emphasize the need for further studies to discuss alternative hypotheses regarding such differences. Am J Phys Anthropol 143:298–305, 2010. © 2010 Wiley‐Liss, Inc.     

Morphometric cranial identity of prehistoric Malawians in the light of sub-Saharan African diversity

Little has been described of the Holocene populations of South‐Central Africa, despite the region demonstrating major subsistence shifts relating to dispersals of agriculturalists at least 2,000 years ago. Seven sites with associated human skeletal remains were selected. Hora, Chencherere, Fingura, and Mtuzi represent the Middle Holocene (2,000–5,000 years ago), and Phwadze, Mtemankhokwe, and Nkudzi Bay represent the Late Holocene and the arrival of agriculturalists between 500–2,000 years ago. Focusing on the identity of Hora and Chencherere specimens, two questions were addressed: are the various Holocene Malawians similar to each other, or do they suggest morphological change over time? What modern populations are closest to the prehistoric specimens? The archaeological sample was compared to modern sub‐Saharan Africans from four regions, plus a historic Khoi‐San foraging group. Factor analyses were performed in order to identify complex patterns of variation in metric traits of the skull. According to the results, prehistoric Malawians showed only slight differences between the Late and Middle Holocene, suggesting a population change without any major discontinuity. Later Stone Age skulls did not exclusively show similarities with the Khoi‐San, as they frequently fit well within the variation of modern Bantu‐speaking groups, especially West‐Central Africa. Therefore, we reject the hypothesis that Middle Holocene South‐Central Africans have an exclusively Khoi‐San ancestry, and support an alternative hypothesis that both Middle and Late Holocene groups share a common biological heritage originating in West‐Central Africa in earlier times. Am J Phys Anthropol, 2006. © 2005 Wiley‐Liss, Inc.     

Functional and morphological correlates of mandibular symphyseal form in a living human sample

Variation in recent human mandibular form is often thought to reflect differences in masticatory behavior associated with variation in food preparation and subsistence strategies. Nevertheless, while mandibular variation in some human comparisons appear to reflect differences in functional loading, other comparisons indicate that this relationship is not universal. This suggests that morphological variation in the mandible is influenced by other factors that may obscure the effects of loading on mandibular form. It is likely that highly strained mandibular regions, including the corpus, are influenced by well‐established patterns of lower facial skeletal integration. As such, it is unclear to what degree mandibular form reflects localized stresses incurred during mastication vs. a larger set of correlated features that may influence bone distribution patterns. In this study, we examine the relationship between mandibular symphyseal bone distribution (i.e., second moments of area, cortical bone area) and masticatory force production (i.e., in vivo maximal bite force magnitude and estimated symphyseal bending forces) along with lower facial shape variation in a sample of n = 20 living human male subjects. Our results indicate that while some aspects of symphyseal form (e.g., wishboning resistance) are significantly correlated with estimates of symphyseal bending force magnitude, others (i.e., vertical bending resistance) are more closely tied to variation in lower facial shape. This suggests that while the symphysis reflects variation in some variables related to functional loading, the complex and multifactorial influences on symphyseal form underscores the importance of exercising caution when inferring function from the mandible especially in narrow taxonomic comparisons. Am J Phys Anthropol 153:387–396, 2014. © 2013 Wiley Periodicals, Inc.     

In vivo bone strain and bone functional adaptation

Mechanistic interpretations of bone cross-sectional shapes are based on the paradigm of shape optimization such that bone offers maximum mechanical resistance with a minimum of material. Recent in vivo strain studies (Demes et al., Am J Phys Anthropol 106 (1998) 87-100, Am J Phys Anthropol 116 (2001) 257-265; Lieberman et al., Am J Phys Anthropol 123 (2004) 156-171) have questioned these interpretations by demonstrating that long bones diaphyses are not necessarily bent in planes in which they offer maximum resistance to bending. Potential limitations of these in vivo studies have been pointed out by Ruff et al. (Am J Phys Anthropol 129 (2006) 484-498). It is demonstrated here that two loading scenarios, asymmetric bending and buckling, would indeed not lead to correct predictions of loads from strain. It is also shown that buckling is of limited relevance for many primate long bones. This challenges a widely held view that circular bone cross sections make loading directions unpredictable for bones which is based on a buckling load model. Asymmetric bending is a potentially confounding factor for bones with directional differences in principal area moments (I(max) > I(min)). Mathematical corrections are available and should be applied to determine the bending axis in such cases. It is concluded that loads can be reliably extrapolated from strains. More strain studies are needed to improve our understanding of the relationships between activities, bone loading regimes associated with them, and the cross-sectional geometry of bones.     

Brief communication: Paleohistopathological analysis of pathology museum specimens: Can periosteal reaction microstructure explain lesion etiology?

The assertion that the microstructure of periosteal new bone formation can be used to differentiate between disease etiologies (Schultz: Yrbk Phys Anthropol 44 2001 106–147; Schultz: Identification of pathological conditions in human skeletal remains, 2nd ed. London: Academic Press 2003 73–109) was tested in a pilot‐study, using diagnosed bone specimens from St George's Hospital Pathology Museum, London, UK. Embedded bone specimens exhibiting pathological periosteal new bone formation were examined using scanning electron microscopy in back‐scattered electron imaging mode (SEM‐BSE). The results suggest that several histological features (i.e. Grenzstreifen, Polsters, and sinuous lacunae) deemed to be diagnostic of specific pathological conditions are of no specific diagnostic value, as they are encountered in pathological conditions of differing disease etiology. These results tie in with a previous investigation demonstrating a lack of diagnostic qualitative or quantitative characteristics seen in the macroscopic and radiographic appearance of periosteal reactions (Weston: Am J Phys Anthropol 137 2008 48–59). Am J Phys Anthropol, 2009. © 2009 Wiley‐Liss, Inc.     

Brief Communication: Quantitative‐ and molecular‐genetic differentiation in humans and chimpanzees: Implications for the evolutionary processes underlying cranial diversification

Estimates of the amount of genetic differentiation in humans among major geographic regions (e.g., Eastern Asia vs. Europe) from quantitative‐genetic analyses of cranial measurements closely match those from classical‐ and molecular‐genetic markers. Typically, among‐region differences account for ～10% of the total variation. This correspondence is generally interpreted as evidence for the importance of neutral evolutionary processes (e.g., genetic drift) in generating among‐region differences in human cranial form, but it was initially surprising because human cranial diversity was frequently assumed to show a strong signature of natural selection. Is the human degree of similarity of cranial and DNA‐sequence estimates of among‐region genetic differentiation unusual? How do comparisons with other taxa illuminate the evolutionary processes underlying cranial diversification? Chimpanzees provide a useful starting point for placing the human results in a broader comparative context, because common chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are the extant species most closely related to humans. To address these questions, I used 27 cranial measurements collected on a sample of 861 humans and 263 chimpanzees to estimate the amount of genetic differentiation between pairs of groups (between regions for humans and between species or subspecies for chimpanzees). Consistent with previous results, the human cranial estimates are quite similar to published DNA‐sequence estimates. In contrast, the chimpanzee cranial estimates are much smaller than published DNA‐sequence estimates. It appears that cranial differentiation has been limited in chimpanzees relative to humans. Am J Phys Anthropol 154:615–620, 2014. © 2014 Wiley Periodicals, Inc.     

Systematics and evolution of the Jacchus group of marmosets (Platyrrhini)

Interspecific differentiation and geographic variation patterns in 39 skull traits of eastern Brazilian marmosets are analyzed. Eastern Callithrix taxa are distinct morphologically, and no evidence of intergradation among taxa is observed. Instead, there are sharp, stepped morphological boundaries among taxa, consistent with species‐level distinction. The morphological similarity cluster diagram obtained from Mahalanobis distances is different from available molecular trees, and the general picture emerging is that the eastern Callithrix taxa should be considered as good species arising recently in South American history. In particular, C. kuhlii is morphologically distinct from other marmoset taxa, including C. geoffroyi and C. penicillata, which were previously hypothesized to be the parental populations that formed C. kuhlii by hybridization. Furthermore, C. kuhlii populations from southeastern Bahia do not overlap morphologically with any C. penicillata population, including the upper São Francisco River populations that display skin colors and pelage patterns to some extent similar to true Kuhli's marmosets. There is a negative, though insignificant, correlation between the morphological distance matrix and a Mahalanobis distance matrix estimated from nine climatic variables, a pattern opposite to that expected under a parapatric speciation model. This result, together with the lack of clinal variation in skull traits, suggests that an allopatric model of speciation might best explain eastern marmoset diversification. Am J Phys Anthropol 2003. © 2002 Wiley‐Liss, Inc.     

A quantitative and qualitative reanalysis of the endocast from the juvenile Paranthropus specimen l338y-6 from Omo, Ethiopia

Based on an analysis of its endocast, Holloway (1981 Am J Phys Anthropol 53:109-118) attributed the juvenile Omo L338y-6 specimen to Australopithecus africanus (i.e., gracile australopithecines) rather than to Paranthropus (Australopithecus) boisei (robust australopithecines) favored by other workers (Rak and Howell [1978] Am J Phys Anthropol 48:345-366). Holloway's attribution was based on the specimen's (1) low cranial capacity, (2) gracile-like meningeal vessels, (3) gracile-like cerebellar hemispheres, and (4) absence of an enlarged occipital/marginal (O/M) sinus system. Recent work, however, has shown that criteria 1 and 2 are not useful for sorting gracile from robust australopithecines (Culotta [1999] Science 284:1109-1111; Falk [1993] Am J Phys Anthropol 92:81-98). In this paper, we test criterion 3 by quantifying the endocranial cerebellar and occipital morphology reproduced on the Omo L338y-6 endocast, and comparing it to seven endocasts from South and East African early hominids. Our preliminary results show that metric analysis of this specimen cannot be used to sort it preferentially with either robust or gracile australopithecines. Finally, we demonstrate that, contrary to previous reports, the Omo L338y-6 endocast reproduces an enlarged left occipital sinus (criterion 4). This observation is consistent with the original attribution of the Omo specimen to robust australopithecines (Rak and Howell [1978] Am J Phys Anthropol 48:345-366). Furthermore, if Omo L338y-6 was a robust australopithecine, this discovery extends the occurrence of an enlarged O/M sinus system to one of the earliest known paranthropines. Am J Phys Anthropol 110:399-406, 1999.     

“Missing perikymata”—fact or fiction? A study on chimpanzee (Pan troglodytes verus) canines

Recently, a lower than expected number of perikymata between repetitive furrow‐type hypoplastic defects has been reported in chimpanzee canines from the Fongoli site, Senegal (Skinner and Pruetz: Am J Phys Anthropol 149 (2012) 468–482). Based on an observation in a localized enamel fracture surface of a canine of a chimpanzee from the Taï Forest (Ivory Coast), these authors inferred that a nonemergence of striae of Retzius could be the cause for the “missing perikymata” phenomenon in the Fongoli chimpanzees. To check this inference, we analyzed the structure of outer enamel in three chimpanzee canines. The teeth were studied using light‐microscopic and scanning‐electron microscopic techniques. Our analysis of the specimen upon which Skinner and Pruetz (Am J Phys Anthropol 149 (2012) 468–482) had made their original observation does not support their hypothesis. We demonstrate that the enamel morphology described by them is not caused by a nonemergence of striae of Retzius but can be attributed to structural variations in outer enamel that result in a differential fracture behavior. Although rejecting the presumed existence of nonemergent striae of Retzius, our study provided evidence that, in furrow‐type hypoplastic defects, a pronounced tapering of Retzius increments can occur, with the striae of Retzius forming acute angles with the outer enamel surface. We suggest that in such cases the outcrop of some striae of Retzius is essentially unobservable at the enamel surface, causing too low perikymata counts. The pronounced tapering of Retzius increments in outer enamel presumably reflects a mild to moderate disturbance of the function of late secretory ameloblasts. Am J Phys Anthropol 157:276–283, 2015. © 2015 Wiley Periodicals, Inc.     

Reawakening Malthus: empirical support for the Smail scenario

Editor's Note: In the November issue of the Journal, Kenneth Smail presented the third and final part of his series on Malthus and human population growth. Here, Jeffrey McKee offers additional thoughts on population and its impact on biodiversity and extinction history. Am J Phys Anthropol 122: 371–374, 2003. © 2003 Wiley‐Liss, Inc.     

Brief Communication: Calculating hominin and nonhuman anthropoid femoral head diameter from acetabular size

Femoral head size provides important information on body size in extinct species. Although it is well‐known that femoral head size is correlated with acetabular size, the precision with which femoral head size can be estimated from acetabular size has not been quantified. The availability of accurate 3D surface models of fossil acetabular remains opens the possibility of obtaining accurate estimates of femoral head size from even fragmentary fossil remains [Hammond et al.,: Am J Phys Anthropol 150 (2013) 565–578]. Here we evaluate the relationship between spheres fit to surface models of the femoral head and acetabulum of a large sample of extant anthropoid primates. Sphere diameters are tightly correlated and scale isometrically. In spite of significant taxonomic and possibly functional differences in the relationship between femoral head size and acetabulum size, percent prediction errors of estimated femoral head size remain low regardless of the taxonomic composition of the reference sample. We provide estimates of femoral head size for a series of fossil hominins and monkeys. Am J Phys Anthropol 155:469–475, 2014. © 2014 Wiley Periodicals, Inc.     

Brief communication: Investigation of the semicircular canal variation in the Krapina Neandertals

Previous studies comparing bony labyrinth morphology in geographically‐dispersed samples of Neandertals and modern Homo sapiens (H. sapiens) showed that Neandertals generally have smaller semicircular canals than modern H. sapiens (Hublin et al., 1996 ; Spoor et al., 2003 ; Glantz et al., 2008 ). Here we analyze the morphology of a single group of Neandertal specimens from one locale, the Krapina site, to determine the intraspecific variation in Neandertal semicircular canal sizes. Dimensions of the semicircular canals were collected from computed tomography scans of nine temporal bones. With the rare exception, the dimensions of the semicircular canals in the Krapina sample are similar to those previously reported across a geographically‐dispersed sample of Neandertals, further supporting previous studies that suggest low levels of variation in the semicircular canals for Neandertals. Am J Phys Anthropol 154:302–306, 2014. © 2014 Wiley Periodicals, Inc.     

Composition of the founding population of Iceland: biological distance and morphological variation in early historic Atlantic Europe

We examined the composition of the founding population of Iceland through the study of morphological traits in skeletons from Iceland, Ireland, Norway, and Greenland. This is the first study to address this issue from the Settlement Period of Iceland and contemporary samples from Ireland. We pose the following questions: 1) Was the founding population of Iceland of mixed or homogeneous origin? 2) Is there evidence for a significant Irish cohort in the founding population, as suggested in medieval Icelandic literature? Analysis of biodistance revealed that both Settlement Age and later samples from Iceland showed a greater degree of phenetic similarity to contemporary Viking Age Norwegians than to samples obtained from early medieval Ireland. Analysis of among‐individual morphological variation showed that the Settlement Age population of Iceland did not exhibit an increase in variation in comparison to other populations in the sample, suggesting a relatively homogenous origin. However, estimation of admixture between the Irish and Norwegian populations indicated that 66% of the Icelandic settlers were of Norwegian origin. Comparison of the Icelandic samples to hybrid samples produced by resampling the Viking Age Norwegian and early medieval Irish samples revealed that the Icelandic samples are much closer to the Norwegian samples than expected, based on a 66:34 mixture of Norwegian and Irish settlers. We conclude that the Settlement Age population of Iceland was predominantly (60–90%) of Norwegian origin. Although this population was relatively homogenous, our results do not preclude significant contributions from Ireland as well as other sources not represented in our analysis. Am J Phys Anthropol, 2003. © 2003 Wiley‐Liss, Inc.     

An interspecific analysis of relative jaw‐joint height in primates

Jaw‐joint height (JJH) above the occlusal plane is thought to be influenced by cranial base angle (CBA) and facial angulation during growth. To better understand how JJH relates to midline craniofacial form, we test the hypothesis that relative increases in JJH are correlated with increasing CBA flexion and facial kyphosis (i.e., ventral bending) across primates. We compared JJH above the occlusal plane to CBA and the angle of facial kyphosis (AFK) across adults from 82 species. JJH scales with positive allometry relative to a skull geometric mean in anthropoids and most likely strepsirrhines. Anthropoid regressions for JJH are elevated above strepsirrhines, whereas catarrhines exhibit a higher slope than platyrrhines. Semipartial correlations between relative JJH and both CBA and AFK show no association across a small strepsirrhine sample, limited associations among catarrhines and anthropoids, but strong correlations in platyrrhines. Contrary to our hypothesis, however, increases in relative JJH are correlated with relatively less flexed basicrania and more airorhynch faces (i.e., reduced ventral bending) in platyrrhines. The mosaic pattern of relationships involving JJH across primate clades points to multiple influences on JJH across primates. In clades showing little association with basicranial and facial angles, such as strepsirrhines, the potential morphological independence of JJH may facilitate a relative freedom for evolutionary changes related to masticatory function. Finally, failure to associate relative JJH and basicranial flexion in most clades suggests that the relatively taller JJH and more flexed basicrania of anthropoids compared to strepsirrhines may have evolved as an isolated event during the origin of anthropoids. Am J Phys Anthropol 142:519–530, 2010. © 2010 Wiley‐Liss, Inc.     

Canine tip wear in male and female anthropoids

One component of the “dual selection hypothesis” (Greenfield [1992a] Year. Phys. Anthropol. 35:153–185) is that the tips of female canines are commonly blunted and more frequently so than those of conspecific males. Data derived from two randomly selected age-graded samples of Macaca fascicularis (n = 70) and Colobus badius (n = 59) show that at least 80% of the females exhibit tip blunting on one or both canines and that frequencies of blunting are far greater than those of conspecific males in both jaws. Sexual dimorphism in mandibular canine morphology and wear and other recently critiqued aspects of the “dual selection hypothesis” (Plavcan and Kelley [1996] Am. J. Phys. Anthropol. 99:379–387.) are also discussed. Am J Phys Anthropol 107:87–97. © 1998 Wiley-Liss, Inc.     

Ontogenetic perspective on mechanical and nonmechanical models of primate circumorbital morphology

Dimensions of the supraorbital torus, postorbital bar, and postorbital septum were collected in an ontogenetic series of Macaca fascicularis and compared with expectations based on models that attribute morphological variation in these features to spatial factors, allometry, anterior dental loading, and neurofacial torsion. Each model was evaluated using correlation, partial correlation, and regression techniques (model I/least squares; model II/reduced major axis) applied to log-transformed and size-corrected data. Results indicate clearly that face or skull size is the primary determinant of variation in circumorbital structures. Strong support is found for the influence of spatial influences on anteroposterior supraorbital torus development (Moss and Young, Am. J. Phys. Anthropol. 18:281-292, 1960). Only minor support is noted for the neurofacial torsion model of Greaves (J. Zool. 207:125-136, 1985), and no support is indicated for the anterior dental loading model. The sexes do not differ significantly in any relevant comparisons of ontogenetic trajectories.     

Cranial airways and the integration between the inner and outer facial skeleton in humans

The cranial airways are in the center of the human face. Therefore variation in the size and shape of these central craniofacial structures could have important consequences for the surrounding midfacial morphology during development and evolution. Yet such interactions are unclear because one school of thought, based on experimental and developmental evidence, suggests a relative independence (modularity) of these two facial compartments, whereas another one assumes tight morphological integration. This study uses geometric morphometrics of modern humans (N = 263) and 40 three‐dimensional‐landmarks of the skeletal nasopharynx and nasal cavity and outer midfacial skeleton to analyze these questions in terms of modularity. The sizes of all facial compartments were all strongly correlated. Shape integration was high between the cranial airways and the outer midfacial skeleton and between the latter and the anterior airway openings (skeletal regions close to and including piriform aperture). However, no shape integration was detected between outer midface and posterior airway openings (nasopharynx and choanae). Similarly, no integration was detected between posterior and anterior airway openings. This may reflect functional modularization of nasal cavity compartments related to respiratory physiology and differential developmental interactions with the face. Airway size likely relates to the energetics of the organism, whereas airways shape might be more indicative of respiratory physiology and climate. Although this hypothesis should be addressed in future steps, here we suggest that selection on morphofunctional characteristics of the cranial airways could have cascading effects for the variation, development, and evolution of the human face. Am J Phys Anthropol 152:287–293, 2013. © 2013 Wiley Periodicals, Inc.