Variation in human limb joint articular morphology.

OBJECTIVES: Synovial joints in human limbs strike a balance between mobility, stability, and articular fit, yet little is known about how these conflicting demands pattern intraspecific variation in articular shape. In this study, we use geometric morphometrics to establish the apportionment and magnitude of morphological variance of the articular surfaces of the human shoulder, elbow, hip, and knee. We hypothesize that variances will be comparable between articulating surfaces within a joint and will be larger in joints with smaller ranges of motion, given their plurality of functional demands. MATERIALS AND METHODS: Three-dimensional landmarks were taken on the articular surfaces of the glenohumeral, humeroulnar, acetabulofemoral, and tibiofemoral joints from CT scans of 200 skeletons from the University of Tennessee Donated Skeletal Collection (84 females, 116 males). Root mean-squared distances between articulations calculated from Procrustes shape coordinates were used to determine variance distributions. RESULTS: We found no difference in variances for each articular surface between the sexes or between left and right articular surfaces. A high range of motion is associated with greater morphological variance; however, this pattern is largely driven by the concave articular surfaces of each joint, which consistently exhibit statistically greater variance than their convex counterparts. DISCUSSION: The striking pattern of differential variance between articulating morphologies points to potential disparities in development between them. Consistently higher variance in concave surfaces may relate to chondral modeling theory for the formation of joints. Establishing intraspecific morphological variance patterns is a first step in understanding coordinated evolution among articular features.

Morphology, evolution, and the whole organism imperative: Why evolutionary questions need multi-trait evolutionary quantitative genetics.

Since Washburn's New Physical Anthropology, researchers have sought to understand the complexities of morphological evolution among anatomical regions in human and non-human primates. Researchers continue, however, to preferentially use comparative and functional approaches to examine complex traits, but these methods cannot address questions about evolutionary process and often conflate function with fitness. Moreover, researchers also tend to examine anatomical elements in isolation, which implicitly assumes independent evolution among different body regions. In this paper, we argue that questions asked in primate evolution are best examined using multiple anatomical regions subjected to model-bound methods built from an understanding of evolutionary quantitative genetics. A nascent but expanding number of studies over the last two decades use this approach, examining morphological integration, evolvability, and selection modeling. To help readers learn how to use these methods, we review fundamentals of evolutionary processes within a quantitative genetic framework, explore the importance of neutral evolutionary theory, and explain the basics of evolutionary quantitative genetics, namely the calculation of evolutionary potential for multiple traits in response to selection. Leveraging these methods, we demonstrate their use to understand non-independence in possible evolutionary responses across the limbs, limb girdles, and basicranium of humans. Our results show that model-bound quantitative genetic methods can reveal unexpected genetic covariances among traits that create a novel but measurable understanding of evolutionary complexity among multiple traits. We advocate for evolutionary quantitative genetic methods to be a standard whenever appropriate to keep studies of primate morphological evolution relevant for the next seventy years and beyond.

The developmental impacts of natural selection on human pelvic morphology.

Evolutionary responses to selection for bipedalism and childbirth have shaped the human pelvis, a structure that differs substantially from that in apes. Morphology related to these factors is present by birth, yet the developmental-genetic mechanisms governing pelvic shape remain largely unknown. Here, we pinpoint and characterize a key gestational window when human-specific pelvic morphology becomes recognizable, as the ilium and the entire pelvis acquire traits essential for human walking and birth. We next use functional genomics to molecularly characterize chondrocytes from different pelvic subelements during this window to reveal their developmental-genetic architectures. We then find notable evidence of ancient selection and genetic constraint on regulatory sequences involved in ilium expansion and growth, findings complemented by our phenotypic analyses showing that variation in iliac traits is reduced in humans compared to African apes. Our datasets provide important resources for musculoskeletal biology and begin to elucidate developmental mechanisms that shape human-specific morphology.

Morphological integration and evolutionary potential of the primate shoulder: Variation among taxa and implications for genetic covariances with the basicranium, pelvis, and arm.

Within the primate order, the morphology of the shoulder girdle is immensely variable and has been shown to reflect the functional demands of the upper limb. The observed morphological variation among extant primate taxa consequently has been hypothesized to be driven by selection for different functional demands. Evolutionary analyses of the shoulder girdle often assess this anatomical region, and its traits, individually, therefore implicitly assuming independent evolution of the shoulder girdle. However, the primate shoulder girdle has developmental and functional covariances with the basicranium and pelvic girdle that have been shown to potentially influence its evolution. It is unknown whether these relationships are similar or even present across primate taxa, and how they may affect morphological variation among primates. This study evaluates the strength of covariance and evolutionary potential across four anatomical regions: shoulder girdle, basicranium, pelvis, and distal humerus. Measures of morphological integration and evolutionary potential (conditioned covariance and evolutionary flexibility) are assessed across eight anthropoid primate taxa. Results demonstrate a consistent pattern of morphological constraint within paired anatomical regions across primates. Differences in evolutionary flexibility are observed among primate genera, with humans having the highest evolutionary potential overall. This pattern does not follow functional differences, but rather a separation between monkeys and apes. Therefore, evolutionary hypotheses of primate shoulder girdle morphological variation that evaluate functional demands alone may not account for the effect of these relationships. Collectively, our findings suggest differences in genetic covariance among anatomical regions may have contributed to the observable morphological variation among taxa.

Using Optimal Transport to Improve Spherical Harmonic Quantification of Complex Biological Shapes.

The knowledge of the anatomical shape of both gross and microscopic structures is the key to understanding the effects of disease processes on cellular structure. Geometric morphometric methods, such as Procrustes superimposition, and Spherical Harmonics (SPHARM), have been used to capture the biological shape variation and group differences in morphology. Previous SPHARM-MAT techniques use the CALD algorithm to parameterize the mesh surface. It starts from initial mapping and performs local and global smoothing methods alternately to control the area and length distortions simultaneously. However, this parameterization may not be sufficient in complex morphological cases. To bridge this gap, we propose SPHARM-OT, an enhanced SPHARM surface modeling method using optimal transport (OT) for spherical parameterization. First, the genus 0 3D objects are conformally mapped onto a sphere. Then the optimal transport theory via spherical power diagram is introduced to minimize the area distortion. This new algorithm can effectively reduce the area distortion and lead to a better reconstruction result. We demonstrate the effectiveness of the method by applying it to the human sphenoidal paranasal sinuses.

Variation in obstetric dimensions of the human bony pelvis in relation to age-at-death and latitude.

OBJECTIVES: Previous studies associate females who died in young adulthood with narrower obstetric pelvic dimensions, presumably in association with obstetric insufficiency (though this causal relationship is unresolved). In this study, we examine whether females within groups living at higher latitudes present this pattern, as high-latitude groups have larger pelvic dimensions than groups previously examined. These patterns are compared with males. We assess whether there is evidence for younger ages-at-death in females to have been in response to natural selection against narrower true pelvis dimensions. METHODS: We measured 14 pelvic dimensions in 327 adults (188 females, 139 males), representing archaeological sites from mid-latitude and high-latitude North America. Individuals were placed into a "young" or "not young" age-at-death category. Latitude, sex, and age-at-death groups were compared using ANOVAs and scaled variance, and evidence for selection was examined with F-tests. RESULTS: Pelvic dimensions were larger in high-latitude females and males. Females but not males who died at younger ages had smaller pelvic canals than older individuals, especially in the mediolateral inlet and anteroposterior outlet dimensions. Variance in all pelvic dimensions is equal between the two female age-at-death groups. CONCLUSIONS: We found narrower obstetrical dimensions in the female pelvis among individuals who died at younger ages. However, statistically equivalent variances in the two female age-at-death groups does not support natural selection on pelvic dimensions as leading to younger ages at death. We instead argue that this difference is result of continued growth due to remodeling in the pelvis occurring in females, but not males, after early adulthood.

Morphological Integration of the Human Pelvis with Respect to Age and Sex.

Considerable research has shown that modern human pelvic dimensions, especially of the birth canal, are sexually dimorphic. Studies also suggest that females with younger ages-at-death have narrower canal dimensions than those who die at older ages, possibly due to continued independent growth of the pubis. A recent examination of this pattern argued that it is unlikely that these differences relate to mortality, but the source of the difference in pelvic dimensions with age remains unresolved. We use pelvic dimensions to assess differences in magnitudes of morphological integration between adult females and males across ages-at-death. We first ascertain whether the sexes have different strengths of integration, and then assess if differences in magnitudes of integration are associated with age-at-death. Pelvic dimensions of all groups were moderately integrated. Females and males have similar magnitudes of integration, and there is no change in the strength of integration with age. Examining individual regions of the pelvis indicates that the ilium, pubis, and pelvic inlet and outlet have stronger integration than the overall pelvis. This was particularly true of the pelvic outlet, which demonstrated the strongest integration. Our findings suggest that regions of the pelvis are more strongly integrated internally, and less integrated with each other, which would allow for proportional growth among regions of the pelvis with age that do not affect its overall integration. No single region of the pelvis appears to be motivating the difference in pelvic dimensions between age groups. We further consider the implications of these findings on evolutionary constraints. Anat Rec, 300:666-674, 2017. © 2017 Wiley Periodicals, Inc.

The relative position of the human fibula to the tibia influences cross-sectional properties of the tibia.

OBJECTIVES: The fibula transmits loads within the lower limb of hominids. The few studies of variation in the cross-sectional geometric (CSG) properties of the fibula have established differences in its rigidity among groups engaged in distinct habitual loading activities. This study adds to this research by considering the relationship between CSG properties and the anatomical position of the fibula relative to the tibia among groups with differences in documented activity patterns. MATERIAL AND METHODS: We used pQCT scans taken at 50% of the length of the lower leg in 83 healthy young adult collegiate-aged individuals divided into five activity groups: runners, swimmers, cricketers, field hockey players, and non-athletes. We compared variation in calculated CSG properties against the distance between fibular and tibial centroids, as well as the angle of that plane relative to the plane of tibial Imax . RESULTS: Tibial and fibular CSG properties vary with respect to the relative position of the two bones. Tibial CSG properties differ in concert with the relative angle of the fibula to tibial Imax , while fibular CSG properties differ with the distance between the elements. Fibulae are more posterior-medially positioned in groups engaged in terrestrial athletics than among swimmers. DISCUSSION: The tibia and fibula experience different loads. The relative position of the two bones leads to compensatory differences in their CSG properties, perhaps due to increased resistance to bending in fibulae with greater distances from the tibia. Examinations of tibial CSG properties without considering the fibula limits interpretations about activity.

Constraint, natural selection, and the evolution of human body form.

Variation in body form among human groups is structured by a blend of natural selection driven by local climatic conditions and random genetic drift. However, attempts to test ecogeographic hypotheses have not distinguished between adaptive traits (i.e., those that evolved as a result of selection) and those that evolved as a correlated response to selection on other traits (i.e., nonadaptive traits), complicating our understanding of the relationship between climate and morphological distinctions among populations. Here, we use evolutionary quantitative methods to test if traits previously identified as supporting ecogeographic hypotheses were actually adaptive by estimating the force of selection on individual traits needed to drive among-group differentiation. Our results show that not all associations between trait means and latitude were caused by selection acting directly on each individual trait. Although radial and tibial length and biiliac and femoral head breadth show signs of responses to directional selection matching ecogeographic hypotheses, the femur was subject to little or no directional selection despite having shorter values by latitude. Additionally, in contradiction to ecogeographic hypotheses, the humerus was under directional selection for longer values by latitude. Responses to directional selection in the tibia and radius induced a nonadaptive correlated response in the humerus that overwhelmed its own trait-specific response to selection. This result emphasizes that mean differences between groups are not good indicators of which traits are adaptations in the absence of information about covariation among characteristics.

Fluctuating and directional asymmetry in the long bones of captive cotton-top tamarins (Saguinus oedipus).

OBJECTIVES: Skeletal asymmetries reflect developmental stability and mechanical, functional, and physiological influences on bone growth. In humans, researchers have documented the greatest limb bone bilateral asymmetry in diaphyseal breadths, with less asymmetry in articular and maximum length dimensions. However, it remains unclear as to whether the pattern observed for humans is representative of nonhuman primates, wherein bilateral loading may minimize directional asymmetry. This study adds to the small body of asymmetry data on nonhuman primates by investigating patterns of long bone asymmetry in a skeletal sample of Saguinus oedipus (cotton-top tamarin). MATERIALS AND METHODS: Humeri, radii, ulnae, femora, and tibiae of 76 adult captive cotton-top tamarin skeletons (48 males, 28 females) were measured bilaterally. We included maximum length, midshaft diaphyseal breadths, and at least one articular measurement for each bone to assess directional (DA) and fluctuating asymmetry (FA) in each dimension. RESULTS: Most dimensions exhibit significant FA, and very few have significant DA; DA is limited to the lower limb, especially in knee dimensions. Overall, the magnitudes of asymmetry in tamarins have a consistent ranking that follows the same pattern as found in humans. DISCUSSION: This first study of DA and FA among multiple dimensions throughout the limbs of a non-hominoid primate suggests that previously-reported patterns of human bilateral asymmetry are not exclusive to humans. The results further indicate potential underlying differences in constraints on variation within limb bones. While processes shaping variation await further study, our results argue that different long bone dimensions may reflect dissimilar evolutionary processes.

Ecogeography, genetics, and the evolution of human body form.

Genetic resemblances among groups are non-randomly distributed in humans. This population structure may influence the correlations between traits and environmental drivers of natural selection thus complicating the interpretation of the fossil record when modern human variation is used as a referential model. In this paper, we examine the effects of population structure and natural selection on postcranial traits that reflect body size and shape with application to the more general issue of how climate - using latitude as a proxy - has influenced hominin morphological variation. We compare models that include terms reflecting population structure, ascertained from globally distributed microsatellite data, and latitude on postcranial phenotypes derived from skeletal dimensions taken from a large global sample of modern humans. We find that models with a population structure term fit better than a model of natural selection along a latitudinal cline in all cases. A model including both latitude and population structure terms is a good fit to distal limb element lengths and bi-iliac breadth, indicating that multiple evolutionary forces shaped these morphologies. In contrast, a model that included only a population structure term best explained femoral head diameter and the crural index. The results demonstrate that population structure is an important part of human postcranial variation, and that clinally distributed natural selection is not sufficient to explain among-group differentiation. The distribution of human body form is strongly influenced by the contingencies of modern human origins, which calls for new ways to approach problems in the evolution of human variation, past and present.

Neandertal clavicle length.

The Late Pleistocene archaic humans from western Eurasia (the Neandertals) have been described for a century as exhibiting absolutely and relatively long clavicles. This aspect of their body proportions has been used to distinguish them from modern humans, invoked to account for other aspects of their anatomy and genetics, used in assessments of their phylogenetic polarities, and used as evidence for Late Pleistocene population relationships. However, it has been unclear whether the usual scaling of Neandertal clavicular lengths to their associated humeral lengths reflects long clavicles, short humeri, or both. Neandertal clavicle lengths, along with those of early modern humans and latitudinally diverse recent humans, were compared with both humeral lengths and estimated body masses (based on femoral head diameters). The Neandertal do have long clavicles relative their humeri, even though they fall within the ranges of variation of early and recent humans. However, when scaled to body masses, their humeral lengths are relatively short, and their clavicular lengths are indistinguishable from those of Late Pleistocene and recent modern humans. The few sufficiently complete Early Pleistocene Homo clavicles seem to have relative lengths also well within recent human variation. Therefore, appropriately scaled clavicular length seems to have varied little through the genus Homo, and it should not be used to account for other aspects of Neandertal biology or their phylogenetic status.

A three-dimensional analysis of bilateral directional asymmetry in the human clavicle.

This study presents a novel three-dimensional analysis using statistical atlases and automated measurements to assess diaphyseal morphology of the clavicle and its relationship to muscle asymmetry. A sample of 505 individuals (285 males, 220 females) from the William McCormick Clavicle Collection was CT scanned, segmented, and added to a statistical bone atlas that captures correspondence between homologous points on the bone surfaces. Muscle attachment sites were localized on the atlas and then propagated across the entire population. Cross-sectional contours were extracted at 5% increments along the entire bone, as well as at muscle attachment sites and the clavicle waist; maximum and minimum dimensions of each cross-sectional contour were calculated. In addition, the entire three-dimensional surface was examined for asymmetry by analyzing the magnitude and directional differences between homologous points across all bone surfaces in the dataset. The results confirm the existing studies on clavicle asymmetry, namely that the left clavicle is longer than the right, but the right is more robust than the left. However, the patterns of asymmetry are sexually dimorphic. Males are significantly asymmetric in all dimensions and at muscle and ligament attachment sites (P < 0.05), whereas female asymmetry is more variable. We hypothesize that this is related to absolute and relative differences in male muscle strength compared to females. However, an area with no muscle attachments on the posterior midshaft was significantly asymmetric in both sexes. We suggest that this is a curvature difference caused by opposing muscle actions at the medial and lateral ends of the bone.

Skeletal variation among early Holocene North American humans: implications for origins and diversity in the Americas.

The movement of humans into the Americas remains a major topic of debate among scientific disciplines. Central to this discussion is ascertaining the timing and migratory routes of the earliest colonizers, in addition to understanding their ancestry. Molecular studies have recently argued that the colonizing population was isolated from other Asian populations for an extended period before proceeding to colonize the Americas. This research has suggested that Beringia was the location of this "incubation," though archaeological and skeletal data have not yet supported this hypothesis. This study employs the remains of the five most complete North American male early Holocene skeletons to examine patterns of human morphology at the earliest observable time period. Stature, body mass, body breadth, and limb proportions are examined in the context of male skeletal samples representing the range of morphological variation in North America in the last two millennia of the Holocene. These are also compared with a global sample. Results indicate that early Holocene males have variable postcranial morphologies, but all share the common trait of wide bodies. This trait, which is retained in more recent indigenous North American groups, is associated with adaptations to cold climates. Peoples from the Americas exhibit wider bodies than other populations sampled globally. This pattern suggests the common ancestral population of all of these indigenous American groups had reduced morphological variation in this trait. Furthermore, this provides support for a single, possibly high latitude location for the genetic isolation of ancestors of the human colonizers of the Americas.

Developmental variation in ecogeographic body proportions.

While ecogeographic variation in adult human body proportions has been extensively explored, relatively less attention has been paid to the effect of Bergmann's and Allen's rules on human body shape during growth. The relationship between climate and immature body form is particularly important, as immature mortality is high, mechanisms of thermoregulation differ between young and mature humans, and immature body proportions fluctuate due to basic parameters of growth. This study explores changes in immature ecogeographic body proportions via analyses of anthropometric data from children included in Eveleth and Tanner's (1976) Worldwide Variation in Human Growth, as well as limb proportion measurements in eight different skeletal samples. Moderate to strong correlations exist between climatic data and immature stature, weight, BMI, and bi-iliac breadth; these relationships are as strong, if not stronger, in immature individuals as they are in adults. Correlations between climate and trunk height relative to stature are weak or nonexistent. Altitude also has significant effects on immature body form, with children from higher altitudes displaying smaller statures and lower body weights. Brachial and crural indices remain constant over the course of growth and display consistent, moderate correlations with latitude across ontogeny that are just as high as those detected in adults. The results of this study suggest that while some features of immature body form, such as bi-iliac breadth and intralimb indices, are strongly dictated by ecogeographic principles, other characteristics of immature body proportions are influenced by intrinsic and extrinsic factors such as nutrition and basic constraints of growth.

Allometry and apparent paradoxes in human limb proportions: Implications for scaling factors.

It has been consistently demonstrated that human proximal limb elements exhibit negative allometry, while distal elements scale with positive allometry. Such scaling implies that longer limbs will have higher intralimb indices, a phenomenon not borne out by empirical analyses. This, therefore, creates a paradox within the limb allometry literature. This study shows that these apparently conflicting results are the product of two separate phenomena. First, the use of the geometric mean of limb elements produces allometry coefficients that are not independent, and that when using ordinary least squares regression must yield an average slope of one. This phenomenon argues against using the geometric mean as a size variable when examining limb allometry. While the employment of relevant dimensions independent of those under analysis to calculate the geometric mean--as suggested by Coleman (Am J Phys Anthropol 135 (2008) 404-415)--may be a partial method for resolving the problem, an empirically determined, independent and biologically relevant size variable is advocated. If stature is used instead of the geometric mean as an independent size variable, all major limb elements scale with positive allometry. Second, while limb allometry coefficients do indicate differential allometry in limb elements, and thus should lead to some intralimb index allometry, this pattern appears to be attenuated by other sources of limb element length variation.

Methods for estimating missing human skeletal element osteometric dimensions employed in the revised fully technique for estimating stature.

One of the greatest limitations to the application of the revised Fully anatomical stature estimation method is the inability to measure some of the skeletal elements required in its calculation. These element dimensions cannot be obtained due to taphonomic factors, incomplete excavation, or disease processes, and result in missing data. This study examines methods of imputing these missing dimensions using observable Fully measurements from the skeleton and the accuracy of incorporating these missing element estimations into anatomical stature reconstruction. These are further assessed against stature estimations obtained from mathematical regression formulae for the lower limb bones (femur and tibia). Two thousand seven hundred and seventeen North and South American indigenous skeletons were measured, and subsets of these with observable Fully dimensions were used to simulate missing elements and create estimation methods and equations. Comparisons were made directly between anatomically reconstructed statures and mathematically derived statures, as well as with anatomically derived statures with imputed missing dimensions. These analyses demonstrate that, while mathematical stature estimations are more accurate, anatomical statures incorporating missing dimensions are not appreciably less accurate and are more precise. The anatomical stature estimation method using imputed missing dimensions is supported. Missing element estimation, however, is limited to the vertebral column (only when lumbar vertebrae are present) and to talocalcaneal height (only when femora and tibiae are present). Crania, entire vertebral columns, and femoral or tibial lengths cannot be reliably estimated. Further discussion of the applicability of these methods is discussed.

Stature estimation formulae for indigenous North American populations.

Stature estimation methods for adult indigenous humans from the Americas have generally relied on a limited number of regression equations. The available equations, however, are not broadly applicable to the diversity of the populations that lived in the New World prior to European colonization. Furthermore, some equations that have been used were originally derived from inappropriate reference samples, such as the "Mongoloid" group measured by Trotter and Gleser (Am J Phys Anthropol 16 [1958] 79-123). This study develops new stature estimation equations for long bones of the lower limb from a geographically diverse sample of North American archaeological sites. Statures were reconstructed from 967 skeletons from 75 archaeological sites using the revised Fully anatomical technique (Raxter et al., Am J Phys Anthropol 130 [2006] 374-384). Archaeological samples were grouped according to general body proportions, using relative tibia and femur length to stature as guides. On the basis of differences in these proportions, three broad groupings were identified: a high latitude "arctic" group, a general "temperate" group, and a Great Plains group. Sex-specific ordinary least squares regression formulae were developed based on femoral and tibial lengths for each of these groups. Comparisons of the new stature estimation equations with previously available equations were conducted using several archaeological test samples. In most cases, the new stature estimation equations are more precise than those previously available, and we recommend their use throughout most of North America. The equations developed by Genovés for Mesoamerican and US Southwest samples are a useful alternative for these regions. Applicability of the new equations to South American samples awaits further testing.

Variation in limb proportions between Jomon foragers and Yayoi agriculturalists from prehistoric Japan.

Variation in limb proportions between prehistoric Jomon and Yayoi people of Japan are explored by this study. Jomon people were the descendents of Pleistocene nomads who migrated to the Japanese Islands around 30,000 yBP. Phenotypic and genotypic evidence indicates that Yayoi people were recent migrants to Japan from continental Northeast Asia who likely interbred with Jomon foragers. Limb proportions of Jomon and Yayoi people were compared using RMA regression and "Quick-Test" calculations to investigate relative variability between these two groups. Cluster and principal components analyses were performed on size-standardized limb lengths and used to compare Jomon and Yayoi people with other groups from various climatic zones. Elongated distal relative to proximal limb lengths were observed among Jomon compared to Yayoi people. Jomon limb proportions were similar to human groups from temperate/tropical climates at lower latitudes, while Yayoi limb proportions more closely resemble groups from colder climates at higher latitudes. Limb proportional similarities with groups from warmer environments among Jomon foragers likely reflect morphological changes following Pleistocene colonization of the Japanese Islands. Cold-derived limb proportions among the Yayoi people likely indicate retention of these traits following comparatively recent migrations to the Japanese Islands. Changes in limb proportions experienced by Jomon foragers and retention of cold-derived limb proportions among Yayoi people conform to previous findings that report changes in these proportions following long-standing evolution in a specific environment.

Patterns of clavicular bilateral asymmetry in relation to the humerus: variation among humans.

Studies of directional asymmetry in the human upper limb have extensively examined bones of the arm, forearm, and hand, but have rarely considered the clavicle. Physiologically, the clavicle is an integrated element of the upper limb, transmitting loads to the axial skeleton and supporting the distal bones. However, clavicles develop in a manner that is unique among the bones of the upper limb. Previous studies have indicated that the clavicle has a right-biased asymmetry in diaphyseal breadth, as in humeri, radii, ulnae, and metacarpals, but unlike these other elements, a left-biased length asymmetry. Few studies have assessed how clavicular asymmetry relates to these other bones of the upper limb. Bilateral directional asymmetry of the clavicle is examined in relation to the humerus in a large, geographically diverse human sample, comparing lengths and diaphyseal breadths. Dimensions were converted into percentage directional (%DA) and absolute (%AA) asymmetries. Results indicate that humans have same-side %DA bias in the clavicles and humeri, and contralateral length %DA between these elements. Diaphyseal breadths in both clavicles and humeri are more asymmetric-both in direction and amount-than lengths. Differences in diaphyseal asymmetry are shown to relate to variation in physical activities among groups, but a relationship between activity and length asymmetry is not supported. This further supports previous research, which suggests different degrees of sensitivity to loading between diaphyseal breadths and maximum lengths of long bones. Differences in lateralized behavior and the potential effects of different bone development are examined as possible influences on the patterns observed among human groups.