Extant Humans Research Articles

The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism

Functional morphology of the atlas reflects multiple aspects of an organism’s biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 (‘Little Foot’) skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution.

The study of the lower limb entheses in the Neanderthal sample from El Sidrón (Asturias, Spain): How much musculoskeletal variability did Neanderthals accumulate?

Entheses have rarely been systematically studied in the field of human evolution. However, the investigation of their morphological variability (e.g., robusticity) could provide new insight into their evolutionary significance in the European Neanderthal populations. The aim of this work is to study the entheses and joint features of the lower limbs of El Sidrón Neanderthals (Spain; 49 ka), using standardized scoring methods developed on modern samples. Paleobiology, growth, and development of both juveniles and adults from El Sidrón are studied and compared with those of Krapina Neanderthals (Croatia, 130 ka) and extant humans. The morphological patterns of the gluteus maximus and vastus intermedius entheses in El Sidrón, Krapina, and modern humans differ from one another. Both Neanderthal groups show a definite enthesis design for the gluteus maximus, with little intrapopulation variability with respect to modern humans, who are characterized by a wider range of morphological variability. The gluteus maximus enthesis in the El Sidrón sample shows the osseous features of fibrous entheses, as in modern humans, whereas the Krapina sample shows the aspects of fibrocartilaginous ones. The morphology and anatomical pattern of this enthesis has already been established during growth in all three human groups. One of two and three of five adult femurs from El Sidrón and from Krapina, respectively, show the imprint of the vastus intermedius, which is absent among juveniles from those Neanderthal samples and in modern samples. The scant intrapopulation and the high interpopulation variability in the two Neanderthal samples is likely due to a long-term history of small, isolated populations with high levels of inbreeding, who also lived in different ecological conditions. The comparison of different anatomical entheseal patterns (fibrous vs. fibrocartilaginous) in the Neanderthals and modern humans provides additional elements in the discussion of their functional and genetic origin.

Cortical bone distribution in the femoral neck of Paranthropus robustus

Studies of the australopith (Australopithecus and Paranthropus) proximal femur have increasingly integrated information from the local arrangement of the cortical and cancellous bone to allow functional-biomechanical inferences on the locomotor behavioral patterns. In Australopithecus africanus and Paranthropus robustus, the cancellous bone organization at the center of the femoral head shows principal strut orientation similar to that of fossil and recent humans, which indicates that australopiths were human-like in many aspects of their bipedalism. However, by combining outer morphology with superoinferior asymmetry in cortical bone thickness at the base of neck and mid-neck, it has been suggested that, while adapted for terrestrial bipedality, australopiths displayed a slightly altered gait kinematics compared to Homo. We used techniques of 2D and 3D virtual imaging applied to an X-ray microtomographic record to assess cortical bone distribution along the entire femoral neck compartment in four upper femora from Swartkrans, South Africa (SK 82, SK 97, SK 3121, and SWT1/LB-2) and compared the results to the extant human and chimpanzee conditions. Our results support and extend previous evidence for more symmetric superior and inferior femoral neck cortical thicknesses in P. robustus than in modern humans and show that the differences are even greater than previously reported. However, P. robustus and humans still share a trend of lateral-to-medial decrease in asymmetry of the superior/inferior cortical thickness ratio, while this pattern is reversed in chimpanzees. We also identified two features uniquely characterizing P. robustus: an accentuated contrast between the relatively thicker anterior and the thinner posterior walls, and a more marked lateral-to-medial thinning of both cortices compared to extant humans and chimpanzees, which indicate wider interspecific differences among hominids in structural organization of the proximal femur than previously reported. It remains to be ascertained if, and to what extent, these features also characterize the femoral neck of Australopithecus.

Comparative morphology and ontogeny of the thoracolumbar transition in great apes, humans, and fossil hominins

Variation among extant hominoid taxa in the anatomy of the thoracolumbar vertebral transition is well-established and constitutes an important framework for making inferences about posture and locomotion in fossil hominins. However, little is known about the developmental bases of these differences, posing a challenge when interpreting the morphology of juvenile hominins. In this study, we investigated ontogenetic variation in the thoracolumbar transition of juvenile and adult great apes, humans, and fossils attributed to Australopithecus and early Pleistocene Homo erectus. For each vertebra involved in the transition, we quantified functionally relevant aspects of zygapophyseal form: facet curvature in the transverse plane, facet orientation relative to midline, and the shift in these variables across the thoracolumbar transition, from the antepenultimate rib-bearing thoracic to the first lumbar vertebra (L1). Among extant hominids, adult individuals of Pan and Homo exhibit a greater shift in facet morphology across the thoracolumbar transition in comparison to Gorilla and Pongo. This pattern is driven by interspecific differences in the L1 facets, with those of chimpanzees and humans being more curved and more sagittally oriented. Chimpanzees and humans also experience more change in facet morphology during development relative to gorillas and orangutans. Humans differ from chimpanzees in achieving their adultlike configuration much earlier in development. The fossil specimens indicate that early hominins had adult morphologies that were similar to those of extant Homo and Pan, and that they achieved their adult morphologies early in development, like extant humans. Although it is unclear why adult chimpanzees and hominins share an adult morphology, we speculate that the early acquisition of adultlike L1 zygapophyseal morphology in hominins is an evolutionary novelty related to conferring stability to a relatively long lumbar spine as young individuals are learning to walk bipedally.

Intersectionality of Migration and Metahumanism, with a Special Focus on Lagrangian Space Settlements

ABSTRACT Lagrangian points in orbit about the Earth and gravitationally stabilized by the Moon have been identified as optimal settlement locations for many millions of migrants from the Earth. The motivation of many extant humans to make this migration can be satisfied via their transition into cybernetic biostasis prior to a medical determination of legal death, subsequent revival from such biostasis as legally alive cyberconscious mindclones in cyberspace, and ultimate reinstantiation of such mindclones into manufactured physical bodies on Lagrangian space settlements. This article analyzes the intersectionality of the metahumanistic status of these human migration efforts.

Hominin cranial fragments from Milner Hall, Sterkfontein, South Africa

The Sterkfontein Caves site is one of the richest early hominin localities in Africa. In addition to significant fossil assemblages from Members 2 and 4 of the Sterkfontein Formation, recent excavations have revealed hominin-bearing sedimentary deposits in the lesser-known Milner Hall. We describe two hominin cranial fragments excavated from the Milner Hall in 2015 and present the results of a high-resolution microtomographic-based approach to diagnosing the anatomical and taxonomical origin of these specimens. Based on external morphology, StW 671 and StW 672 are tentatively identified as frontal and occipital fragments, respectively. Our non-invasive bi-dimensional quantitative investigation of the two cranial fragments reveals a mean cranial thickness of 8.8 mm and 5.6 mm for StW 671 and StW 672 respectively and a contribution of the diploic layer to the cumulative cranial thickness that is less than 50%. While the mean cranial thickness of StW 671 falls within the range of Homo, the relative proportion of the diploë in both StW 671 and StW 672 is lower than in Australopithecus (>60%) and extant humans (>50%). Accordingly, in terms of both cranial thickness and inner structural organization, the Milner Hall hominins combine derived and unique traits, consistent with the condition of other postcranial and dental material already described from the deposit. Moreover, our study opens interesting perspectives in terms of analysis of isolated cranial fragments, which are abundant in the hominin fossil record.
 Significance:
 
 The Sterkfontein Caves have widely contributed to our understanding of human evolution.
 Besides the well-known Members 4 and 2, where the iconic ‘Mrs Ples’ and ‘Little Foot’ have been found, in this study we suggest that the Milner Hall locality represents an additional, stratigraphically associated source of not only fossil hominins, but also Oldowan stone tools.
 In particular, we describe for the first time two cranial fragments, StW 671 and StW 672, identified as frontal and occipital bones, respectively.
 Our microtomographic-based analysis of these materials reveals some affinities with Homo combined with unique characters.
 In this context, our study suggests an intriguing mosaicism consistent with the description of the two fossil hominins found in the Milner Hall.

Exploring bone volume and skeletal weight in the Middle Pleistocene humans from the Sima de los Huesos site (Sierra de Atapuerca, Spain).

Body mass estimation in fossil human species is a crucial topic in paleoanthropology as it yields information about ecologically relevant characteristics. Nevertheless, variables crucial to body mass estimation such as bone volume and skeletal weight have never before been calculated in a fossil human species. The exceptional state of preservation of several fossil human long bones from the Sima de los Huesos (SH) Middle Pleistocene site, in the Sierra de Atapuerca, makes it possible to calculate for the first time the absolute bone volume in five complete long bones (two femora and three humeri) of a fossil human species, an approach not possible in fragmentary or poorly preserved fossils. We have relied on computed tomography scans and 3D reconstructions to calculate bone volume. A sample of 62 complete bones of robust recent humans was also used for comparative purposes. The male SH femora (weight-bearing bones) and humeri (non-weight-bearing bones) have, relative to their size, greater bone volume (volume of bone tissue over total bone volume) than the equivalent bones in our recent human sample. As mass is volume × density, and bone tissue density (as a material) is similar across mammals, we calculate bone mass, and our results show that the SH hominins had on average heavier long bones than extant humans of the same size. From the femoral weight at hand, we have estimated the total skeletal weight in two SH individuals, which is about 36% heavier than in the recent humans of the equivalent body size. Using different methods and skeletal variables, including skeletal weight, to estimate body mass in these two SH humans, we highlight the considerable differences in body mass estimates we obtained, and that the largest body mass estimate is the one based on the skeletal weight. Our results suggest that we cannot assume the same relative proportion of bone volume and bone and skeletal weight characterized the entire genus Homo. Given that skeletal weight has a significant influence on body mass, current body mass estimates of fossil Homo specimens could be systematically underestimated. Thus, the significantly larger bone volume and heavier bones, probably throughout the entire skeleton, of SH humans could have had consequences for many biological parameters in this Pleistocene population and considerable importance for studies focusing on adaptive and ecologically relevant characteristics. Although more recent human samples should be analyzed, in our view, the high skeletal robusticity of the SH sample, including larger bone volume and skeletal weight, is part of their adaptive body type selected for throughout the Pleistocene to support different mechanical and activity regimes and formed under tight genetic control, including control over bone formative and regulatory processes.

Warfare, Ethics, Ethology

The aim of this article is to set a macro-historical narrative concerning the emergence of warfare and social ethics as symplesiomorphic features in the lineage of Homo sapiens. This means that these two behavioral aspects, representative of a very selected branch in the phylogenetic tree of the Primate order, are shared by the two lineages of great African apes that diverged from a common ancestor around six million years in the past, leading to extant humans and chimpanzees. Therefore, this article proposes an ethological understanding of warfare and social ethics, as both are innate to the social high-specialized modular mind present in the species of genera Pan and Homo. However behavioral restraints to intersocietal coalitionary violence seems to be an exclusive aspect of the transdominial modular cognition that characterizes modern humans. Thus, if in the evolutionary long duree, warfare and restrictions to intrasocial violence both appear to be ethologically common to humans and chimpanzees to a certain extent, an ethics of warfare - and, of course, the cognitive capability for intersocietal peace - seems to be distinctly human.

Could Wading in Shallow Water Account for the Unique Shape of the <i>Australopithecus afarensis</i> Pelvis?

A 3D Geometric Morphometric (GM) analysis of the shape of the pelvis and femur of various extinct hominids and extant humans and apes is described. Observed differences in shape are then discussed in the context of the wading hypothesis, a model of the evolution of hominin bipedalism that has rarely been seriously considered despite some compelling arguments in its favour. The general shape of the pelvis of Australopithecus afarensis is confirmed to be fundamentally different from both Homo and extant great apes, and not intermediate between them. Although it includes some human-like traits indicating a strong propensity to bipedalism, there are also sufficient differences to indicate that australopithecines probably exhibited a different type of bipedality to the relatively efficient striding gait associated with modern humans. An analysis of putative muscle lever arm ratios is described, which generated over 135,000 ratios in all. This data was then explored using the Pivot Table feature of Microsoft Excel. Succinct species summaries of broad lever arm groups, such as those pertaining to abduction compared to those pertaining to extension were generated. The results indicate that the australopithecine hip was more adapted, than modern humans or extant great apes, to adduction, abduction and rotation of the thigh during locomotion. It is argued that this apparent lateral biomechanical advantage complements the broad platypelloid shape as a putative adaptation to side-to-side wading. This adds further evidential weight to the wading hypothesis of bipedal origins in addition to the already compelling arguments from extant ape behaviour in shallow water and the favourable evidence of the paleohabitats of the earliest bipeds.

Human-Specific Mutations and Positively Selected Sites in MARCO Confer Functional Changes.

Macrophage Receptor with COllagenous structure (MARCO) is a class A scavenger receptor that binds, phagocytoses, and modifies inflammatory responses to bacterial pathogens. Multiple candidate gene approach studies have shown that polymorphisms in MARCO are associated with susceptibility or resistance to Mycobacterium tuberculosis infection, but how these variants alter function is not known. To complement candidate gene approach studies, we previously used phylogenetic analyses to identify a residue, glutamine 452 (Q452), within the ligand-binding Scavenger Receptor Cysteine Rich domain as undergoing positive selection in humans. Herein, we show that Q452 is found in Denisovans, Neanderthals, and extant humans, but all other nonprimate, terrestrial, and aquatic mammals possess an aspartic acid (D452) residue. Further analysis of hominoid sequences of MARCO identified an additional human-specific mutation, phenylalanine 282 (F282), within the collagenous domain. We show that residue 282 is polymorphic in humans, but only 17% of individuals (rs6761637) possess the ancestral serine residue at position 282. We show that rs6761637 is in linkage disequilibrium with MARCO polymorphisms that have been previously linked to susceptibility to pulmonary tuberculosis. To assess the functional importance of sites Q452 and F282 in humans, we cloned the ancestral residues and loss-of-function mutations and investigated the role of these residues in binding and internalizing polystyrene microspheres and Escherichia coli. Herein, we show that the residues at sites 452 and 282 enhance receptor function.

Relationship between foramen magnum position and locomotion in extant and extinct hominoids

From the Miocene Sahelanthropus tchadensis to Pleistocene Homo sapiens, hominins are characterized by a derived anterior position of the foramen magnum relative to basicranial structures. It has been previously suggested that the anterior position of the foramen magnum in hominins is related to bipedal locomotor behavior. Yet, the functional relationship between foramen magnum position and bipedal locomotion remains unclear. Recent studies, using ratios based on cranial linear measurements, have found a link between the anterior position of the foramen magnum and bipedalism in several mammalian clades: marsupials, rodents, and primates. In the present study, we compute these ratios in a sample including a more comprehensive dataset of extant hominoids and fossil hominins. First, we verify if the values of ratios can distinguish extant humans from apes. Then, we test whether extinct hominins can be distinguished from non-bipedal extant hominoids. Finally, we assess if the studied ratios are effective predictors of bipedal behavior by testing if they mainly relate to variation in foramen magnum position rather than changes in other cranial structures. Our results confirm that the ratios discriminate between extant bipeds and non-bipeds. However, the only ratio clearly discriminating between fossil hominins and other extant apes is that which only includes basicranial structures. We show that a large proportion of the interspecific variation in the other ratios relates to changes in facial, rather than basicranial, structures. In this context, we advocate the use of measurements based only on basicranial structures when assessing the relationship between foramen magnum position and bipedalism in future studies.

Inner structural organization of the distal humerus in Paranthropus and Homo

The taxonomic attribution of isolated hominin distal humeri has been a matter of uncertainty and disagreement notwithstanding their relative abundance in the fossil record. Four taxonomically-based morphotypes, respectively representing P. boisei, P. robustus, non-erectus early Homo and H. erectus, have been identified based on the cross-sectional outer shape variation of an assemblage of Plio-Pleistocene eastern and southern African specimens (Lague, 2015). However, the existence of possible differences between Paranthropus and Homo in the inner structural organisation at this skeletal site remains unexplored. We used noninvasive imaging techniques to tentatively characterize the endostructural organization of five early Pleistocene distal humeri from South Africa (TM 1517g, SK 24600, SKX 10924, SKX 34805) and Ethiopia (Gombore IB), which have been variably attributed to Paranthropus or Homo. While the investigated specimens reveal diverse degrees of inner preservation related to their taphonomic and diagenetic history, in all but SK 24600 from Swartkrans we could comparatively assess some geometric properties at the most distal cross-sectional level (%CA, Ix/Iy, Imax/Imin) and quantify cortical bone thickness topographic variation across the preserved shaft portions by means of a 2-3D Relative Cortical Thickness index. Whenever possible, we also provided details about the site-specific organization of the cancellous network and measured the same parameters in a comparative sample of twelve adult extant humans. For most features, our results indicate two main patterns: the first includes the specimens TM 1517g, SKX 10924 and SKX 34805, while the second endostructural morphotype sets apart the robust Homo aff. erectus Gombore IB specimen from Melka Kunture, which more closely resembles the condition displayed by our comparative human sample. Notably, marked differences in the amount and pattern of proximodistal cortical bone distribution have been detected between Gombore IB and SKX 34805 from Swartkrans. Given its discordant outer and inner signatures, we conclude that the taxonomic status of SKX 34805 deserves further investigations.

Sex-biased dispersal of human ancestors.

Some anthropologists and primatologists have argued that, judging by extant chimpanzees and humans, which are female-biased dispersers, the common ancestors of humans and chimpanzees were also female-biased dispersers. It has been thought that sex-biased dispersal patterns have been genetically transmitted for millions of years. However, this character has changed many times with changes in environment and life-form during human evolution and historical times. I examined life-form and social organization of nonhuman primates, among them gatherers (foragers), hunter-gatherers, agriculturalists, industrialists, and modern and extant humans. I conclude that dispersal patterns changed in response to environmental conditions during primate and human evolution.

Hominoid arcade shape: Pattern and magnitude of covariation

The shape of the dental arcade and canine size distinguish extant humans from all apes. Humans are characterized by a parabolic arcade with short postcanine tooth rows and small canines, whereas apes have long, U-shaped arcades with large canines. The evolutionary and biomechanical mechanisms underlying arcade shape differences between and within groups are not well understood. It is unclear, for example, whether evolutionary changes in the covariation among modules comprising the upper and lower jaws are the cause and/or consequence of different arcade shapes. Here we use 3D geometric morphometric methods to explore to what extent the morphological differences in arcade shape between living hominoids are related to differences in covariation of upper and lower jaws, and the premaxilla and the maxilla. We show that all extant hominoids follow a very similar covariation pattern between upper and lower dental arcades, as well as between the premaxilla and the maxilla. We find comparably high magnitudes of covariation between the premaxilla and the maxilla in all groups. Between the upper and lower jaws, levels of covariation are similar in apes (Pan, Gorilla, Pongo, and Hylobates), but overall lower in extant humans. Our results demonstrate an independence of the pattern of arcade shape covariation from dental spatial arrangements. Importantly, we show that a shared hominoid pattern of covariation between premaxilla and maxilla together with the covariation of upper and lower jaw is consistent with major evolutionary arcade shape changes in hominoids. We suggest that with the reduction of canine and diastema size in hominins, the incisors move posteriorly and the tooth row becomes more parabolic. Our study provides a framework for addressing questions about fossil hominin dentognathic diversity, including inter- and intraspecific variation and associations of upper and lower jaw morphology.

Structural organization and tooth development in a Homo aff. erectus juvenile mandible from the Early Pleistocene site of Garba IV at Melka Kunture, Ethiopian highlands.

The immature partial mandible GAR IVE from the c. 1.7 Ma old Garba IV site at Melka Kunture (Upper Awash Basin, Ethiopia), the earliest human representative from a mountain-like environment, represents one of the oldest early Homo specimens bearing a mixed dentition. Following its first description (Condemi, ), we extended the analytical and comparative record of this specimen by providing unreported details about its inner morphology, tooth maturational pattern and age at death, crown size, and tooth tissue proportions. The new body of quantitative structural information and virtual imaging derives from a medical CT record performed in 2013. Compared to the extant human condition and to some fossil representatives of comparable individual age, the GAR IVE mandible reveals absolutely and relatively thick cortical bone. Crown size of the permanent lateral incisor and the canine fit the estimates of H. erectus s.l., while the dm2 and the M1 more closely approach those of H. habilis-rudolfensis. Molar crown pulp volumes are lower than reported in other fossil specimens and in extant humans. The mineralization sequence of the permanent tooth elements is represented four times in our reference sample of extant immature individuals (N = 795). The tooth developmental pattern displayed by the immature individual from Garba IV falls within the range of variation of extant human populations and is also comparable with that of other very young early fossil hominins. Taken together, the evidence presented here for mandibular morphology and dental development suggest GAR IVE is a robust 2.5- to 3.5-year old early Homo specimen.

Four kinds of humans in past 100000 years and their relations

In the past 100000 years four kinds of humans coexisted. They are early modern humans, Neanderthals, Homo floresiensis , and Denisovans. Early modern humans are ancestors of extant humans and have fundmentally similar morphology to that of ours. Neanderthals are slightly shorter than modern humans. They have robust extremities and larger brain volume, Neanderthals inhabited widely in Europe and also distributed in West and Central Asia. They disappeared around 30000 year ago. Evidence of DNA analysis and morphology verified the gene flow between Neanderthals and early modern humans. The genes with Neanderthal ancestry in the genome of extant humans have definite influences on the physiology, morphology and risk for certain diseases in extant humans. Fossils of Homo floresiensis were unearthed only in Flores Island, Indonesia. This kind of humans is very short, with cranial capacity as small as one third of extant humans. They lived 100000–60000 years ago. The origin and their relation with other kinds of humans are mysteries. Fossils of Denisovans were unearthed only at Denisova Cave of Altai region, south Siberia of Russia and only a finger bone and two teeth were found. Denisovans are identified by DNA analysis and lived around 40000 years ago. In the genome of Melanesians 4.8%± 0.5% are derived from Denisovans. It is suggested that the gene flow with Denisovans was helpful for the adaptation of Tibetans to the high plateau environment. Paleo-DNA study also suggests that perhaps another kind of humans coexisted with our ancestor during this period.

Fetal and neonatal maxillary ontogeny in extant humans and the utility of prenatal maxillary morphology in predicting ancestral affiliation.

The midface of extant Homo sapiens is known to undergo shape changes through fetal and neo-natal ontogeny; however, little work has been done to quantify these shape changes. Further, while midfacial traits which vary in frequency between populations of extant humans are presumed to develop prenatally, patterns of population-specific variation maxillary shape across ontogeny are not well documented. Only one study of fetal ontogeny which included specific discussion of the midface has taken a three-dimensional geometric morphometric approach, and that study was limited to one population (Japanese). The present research project seeks to augment our understanding of fetal maxillary growth patterns, most especially in terms of intraspecific variation. Three-dimensional coordinate landmark data were collected on the right maxillae of 102 fetal and neo-natal individuals from three groups (Euro-American, African-American, "Mixed Ancestry"). Shape changes were seen mainly in the lateral wall of the piriform aperture, the anterior nasal spine, and the subnasal alveolar region. The greatest difference across age groups (second trimester, third trimester, neonates) was between the second and third trimester. Euro-Americans and African-Americans clustered by population and differences in midfacial morphology related to ancestry could be discerned as early as the second trimester (p = .002), indicating that population variation in maxillary morphology appears very early in ontogeny. The midface is a critical region of the skull for assessing ancestry and these results indicate that maxillary morphology may be useful for estimating ancestry for prenatal individuals as young as the second trimester.

Developmental identity versus typology: Lucy has only four sacral segments.

Both interspecific and intraspecific variation in vertebral counts reflect the action of patterning control mechanisms such as Hox. The preserved A.L. 288-1 ("Lucy") sacrum contains five fused elements. However, the transverse processes of the most caudal element do not contact those of the segment immediately craniad to it, leaving incomplete sacral foramina on both sides. This conforms to the traditional definition of four-segmented sacra, which are very rare in humans and African apes. It was recently suggested that fossilization damage precludes interpretation of this specimen and that additional sacral-like features of its last segment (e.g., the extent of the sacral hiatus) suggest a general Australopithecus pattern of five sacral vertebrae. We provide updated descriptions of the original Lucy sacrum. We evaluate sacral/coccygeal variation in a large sample of extant hominoids and place it within the context of developmental variation in the mammalian vertebral column. We report that fossilization damage did not shorten the transverse processes of the fifth segment of Lucy's sacrum. In addition, we find that the extent of the sacral hiatus is too variable in apes and hominids to provide meaningful information on segment identity. Most importantly, a combination of sacral and coccygeal features is to be expected in vertebrae at regional boundaries. The sacral/caudal boundary appears to be displaced cranially in early hominids relative to extant African apes and humans, a condition consistent with the likely ancestral condition for Miocene hominoids. While not definitive in itself, a four-segmented sacrum accords well with the "long-back" model for the Pan/Homo last common ancestor. Am J Phys Anthropol 160:729-739, 2016. © 2016 Wiley Periodicals, Inc.

Strong Selective Sweeps on the X Chromosome in the Human-Chimpanzee Ancestor Explain Its Low Divergence.

The human and chimpanzee X chromosomes are less divergent than expected based on autosomal divergence. We study incomplete lineage sorting patterns between humans, chimpanzees and gorillas to show that this low divergence can be entirely explained by megabase-sized regions comprising one-third of the X chromosome, where polymorphism in the human-chimpanzee ancestral species was severely reduced. We show that background selection can explain at most 10% of this reduction of diversity in the ancestor. Instead, we show that several strong selective sweeps in the ancestral species can explain it. We also report evidence of population specific sweeps in extant humans that overlap the regions of low diversity in the ancestral species. These regions further correspond to chromosomal sections shown to be devoid of Neanderthal introgression into modern humans. This suggests that the same X-linked regions that undergo selective sweeps are among the first to form reproductive barriers between diverging species. We hypothesize that meiotic drive is the underlying mechanism causing these two observations.

The massive fossil humerus from the Oldowan horizon of Gombore I, Melka Kunture (Ethiopia, >1.39 Ma)

A well-preserved distal portion of a left humerus was discovered in 1976 during excavations directed by J. Chavaillon at the Gombore I site, in the Melka Kunture area (Ethiopia). The specimen, labelled Gombore IB-7594 (formally Melka Kunture 3, or MK3), was found in situ within unit 2 of level B, which is dated to >1.39 Ma and includes a rich Oldowan Paleolithic assemblage. Although MK3 has never been described in detail, it appeared in the literature several times and, from a taxonomic point of view, has been alternatively regarded as Homo, Australopithecus or Paranthropus. According to our analysis, MK3 exhibits a suite of features that fit the variability of the genus Homo and does not display any clear Australopithecus/Paranthropus affinity. Nevertheless, MK3 adds a great deal of variability to the genus Homo, at least as far as the Early Pleistocene fossil record is concerned. In particular, our quantitative approach, which combines traditional morphometric analyses and geometric morphometrics, highlights traits that are uncommon among the Plio-Pleistocene fossil record, while affinities with Mid-to-Late Pleistocene representatives of Homo are observed. In addition, the large size of MK3 suggests that this humerus belonged to an individual whose body weight approached 90 kg, far from the range of body size known for Homo representatives in the Early Pleistocene and as big as those of extant humans or even gorillas. We suggest that such peculiar features are of interest when regarded from an ecological perspective; thus, dimension and morphology of MK3 may be considered as an exaptation that became useful when early humans dispersed to high altitudes such as those of the upper Awash basin on the Ethiopian plateau, at heights above 2000 m.