Extant Humans Research Articles

What constitutes Homo sapiens? Morphology versus received wisdom.

Although Linnaeus coined Homo sapiens in 1735, it was Blumenbach forty years later who provided the first morphological definition of the species. Since humans were not then allowed to be ante-Diluvian, his effort applied to the genus, as well. After the Feldhofer Grotto Neanderthal disproved this creationist notion, and human-fossil hunting became legitimate, new specimens were allocated either to sapiens or new species within Homo, or even to new species within new genera. Yet as these taxonomic acts reflected the morphological differences between specimens, they failed to address the question: What constitutes H. sapiens? When in 1950 Mayr collapsed all human fossils into Homo, he not only denied humans a diverse evolutionary past, he also shifted the key to identifying its species from morphology to geological age - a practice most paleoanthropologists still follow. Thus, for example, H. erectus is the species that preceded H. sapiens, and H. sapiens is the species into which H. erectus morphed. In order to deal with a growing morass of morphologically dissimilar specimens, the non-taxonomic terms "archaic" (AS) and "anatomically modern" (AMS) were introduced to distinguish between the earlier and later versions of H. sapiens, thereby making the species impossible to define. In attempting to disentangle fact from scenario, I begin from the beginning, trying to delineate features that may be distinctive of extant humans (ES), and then turning to the fossils that have been included in the species. With the exception of Upper Paleolithic humans - e.g. from Cro-Magnon, Dolni Vestonice, Mladeč - I argue that many specimens regarded as AMS, and all those deemed AS, are not H. sapiens. The features these AMS do share with ES suggest the existence of a sapiens clade. Further, restudy of near-recent fossils, especially from southwestern China (∼11-14.5 ka), reinforces what discoveries such as H. floresiensis indicate: "If it's recent, it's not necessarily H. sapiens".

Brain ontogeny and life history in Pleistocene hominins.

A high level of encephalization is critical to the human adaptive niche and emerged among hominins over the course of the past 2 Myr. Evolving larger brains required important adaptive adjustments, in particular regarding energy allocation and life history. These adaptations included a relatively small brain at birth and a protracted growth of highly dependent offspring within a complex social environment. In turn, the extended period of growth and delayed maturation of the brain structures of humans contribute to their cognitive complexity. The current palaeoanthropological evidence shows that, regarding life history and brain ontogeny, the Pleistocene hominin taxa display different patterns and that one cannot simply contrast an 'ape-model' to a 'human-model'. Large-brained hominins such as Upper Pleistocene Neandertals have evolved along their own evolutionary pathway and can be distinguished from modern humans in terms of growth pattern and brain development. The life-history pattern and brain ontogeny of extant humans emerged only recently in the course of human evolution.

Variations in size, shape and asymmetries of the third frontal convolution in hominids: Paleoneurological implications for hominin evolution and the origin of language

The study of brain structural asymmetries as anatomical substrates of functional asymmetries in extant humans, great apes, and fossil hominins is of major importance in understanding the structural basis of modern human cognition. We propose methods to quantify the variation in size, shape and bilateral asymmetries of the third frontal convolution (or posterior inferior frontal gyrus) among recent modern humans, bonobos and chimpanzees, and fossil hominins using actual and virtual endocasts. These methodological improvements are necessary to extend previous qualitative studies of these features. We demonstrate both an absolute and relative bilateral increase in the size of the third frontal convolution in width and length between Pan species, as well as in hominins. We also observed a global bilateral increase in the size of the third frontal convolution across all species during hominin evolution, but also non-allometric intra-group variations independent of brain size within the fossil samples. Finally, our results show that the commonly accepted leftward asymmetry of Broca's cap is biased by qualitative observation of individual specimens. The trend during hominin evolution seems to be a reduction in size on the left compared with the right side, and also a clearer definition of the area. The third frontal convolution considered as a whole projects more laterally and antero-posteriorly in the right hemisphere. As a result, the left ‘Broca's cap’ looks more globular and better defined. Our results also suggest that the pattern of brain asymmetries is similar between Pan paniscus and hominins, leaving the gradient of the degree of asymmetry as the only relevant structural parameter. As the anatomical substrate related to brain asymmetry has been present since the appearance of the hominin lineage, it is not possible to prove a direct relationship between the extent of variations in the size, shape, and asymmetries of the third frontal convolution and the origin of language in hominins.

The ontogeny of nasal floor shape variation in extant humans.

Variation in nasal floor topography has generated both neontological and paleontological interest. Three categories of nasal floor shape (Franciscus: J Hum Evol 44 (2003) 699-727) have been used when analyzing this trait in extant humans and fossil Homo: flat, sloped, and depressed (or "bi-level"). Variation in the frequency of these configurations within and among extant and fossil humans has been well-documented (Franciscus: J Hum Evol 44 (2003) 699-727; Wu et al.: Anthropol Sci 120 (2012) 217-226). However, variation in this trait in Homo has been observed primarily in adults, with comparatively small subadult sample sizes and/or large age gradients that may not sufficiently track key ontogenetic changes. In this study, we investigate the ontogeny of nasal floor shape in a relatively large cross-sectional age sample of extant humans (n = 382) ranging from 4.0 months fetal to 21 years post-natal. Results indicate that no fetal or young infant individuals possess a depressed nasal floor, and that a depressed nasal floor, when present (ca. 21% of the sample), does not occur until 3.0 years postnatal. A canonical variates analysis of maxillary shape revealed that individuals with depressed nasal floors were also characterized by relatively taller anterior alveolar regions. This suggests that palate remodeling at about 3.0-3.5 years after birth, under the influence of tooth development, strongly influences nasal floor variation, and that various aspects of dental development, including larger crown/root size, may contribute to the development of a depressed nasal floor. These results in extant humans may help explain the high frequency of this trait found in Neandertal and other archaic Homo maxillae.

The Human Semicircular Canals Orientation Is More Similar to the Bonobos than to the Chimpanzees

For some traits, the human genome is more closely related to either the bonobo or the chimpanzee genome than they are to each other. Therefore, it becomes crucial to understand whether and how morphostructural differences between humans, chimpanzees and bonobos reflect the well known phylogeny. Here we comparatively investigated intra and extra labyrinthine semicircular canals orientation using 260 computed tomography scans of extant humans (Homo sapiens), bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Humans and bonobos proved more similarities between themselves than with chimpanzees. This finding did not fit with the well established chimpanzee – bonobo monophyly. One hypothesis was convergent evolution in which bonobos and humans produce independently similar phenotypes possibly in response to similar selective pressures that may be associated with postural adaptations. Another possibility was convergence following a “random walk” (Brownian motion) evolutionary model. A more parsimonious explanation was that the bonobo-human labyrinthine shared morphology more closely retained the ancestral condition with chimpanzees being subsequently derived. Finally, these results might be a consequence of genetic diversity and incomplete lineage sorting. The remarkable symmetry of the Semicircular Canals was the second major finding of this article with possible applications in taphonomy. It has the potential to investigate altered fossils, inferring the probability of post-mortem deformation which can lead to difficulties in understanding taxonomic variation, phylogenetic relationships, and functional morphology.

A Micro Computerized Tomography (X-RAY MICROSCOPY) of the Hand Phalanx of the Denisova Girl*

In 2010, the complete mitochondrial genome of a fossil hominin from Denisova Cave, Altai was sequenced on the basis of mtDNA extracted from the hand phalanx of a girl. Modern micro tomographic techniques mark a new stage in the morphological study of extant and fossil hominins, offering opportunities to work with fragmentary material. We have used the nondestructive method of micro computerized tomography for a comparative histological assessment of the Denisova girl's biological age. The diaphyseal and metaphyseal parts of the phalanx, reflecting different ontogenetic stages, were undergoing rapid growth. The histological pattern of the walls of the diaphysis, specifically the lamellar structure with rare osteons, indicates a stage corresponding to 6–7 years in modern children. An altogether different pattern, resembling that of adults, was earlier observed in a Neanderthal child from Okladnikov Cave, Altai. The resemblance between the Denisova individual and extant humans in certain features of growth and skeletal maturation may point to the very early origin of the modern skeletal growth pattern. The Neanderthal pattern is quite distinct and may have originated after these hominins had branched off from the common stem.

The cognitive ability of extinct hominins: Bringing down the hierarchy using genomic evidences

The availability of the full genomes of Homo sapiens, Homo neanderthalensis, and Denisovans, as well as modern bioinformatic tools, are opening new possibilities for the understanding of the differences and similarities present in these taxa. We searched for cognitive genes, examined their status in the genomes of these three entities. All substitutions present among them were retrieved. We found 93 nonsynonymous substitutions in 51 cognitive genes, in which the derived allele was present in archaic and modern humans and the ancestral allele in other nonhuman primates. The general picture obtained is of similarity in cognitive genes between extinct and extant humans.

Computed tomography in paleoanthropology — an overview

Computed tomography (CT) was first applied in the early 1970s and introduced subsequently a new perspective towards anatomical imaging. In the last decade, high-resolution CT (HR-CT) had a high impact on anthropology and paleoanthropology through its ability to define and explore subtle differences in hard tissue structures in fossil and extant humans and nonhuman primates. CT is very suitable for unique fossil material, because it is destruction free and the original material stays intact while the internal structures are digitized. The imaging yields a virtual copy of the object, which can be used for the generation of detailed copies of original fossil material. CT data allow multiple studies in parallel and independently on specimens which are not commonly accessible. Diverse CT systems with different performance characteristics as designed for different functions can make it difficult for a researcher to choose the most appropriate CT system and to check the image quality of CT scans. The physical principles involved in CT imaging and the principles of signal processing and computer graphics can help to choose the best scan setting and the most suitable CT system for a study. Quantitative and qualitative analysis can also be improved and comparisons between different studies can be facilitated when the above mentioned principles are taken into account. In the following, I will give an overview of the different CT systems and discus both theoretical and practical matters of CT imaging using the example of trabecular bone.

Evolution, development, and plasticity of the human brain: from molecules to bones

Neuroanatomical, molecular, and paleontological evidence is examined in light of human brain evolution. The brain of extant humans differs from the brains of other primates in its overall size and organization, and differences in size and organization of specific cortical areas and subcortical structures implicated into complex cognition and social and emotional processing. The human brain is also characterized by functional lateralizations, reflecting specializations of the cerebral hemispheres in humans for different types of processing, facilitating fast and reliable communication between neural cells in an enlarged brain. The features observed in the adult brain reflect human-specific patterns of brain development. Compared to the brains of other primates, the human brain takes longer to mature, promoting an extended period for establishing cortical microcircuitry and its modifications. Together, these features may underlie the prolonged period of learning and acquisition of technical and social skills necessary for survival, creating a unique cognitive and behavioral niche typical of our species. The neuroanatomical findings are in concordance with molecular analyses, which suggest a trend toward heterochrony in the expression of genes implicated in different functions. These include synaptogenesis, neuronal maturation, and plasticity in humans, mutations in genes implicated in neurite outgrowth and plasticity, and an increased role of regulatory mechanisms, potentially promoting fast modification of neuronal morphologies in response to new computational demands. At the same time, endocranial casts of fossil hominins provide an insight into the timing of the emergence of uniquely human features in the course of evolution. We conclude by proposing several ways of combining comparative neuroanatomy, molecular biology and insights gained from fossil endocasts in future research.

The mesosternum of the Regourdou 1 Neandertal revisited

Fossil hominin mesosterna, while scarce, can provide useful morphological data in addition to rib remains regarding aspects of thoracic size and shape. These data, in turn, can address hypotheses related to respiratory dynamics, climatic adaptation, and ecogeographical patterning. In this study, we re-evaluate the anatomical representation of the mesosternum of the Regourdou 1 Neandertal individual that alters key aspects of the original description of the fossil remains. We compare this specimen together with the mesosterna of the Kebara 2 Neandertal male individual and the Tabun C1 Neandertal female individual to a large extant modern sample. Our study shows that the current evidence available for Neandertals indicates longer mesosterna, reflecting larger thorax sizes among Neandertals, in comparison with extant humans. Additionally, while this study weakens previous suggestions of ecogeographically mediated differences in the size and shape of upper thorax between Neandertals from the Mediterranean Levant and those deriving from Western Europe, we cannot unambiguously disprove the notion of such clinal differences.

Age‐related changes of digital endocranial volume during human ontogeny: Results from an osteological reference collection

Endocranial volume (EV) estimation is widely used in physical anthropology for assessing brain size differences between taxa and monitoring the emergence of brain growth patterns in modern humans. However, to date, no reference data are available for modern human EV ontogeny. We measured 94 skulls with known sex and age (ranging from 0 to 7.5 years) from the osteological collection of Strasbourg University (OCSU) by using an accurate digital active contour model algorithm on 3D virtual models, reconstructed by CT. The OCSU data also allow us to propose improved equations for estimating EV in immature individuals from dry skull diameters (length, width, and height). Aside from the EV, the average proportional endocranial volume (PEV), corresponding to the ratio of EV at a given age to the average EV in the corresponding adult population, was also computed. EV nearly doubles during the first year of life, and later continues to expand more slowly, at least until 7 years of age. No sex differences can be demonstrated between the EV distributions of boys and girls in this sample. However, although PEV at birth is identical in girls and boys, it later displays significantly higher values in the girls of our series. PEV obtained at birth is 22%, which is quite different from values established for the brain itself from autopsied individuals, or MRI data. This suggests that assessments of EV and PEV values in fossil specimens should be conducted by using identical measures in comparative samples of extant humans and apes.

Moment coefficients of skewness in the femoral neck cortical bone distribution of BAR 1002’00

The cortical bone distributions in the femoral necks of apes and humans differ as a result of different loading environments caused by the realignment of the hip abductor apparatus. Femoral neck cortical bone in extant humans is very thin superiorly and thicker inferiorly, while the cortical bone in apes tends to be more uniformly thick. The unique internal anatomy of extant humans allows inferences to be made about primary locomotor function from incomplete femora. Here the differences in cortical bone distributions are quantified using moment coefficient of skewness. Skewness coefficients at two locations along the neck of the 6 million years old African femoral specimen BAR 1002’00 were compared to samples of 9 extant adult humans and 10 adult chimpanzees. The skewness coefficients of cortical bone in the femoral neck of BAR 1002’00 are more similar to those of chimpanzees than to humans, although the contrast is less pronounced in the region closer to the neck–shaft junction than more proximally toward the femoral head; this pattern indicates that in at least one respect this specimen attributed to Orrorin tugenensis manifests structural features suggesting influences of a hip abductor apparatus that had not yet evolved to the same extent as in extant humans.

First evidence of a bipartite medial cuneiform in the hominin fossil record: a case report from the Early Pleistocene site of Dmanisi

A medial cuneiform exhibiting complete bipartition was discovered at the Early Pleistocene site of Dmanisi, Georgia. The specimen is the oldest known instance of this anatomical variant in the hominin fossil record. Here we compare developmental variation of the medial cuneiform in fossil hominins, extant humans and great apes, and discuss potential implications of bipartition for hominin foot phylogeny and function. Complete bipartition is rare among modern humans (< 1%); incomplete bipartition was found in 2 of 200 examined great ape specimens and also appears in the form of a divided distal articular surface in the Stw573c Australopithecus africanus specimen. Although various developmental pathways lead to medial cuneiform bipartition, it appears that the bipartite bone does not deviate significantly from normal overall morphology. Together, these data indicate that bipartition represents a phyletically old developmental variant of the medial cuneiform, which does not, however, affect the species-specific morphology and function of this bone.

The Function and Evolution of the Mastoid Process and Clavicle in Humans: A Natural Experiment

We hypothesize that, in humans, the evolution of a uniquely enlarged mastoid process and clavicle was connected to the acquisition of an upright posture and of freed hands. Our biomechanical model shows that when the arms are moved or loaded, the contracting upper trapezius muscles counterbalance the added weight on the shoulders. The synchronously contracting sternocleidomastoid muscles prevent the head from being retroflexed. To test our hypotheses, we analyzed the asymmetry of biomechanically relevant character pairs in 54 right‐handed male skeletons, based on the premise that bones subjected to greater forces are larger. The humerus diameter, mastoid process width, and rise of the superior nuchal line were more often larger on the right side, while the clavicle length and scapular breadth were so on the left side. These biomechanically relevant characteristics were found in varying aggregates of 2–5 for each individual. This variability may be due to differing occupations. The forces that mold the mastoid process and clavicle in extant humans probably also selected for their robust size within the Homo lineage.Funding: LSU Robert C. West Fund to MLO; LSU Foundation to DGH

The enamel–dentine junction in the postcanine dentition of Australopithecus africanus: intra‐individual metameric and antimeric variation

We used micro-computed tomography and virtual tools to study metric and morphological features at the enamel-dentine junction and on the outer enamel surface in the postcanine dentition of an exceptionally well-preserved maxilla and mandible of an early hominin. The fossil, Sts 52 from Sterkfontein, South Africa, is attributed to Australopithecus africanus and is about 2.5 million years old. For comparative purposes in this exploratory study, we also used micro-computed tomography to analyse the dentition of a common chimpanzee (Pan troglodytes), a pygmy chimpanzee (Pan paniscus) and three extant humans. Metameric variation of the 3D enamel-dentine junction in the two chimpanzee mandibles was much smaller than in extant humans. Variation in metameric shape was high and complex. Notably, the mandibular metameric variation in extant humans can be greater within individuals, as compared with variation between individuals, with differences in shape appearing greater for M2 compared with M1. We recommend the use of a new approach in which individual metameric variation is systematically assessed before making inferences about differences between fossil hominin species. The fossil hominin examined in this study showed a metameric pattern of mandibular variation in shape that was comparable to the pattern seen in two chimpanzees. This degree of metameric variation appeared relatively small compared with the much larger patterns of variation observed within and between extant humans.

The meaning of Neandertal skeletal morphology

A procedure is outlined for distinguishing among competing hypotheses for fossil morphology and then used to evaluate current views on the meaning of Neandertal skeletal morphology. Three explanations have dominated debates about the meaning of Neandertal cranial features: climatic adaptation, anterior dental loading, and genetic drift. Neither climatic adaptation nor anterior dental loading are well supported, but genetic drift is consistent with the available evidence. Climatic adaptation and activity patterns are the most discussed explanations for Neandertal postcranial features. Robust empirical relationships between climate and body form in extant humans and other endotherms currently make climatic adaptation the most plausible explanation for the wide bodies and relatively short limbs of Neandertals, and many additional postcranial features are likely secondary consequences of these overall skeletal proportions. Activity patterns may explain certain Neandertal postcranial features, but unlike the situation for climate, relationships in extant humans between morphology and activities are typically not well established. For both the cranium and the postcranium, changes in diet or activity patterns may underlie why Neandertals and Pleistocene modern humans tend to be more robust than Holocene humans.

Biomechanical Inferences about the Origins of Bipedal Locomotion from Ancient African Femora

Reported here are the diaphyseal cross-sectional geometrical properties of two fossilized femora from Kenya dated at 6Ma (million years ago), designated as the taxon Orrorin tugenensis. These femora are sampled at a temporal level near the divergence of ape and human lineages. The geometric properties of the bones are compared with previous findings from SK 82 and SK 97, hominid fossils from 2 million years later, and a sample of extant humans from Pecos Pueblo, N. M. The Orrorin femoral specimens BAR 1002’00 and BAR 1003’00 are notably robust, exhibiting cross-sectional areas with relatively high percentages of cortical bone in comparison to a contemporary sample. Total cross-sectional area is markedly smaller for BAR 1002’00 and BAR 1003’00 than for either of the Swartkrans early hominid proximal femora or the Pecos Pueblo sample, but %CA of the two Kenyan fossil femurs are comparable to those from Swartkrans and one third higher than the modern sample. Despite their relatively thicker cortical areas BAR 1002’00 and BAR 1003’00 do not exhibit higher bending resistance (Imax and Imin) or torsional rigidity (J) in compared to a modern human sample. These findings extend our knowledge of internal femoral structure related to the origins of human locomotion.

The prehistory of compassion

Beyond the biological traits that differentiate ancient species of hominins from extant humans, differences in social organization remain an important yet difficult issue to assess. Psychological features and interindividual relations, in particular, are among the least accessible aspects of past behaviors. Although the fossil and archaeological records do not easily allow us to tackle these kinds of issues, contrasting views on the cognitive capabilities and behavioral sophistication of fossil human species have often been expressed. In this issue of PNAS, Gracia et al. (1) provide new evidence on the survival of an abnormal individual with possible cognitive deficits from a group of pre-Neandertal Pleistocene hunter-gatherers, currently assigned to a geological age of >500 ka. The cranium SH14 from the Sima de los Huesos (Sierra de Atapuerca, Spain) is the earliest documented case of human neurocranial and brain deformity in the fossil record to date. Despite her/his pathological condition, this individual was not rejected at birth and survived until at least 5 years of age, apparently receiving the same attention as other children from the group.

Size variation in early human mandibles and molars from Klasies River, South Africa: Comparison with other middle and late Pleistocene assemblages and with modern humans

Previous studies of the Middle Stone Age human remains from Klasies River have concluded that they exhibited more sexual dimorphism than extant populations, but these claims have not been assessed statistically. We evaluate these claims by comparing size variation in the best-represented elements at the site, namely the mandibular corpora and M(2)s, to that in samples from three recent human populations using resampling methods. We also examine size variation in these same elements from seven additional middle and late Pleistocene sites: Skhūl, Dolní Vestonice, Sima de los Huesos, Arago, Krapina, Shanidar, and Vindija. Our results demonstrate that size variation in the Klasies assemblage was greater than in recent humans, consistent with arguments that the Klasies people were more dimorphic than living humans. Variation in the Skhūl, Dolní Vestonice, and Sima de los Huesos mandibular samples is also higher than in the recent human samples, indicating that the Klasies sample was not unusual among middle and late Pleistocene hominins. In contrast, the Neandertal samples (Krapina, Shanidar, and Vindija) do not evince relatively high mandibular and molar variation, which may indicate that the level of dimorphism in Neandertals was similar to that observed in extant humans. These results suggest that the reduced levels of dimorphism in Neandertals and living humans may have developed independently, though larger fossil samples are needed to test this hypothesis.

New Insights Into Mid‐Late Pleistocene Fossil Hominin Paranasal Sinus Morphology

Mid-late Pleistocene fossil hominins such as Homo neanderthalensis and H. heidelbergensis are often described as having extensively pneumatized crania compared with modern humans. However, the significance of pneumatization in recognizing patterns of phyletic diversification and/or functional specialization has remained controversial. Here, we test the null hypothesis that the paranasal sinuses of fossil and extant humans and great apes can be understood as biological spandrels, i.e., their morphology reflects evolutionary, developmental, and functional constraints imposed onto the surrounding bones. Morphological description of well-preserved mid-late Pleistocene hominin specimens are contrasted with our comparative sample of modern humans and great apes. Results from a geometric morphometric analysis of the correlation between paranasal sinus and cranial dimensions show that the spandrel hypothesis cannot be refuted. However, visualizing specific features of the paranasal sinus system with methods of biomedical imaging and computer graphics reveals new aspects of patterns of growth and development of fossil hominins.