Afarensis Research Articles

Middle Pliocene hominin distribution patterns in Eastern Africa

Abundance distributions of large mammals are underused in exploring how ecological pressures vary across contemporaneous sites in the fossil record. To investigate variation in relative abundance across contemporaneous Pliocene mammal communities, we examine the time interval between ∼3.6 and 3.22 Ma at four sites in the Afar and Turkana basins: Hadar and the lower Omo Valley in Ethiopia and East Turkana and West Turkana in Kenya. Taphonomic and collection biases are examined using skeletal parts, body size, and taxonomic data from database collections. Taphonomic biases due to geologic conditions and fossil collection affected all sites, but those in the Turkana Basin appeared particularly affected by collecting bias. As a result, hominin relative abundance is calculated separately using a taphonomic control taxon, which shares similar collection biases and size. Comparisons of mammalian taxonomic groups revealed that the Omo region was dominated by suids and cercopithecids. The other sites are dominated by open habitat and mixed habitat associated bovids. Hominins had higher abundance wherein the dominant mammal taxa indicate a mix of woodland and grassland environments (Hadar) and were rarer at sites where the majority of taxa are associated with woodland vegetation (the Omo Valley). West Turkana is characterized by mixed habitats and the highest relative abundance of hominins relative to control taxa, but sampling issues due to the collection and reporting of papionins likely drive this result. East Turkana has few hominins relative to the control taxon and has dominant habitats indicative of floodplain grasslands but has a small sample size compared with the other sites. These analyses suggest that Kenyanthropus platyops and Australopithecus afarensis inhabited similar types of habitats across different rift basins. Most convincingly, this study contributes to a growing body of evidence suggesting that early hominins diverged from their great ape counterparts by abandoning woodland-dominated habitats.

Associated Australopithecusafarensis second and third metatarsals (A.L. 333-133) from Hadar, Ethiopia

Associated Australopithecusafarensis second and third metatarsals (A.L. 333-133) from Hadar, Ethiopia

Hominin dental remains from the Pliocene localities at Lomekwi, Kenya (1982–2009)

Increasing evidence for both taxonomic diversity and early stone manufacture during the Pliocene highlights the importance of the hominin fossil record from this epoch in eastern Africa. Here, we describe dental remains from Lomekwi (West Turkana, Kenya), which date from between 3.2 and 3.5 Ma. The sample was collected between 1982 and 2009 and includes five gnathic specimens and a total of 67 teeth (mostly isolated permanent postcanine teeth). Standard linear dimensions indicate that, although the Lomekwi teeth are relatively small, there is broad overlap in size with contemporary Australopithecus afarensis and Australopithecus deyiremeda specimens at most tooth positions. However, some dental characters differentiate this sample from these species, including a relatively large P4 and M3 compared with the M1, a high incidence of well-developed protostylids, and specific accessory molar cuspules. Owing to a lack of well-preserved tooth crowns (and the complete absence of mandibular teeth) in the holotype and paratype of Kenyanthropus platyops, and limited comparable gnathic morphology in the new specimens, it cannot be determined whether these Lomekwi specimens should be attributed to this species. Attribution of these specimens is further complicated by a lack of certainty about position along the tooth row of many of the molar specimens. More comprehensive shape analyses of the external and internal morphology of these specimens, and additional fossil finds, would facilitate the taxonomic attribution of specimens in this taxonomically diverse period of human evolution.

The morphological affinity of the Early Pleistocene footprints from Happisburgh, England, with other footprints of Pliocene, Pleistocene, and Holocene age

Fossil hominin footprints provide a direct source of evidence of locomotor behavior and allow inference of other biological data such as anthropometrics. Many recent comparative analyses of hominin footprints have used 3D analytical methods to assess their morphological affinities, comparing tracks from different locations and/or time periods. However, environmental conditions can sometimes preclude 3D digital capture, as was the case at Happisburgh (England) in 2013. Consequently, we use here a 2D geometric morphometric approach to investigate the evolutionary context of the Happisburgh tracks. The comparative sample of hominin tracks comes from eight localities that span a broad temporal range from the Pliocene to Late Holocene. The results show disparity in the shapes of tracks ascribed to hominins from the Pliocene (presumably Australopithecus afarensis), Pleistocene (presumably Homo erectus and Homo antecessor), and Holocene (Homo sapiens). Three distinct morphological differences are apparent between time samples: changes in adduction of the hallux, changes in the shape and position of the medial longitudinal arch impression, and apparent changes in foot proportions. Linear dimensions classified the potential H. antecessor tracks from Happisburgh as being most similar to the presumed H. erectus prints from Ileret. We demonstrate using 2D geometric morphometric methods and linear dimensions that the Happisburgh tracks are morphologically similar to other presumed Homo tracks and differ from the Laetoli footprints. The probable functional implications of these results fit well with previous comparative analyses of hominin tracks at other sites.

Fossils from Mille-Logya, Afar, Ethiopia, elucidate the link between Pliocene environmental changes and Homo origins

Several hypotheses posit a link between the origin of Homo and climatic and environmental shifts between 3 and 2.5 Ma. Here we report on new results that shed light on the interplay between tectonics, basin migration and faunal change on the one hand and the fate of Australopithecus afarensis and the evolution of Homo on the other. Fieldwork at the new Mille-Logya site in the Afar, Ethiopia, dated to between 2.914 and 2.443 Ma, provides geological evidence for the northeast migration of the Hadar Basin, extending the record of this lacustrine basin to Mille-Logya. We have identified three new fossiliferous units, suggesting in situ faunal change within this interval. While the fauna in the older unit is comparable to that at Hadar and Dikika, the younger units contain species that indicate more open conditions along with remains of Homo. This suggests that Homo either emerged from Australopithecus during this interval or dispersed into the region as part of a fauna adapted to more open habitats.

Australopithecus afarensis endocasts suggest ape-like brain organization and prolonged brain growth.

Human brains are three times larger, are organized differently, and mature for a longer period of time than those of our closest living relatives, the chimpanzees. Together, these characteristics are important for human cognition and social behavior, but their evolutionary origins remain unclear. To study brain growth and organization in the hominin species Australopithecus afarensis more than 3 million years ago, we scanned eight fossil crania using conventional and synchrotron computed tomography. We inferred key features of brain organization from endocranial imprints and explored the pattern of brain growth by combining new endocranial volume estimates with narrow age at death estimates for two infants. Contrary to previous claims, sulcal imprints reveal an ape-like brain organization and no features derived toward humans. A comparison of infant to adult endocranial volumes indicates protracted brain growth in A. afarensis, likely critical for the evolution of a long period of childhood learning in hominins.

Morphometric analysis of the hominin talus: Evolutionary and functional implications

The adoption of bipedalism is a key benchmark in human evolution that has impacted talar morphology. Here, we investigate talar morphological variability in extinct and extant hominins using a 3D geometric morphometric approach. The evolutionary timing and appearance of modern human–like features and their contributions to bipedal locomotion were evaluated on the talus as a whole, each articular facet separately, and multiple combinations of facets. Distinctive suites of features are consistently present in all fossil hominins, despite the presence of substantial interspecific variation, suggesting a potential connection of these suites to bipedal gait. A modern human–like condition evolved in navicular and lateral malleolar facets early in the hominin lineage compared with other facets, which demonstrate more complex morphological variation within Homininae. Interestingly, navicular facet morphology of Australopithecus afarensis is derived in the direction of Homo, whereas more recent hominin species such as Australopithecus africanus and Australopithecus sediba retain more primitive states in this facet. Combining the navicular facet with the trochlea and the posterior calcaneal facet as a functional suite, however, distinguishes Australopithecus from Homo in that the medial longitudinal arch had not fully developed in the former. Our results suggest that a more everted foot and stiffer medial midtarsal region are adaptations that coincide with the emergence of bipedalism, whereas a high medial longitudinal arch emerges later in time, within Homo. This study provides novel insights into the emergence of talar morphological traits linked to bipedalism and its transition from a facultative to an obligate condition.

Stiffness of the human foot and evolution of the transverse arch.

The stiff human foot enables an efficient push-off when walking or running, and was critical for the evolution of bipedalism1-6. The uniquely arched morphology of the human midfoot is thought to stiffen it5-9, whereas other primates have flat feet that bend severely in the midfoot7,10,11. However, the relationship between midfoot geometry and stiffness remains debated in foot biomechanics12,13, podiatry14,15 and palaeontology4-6. These debates centre on the medial longitudinal arch5,6 and have not considered whether stiffness is affected by the second, transverse tarsal arch of the human foot16. Here we show that the transverse tarsal arch, acting through the inter-metatarsal tissues, is responsible for more than 40% of the longitudinal stiffness of the foot. The underlying principle resembles a floppy currency note that stiffens considerably when it curls transversally. We derive a dimensionless curvature parameter that governs the stiffness contribution of the transverse tarsal arch, demonstrate its predictive power using mechanical models of the foot and find its skeletal correlate in hominin feet. In the foot, the material properties of the inter-metatarsal tissues and the mobility of the metatarsals may additionally influence the longitudinal stiffness of the foot and thus the curvature-stiffness relationship of the transverse tarsal arch. By analysing fossils, we track the evolution of the curvature parameter among extinct hominins and show that a human-like transverse arch was a key step in the evolution of human bipedalism that predates the genus Homo by at least 1.5million years. This renewed understanding of the foot may improve the clinical treatment of flatfoot disorders, the design of robotic feet and the study of foot function in locomotion.

The ecology of Australopithecus anamensis in the early Pliocene of Kanapoi, Kenya

Australopithecus anamensis is a pivotal species in human evolution. It is likely to be the direct ancestor of Australopithecus afarensis and the species that may have given rise to the Homo and Paranthropus lineages. It had a suite of adaptations for habitual bipedalism and a diet that differed from that of earlier hominin species. Under what environmental and ecological conditions did this suite of adaptations arise? The early Pliocene site of Kanapoi in the Lake Turkana Basin of Kenya has the largest sample of A. anamensis in eastern Africa and a rich record of fossil vertebrates. Most Kanapoi fossils are chronologically well constrained by radiometrically dated tephras between the ages of 4.2 and 4.1 million years ago. Sedimentological, isotopic, and faunal data indicate that the environments of Kanapoi during the early Pliocene had a complex range of vegetation types that included closed woodlands, shrubs, and grasslands near a river (for most of the sequence) or lake. These were dynamic landscapes that could shift rapidly from fluvial to lacustrine conditions, and then back. Australopithecus anamensis shared its environments with at least 10 species of very large herbivores, which undoubtedly played a major role in modifying the landscape by opening wooded areas and providing pathways for bipedal hominins. Hominins may have competed for terrestrial resources with abundant suids (Nyanzachoerus and Notochoerus) and for arboreal resources with monkeys (Parapapio being the most common cercopithecid). Kanapoi had a formidable group of predators that included a very abundant species of hyena (Parahyaena howelli), two sabre-tooth felids (Dinofelis and Homotherium), a giant otter (Enhydriodon cf. dikikae), and three species of crocodiles. Various measures of abundance indicate that A. anamensis was an important component of the Kanapoi early Pliocene ecosystems, and that its key adaptations allowed this species to thrive in complex and dynamic landscapes.

A descriptive and comparative study of two Early Pleistocene immature scapulae from the TD6.2 level of the Gran Dolina cave site (Sierra de Atapuerca, Spain)

Here we present the descriptive and comparative study of two immature scapulae recovered from the TD6.2 level of the Gran Dolina cave site (Sierra de Atapuerca, Spain) and assigned to Homo antecessor. This is the first time that data on the morphology and dimensions of the scapulae of a European late Early Pleistocene hominin population are provided. Considering the state of development and the linear dimensions, the scapula ATD6-116 could belong to a child of about 2–4 years. The morphology of ATD6-116 clearly departs from that of the Australopithecus afarensis juvenile specimen DIK-1-1, pointing to functional differences in locomotor behavior between Australopithecus and the late Early Pleistocene hominins. The immature scapula ATD6-118 belonged to an immature individual with a development of the scapula equivalent to that of adolescents of recent human populations. The scapulae ATD6-118 and KNM-WT 15000 present a similar state of development. Although the scapula KNM-WT 15000 is clearly larger than ATD6-118, these two specimens share some characteristics such as their relative narrowness and the value of the axilloglenoid and spinoglenoid angles. The glenoid fossa of ATD6-116 show a lateral orientation, whereas in ATD6-118 the glenoid fossa is slightly cranially oriented, but still within the range of variation of modern humans. The glenoid index of both ATD6-116 and ATD6-118 is low in accordance to the values usually observed in other early hominins, thus showing the primitive condition for this feature. Both scapulae show a ventrally placed axillary sulcus. The presence of this primitive feature in ATD-116 confirms that the shape of the axillary border has a genetic basis and it is not related to physical activity.

Eyasi Plateau Paleontological Expedition, Laetoli, Tanzania, fossil specimen database 1998\u20132005

The Eyasi Plateau Paleontological Expedition (EPPE) Laetoli specimen database contains 13716 records of plant and animal fossils (ca. 28248 specimens) collected by EPPE field teams working at Laetoli, Tanzania between 1998 and 2005. This dataset is a digital version of the original hard-copy specimen catalog, and it documents the discovery, stratigraphic provenience and taxonomic diversity of Plio-Pleistocene fauna and flora in northern Tanzania between 4.4 Ma and >200 ka. Laetoli is renowned for the discovery of important hominin fossils, including the lectotype for Australopithecus afarensis, one of our early hominin ancestors, the first record of Paranthropus aethiopicus outside Kenya-Ethiopia, and an early record of our own species Homo sapiens. This database is one of the few publicly available palaeoanthropological fossil datasets and serves as an example for expanding open access to primary fossil occurrence data in palaeoanthropology. The taxonomic identifications appearing in this dataset are the original field identifications and are provisional. Any taxonomic analysis employing this dataset should refer to updated taxonomic identifications published by specialists.

Visualization of a Juvenile Australopithecus afarensis Specimen: Implications for Functional Foot Anatomy.

Since it was named in 1978, analyses of Australopithecus afarensis have culminated in several dominant theories on how humans acquired many of their unique adaptations. Because bipedal locomotion is one of the earliest characteristics of human functional anatomy to appear in the fossil record, its associated anatomy in early hominins has significant implications for human evolution (Stern 2000). The skeleton and overall morphological characteristics of the foot in Australopithecus afarensis provide important clues about the origins of upright bipedal locomotion. Popularly known as "Selam," the 3.3 million-year-old DIK-1-1 fossil was discovered in Dikika, Ethiopia by Dr. Zeresenay Alemseged and his team in 2000. Selam was an australopithecine who died at three years old, making her the youngest early hominin specimen known today (Alemseged et al. 2006). This discovery allows researchers to investigate not only locomotor patterns of A. afarensis within the context of human evolution, but also to examine what child development may have looked like during this pivotal time. The purpose of this project is to create a 3D animation that accurately reconstructs the anatomy and taphonomy of the Dikika foot. By segmenting CT data, 3D modelling, and animating, this investigation aims to contribute to the breadth of fossil reconstruction techniques in the field of biomedical visualization. This method provides a robust means of communication within, and beyond, the paleoanthropological community about new discoveries and how to visualize them.

Endostructural morphology in hominoid mandibular third premolars: Discrete traits at the enamel-dentine junction

The mandibular third premolar (P3) exhibits substantial differences in size and shape among hominoid taxa, and displays a number of discrete traits that have proven to be useful in studies of hominin taxonomy and phylogeny. Discrete traits at the enamel-dentine junction (EDJ) can be accurately assessed on moderately worn specimens, and often appear sharper than at the outer-enamel surface (OES). Here we use microtomography to image the P3 EDJ of a broad sample of extant apes, extinct hominins and modern humans (n = 100). We present typologies for three important premolar discrete traits at the EDJ (transverse crest, marginal ridge and buccal grooves), and score trait frequencies within our sample. We find that the transverse crest is variable in extant apes, while the majority of hominins display a transverse crest which runs directly between the two major premolar cusps. Some Neanderthals display a unique form in which the transverse crest fails to reach the protoconid. We find that mesial marginal ridge discontinuity is common in Australopithecus anamensis and Australopithecus afarensis while continuous marginal ridges largely characterize Australopithecus africanus and Paranthropus. Interrupted mesial and distal marginal ridges are again seen in Homo sapiens and Neanderthals. Premolar buccal grooves, previously identified at the OES as important for hominin systematics, are again found to show a number of taxon-specific patterns at the EDJ, including a clear difference between Australopithecus and Paranthropus specimens. However, their appearance may be dependent on the morphology of other parts of the crown such as the protoconid crest, and the presence of accessory dentine horns. Finally, we discuss rare variations in the form of dentine horns that underlie premolar cusps, and their potential homology to similar morphologies in other tooth positions.

The deciduous dentition of Homo naledi: A comparative study

In 2013, 2014 new hominin remains were uncovered in the Dinaledi chamber of the Rising Star cave system in South Africa. In 2015 Berger and colleagues identified these remains as belonging to a new species Homo naledi (Berger et al., 2015). Subsequent comparative studies of the skull, postcrania and permanent dentition have supported this taxonomic affiliation (Harcourt-Smith et al., 2015; Kivell et al., 2015; Irish et al., 2018). The deciduous teeth can offer unique insights into hominin evolution. Due to their early onset and rapid development their morphology is thought to be under stronger genetic control and less influenced by environment than are the permanent teeth. In this study we compared the H. naledi deciduous teeth from the 2013–2014 excavations to samples representing much of the hominin clade including Australopithecus afarensis, Australopithecus africanus, Paranthropus boisei, Paranthropus robustus, early Homo, Homo antecessor, Homo erectus s.l., Homo floresiensis, Middle Pleistocene Homo, Homo neanderthalensis, early Homo sapiens and recent H. sapiens from Sub-Saharan Africa. By making such a broad morphological comparison, we aimed to contextualize the Dinaledi hominins and to further assess the validity of their taxonomic assignment. Our analysis of the deciduous teeth revealed a unique combination of features that mirror (but also expand) that found in the permanent teeth. This mosaic includes an asymmetrical lower canine with a distal tubercle, an upper first molar with a large hypocone and epicrista associated with a mesial cuspule, a molarized lower first molar resembling Paranthropus, and upper and lower second molars that resemble later Homo in their lack of accessory cusps. The unique combination of deciduous dental characters supports previous studies assigning H. naledi to a new species, although its phylogenetic position vis-à-vis other Homo species remains ambiguous.

The skull of StW 573, a 3.67 Ma Australopithecus prometheus skeleton from Sterkfontein Caves, South Africa

Here we present the first full anatomical description of the 3.67 million-year-old Australopithecus skull StW 573 that was recovered with its skeleton from the Sterkfontein Member 2 breccia in the Silberberg Grotto. Analysis demonstrates that it is most similar in multiple key morphological characters to a group of fossils from Sterkfontein Member 4 and Makapansgat that are here distinguished taxonomically as Australopithecus prometheus. This taxon contrasts with another group of fossils from those sites assigned to Australopithecus africanus. The anatomical reasons for why these groupings should not be lumped together (as is frequently done for the South African fossils) are discussed in detail. In support of this taxonomy, we also present for the first time a newly reconstructed palate of A. africanus (StW 576 from Sterkfontein Member 4), which has a uniquely complete permanent dentition. The StW 573 skull also has certain similarities with other earlier Australopithecus fossils in East Africa, assigned to Australopithecus afarensis and Australopithecus anamensis, which are discussed. One of its most interesting features is a pattern of very heavy anterior dental wear unlike that found in A. africanus but resembling that found in A. anamensis at 4.17 Ma. Because the StW 573 skull is associated with a near-complete skeleton that is also described for the first time in this special issue, we are now able to use this individual to improve our understanding of more fragmentary finds in the South African fossil record of Australopithecus.

Stratigraphy, mineralogy and geochemistry of the Upper Laetolil tuffs including a new tuff 7 site with footprints of Australopithecus afarensis, Laetoli, Tanzania

The Upper Laetolil marker Tuffs 1 to 8 are mineralogically similar rocks, but heterogeneous in terms of their texture, structure, proportion of primary minerals, volume of cement and degree of low-temperature alteration. Originally they were deposited as crystal and/or vitric ash of evolved melilite-nephelinitic composition and not as melilitite-(natro)carbonatite. Occurrence of carbonate-silicate melt inclusions in primary minerals supports R. Hay's conclusion that the ash could have erupted from a carbonatitic volcanic source. Primary minerals (melilite, clinopyroxene, garnet, perovskite, magnetite) in the tuffs are characterised by wide variations in their compositions and two and even more mineral populations are present within each marker tuff. Thus, any correlation between the tuffs from different localities on the basis of mineral composition is very difficult to impossible. Tuff 7, with footprints of Australopithecus afarensis, is a very heterogeneous unit both vertically and laterally that formed during four major eruption events. Trace-element geochemistry and Sr–Nd isotopic data for Tuffs 6, 7 and 8 suggest that compositionally different volcanic sources were involved in their formation. Initial 87Sr/86Sr and 144Nd/143Nd ratios also show that the Sadiman volcano should not be considered as a source for these three marker tuffs at Laetoli. Only Essimingor and Mosonik volcanoes produced rocks that are mineralogically and geochemically similar to the Upper Laetolil marker tuffs, though these volcanoes lie about 100 km from Laetoli.

The long limb bones of the StW 573 Australopithecus skeleton from Sterkfontein Member 2: Descriptions and proportions

Due to its completeness, the A.L. 288-1 (‘Lucy’) skeleton has long served as the archetypal bipedal Australopithecus. However, there remains considerable debate about its limb proportions. There are three competing, but not necessarily mutually exclusive, explanations for the high humerofemoral index of A.L. 288-1: (1) a retention of proportions from an Ardipithecus-like chimp/human last common ancestor (CLCA); (2) indication of some degree of climbing ability; (3) allometry. Recent discoveries of other partial skeletons of Australopithecus, such as those of Australopithecus sediba (MH1 and MH2) and Australopithecus afarensis (KSD-VP-1/1 and DIK-1/1), have provided new opportunities to test hypotheses of early hominin body size and limb proportions. Yet, no early hominin is as complete (>90%), as is the ∼3.67 Ma ‘Little Foot’ (StW 573) skeleton from Sterkfontein Member 2. Here, we provide the first descriptions of its upper and lower long limb bones, as well as a comparative context of its limb proportions. We found that StW 573 possesses absolutely longer limb lengths than A.L. 288-1, but both skeletons show similar limb proportions. This finding seems to argue against a purely allometric explanation for A.L. 288-1 limb proportions. In fact, our multivariate allometric analysis suggests that limb lengths of Australopithecus, as represented by StW 573 and A.L. 288-1, exhibit a significantly different (p < 0.001) allometric pattern than that which typifies modern humans and African apes. Like some previous analyses, our results also suggest that hominin limb evolution occurred in two stages with: first, a modest increase in lower limb length and a concurrent shortening of the antebrachium between Ardipithecus and Australopithecus, followed by a considerable lengthening of the lower limb along with a decrease of both upper limb elements occurring between Australopithecus and Homo sapiens.

Reply to: Charlier etal. 2018. Mudslide and/or animal attack are more plausible causes and circumstances of death for AL 288 ('Lucy'): a forensic anthropology analysis. Medico-Legal Journal 86(3) 139-142, 2018.

The Pliocene hominin fossil 'Lucy' (A.L. 288-1, Australopithecus afarensis) was discovered in the Afar region of Ethiopia in 1974 and dates to 3.18 million years in age. In Kappelman etal.,1 we presented the results of a detailed investigation of the skeleton that for the first time identified and described unusual bone-into-bone compressive fractures at several of the major long bone joints. Using multiple criteria, we concluded that these fractures are more likely to be perimortem than postmortem in nature. We next evaluated a number of possible mechanisms that could have produced these fractures and, on the basis of all of the evidence, hypothesised that a fall from considerable height, likely out of a tree, with its resulting vertical deceleration event, most closely matched the pattern of fractures preserved in the skeleton and was also the probable cause of death. Charlier etal. disagree with our approach and hypothesis, and instead present what they consider to be better evidence supporting two of the other possible mechanisms for breakage that we also investigated, a mudslide/flood, or an animal attack. We here show that the evidence presented by Charlier etal. is incorrectly interpreted, and that these two alternative hypotheses are less likely to be responsible for the fractures.

Earliest axial fossils from the genus Australopithecus

Australopitheus anamensis fossils demonstrate that craniodentally and postcranially the taxon was more primitive than its evolutionary successor Australopithecus afarensis. Postcranial evidence suggests habitual bipedality combined with primitive upper limbs and an inferred significant arboreal adaptation. Here we report on A. anamensis fossils from the Assa Issie locality in Ethiopia's Middle Awash area dated to ∼4.2 Ma, constituting the oldest known Australopithecus axial remains. Because the spine is the interface between major body segments, these fossils can be informative on the adaptation, behavior and our evolutionary understanding of A. anamensis. The atlas, or first cervical vertebra (C1), is similar in size to Homo sapiens, with synapomorphies in the articular facets and transverse processes. Absence of a retroglenoid tubercle suggests that, like humans, A. anamensis lacked the atlantoclavicularis muscle, resulting in reduced capacity for climbing relative to the great apes. The retroflexed C2 odontoid process and long C6 spinous process are reciprocates of facial prognathism, a long clivus and retroflexed foramen magnum, rather than indications of locomotor or postural behaviors. The T1 is derived in shape and size as in Homo with an enlarged vertebral body epiphyseal surfaces for mitigating the high-magnitude compressive loads of full-time bipedality. The full costal facet is unlike the extant great ape demifacet pattern and represents the oldest evidence for the derived univertebral pattern in hominins. These fossils augment other lines of evidence in A. anamensis indicating habitual bipedality despite some plesiomorphic vertebral traits related to craniofacial morphology independent of locomotor or postural behaviors (i.e., a long clivus and a retroflexed foramen magnum). Yet in contrast to craniodental lines of evidence, some aspects of vertebral morphology in A. anamensis appear more derived than its descendant A. afarensis.

Three-dimensional geometric morphometric analysis of the first metacarpal distal articular surface in humans, great apes and fossil hominins

Understanding the manual abilities of fossil hominins has been a focus of palaeoanthropological research for decades. Of interest are the morphological characteristics of the thumb due to its fundamental role in manipulation, particularly that of the trapeziometacarpal joint. Considerably less attention has been given to the thumb metacarpophalangeal (MCP) joint, which plays a role in stabilizing the thumb during forceful grasps and precision pinching. In this study we use a three-dimensional geometric morphometric approach to quantify the shape of the first metacarpal head in extant hominids (Homo, Pan, Gorilla and Pongo) and six fossil hominin species (Homo neanderthalensis Tabun C1 and La Chappelle-aux-Saints, Homo naledi U.W. 101-1282, Australopithecus sediba MH2, Paranthropus robustus/early Homo SK84, Australopithecus africanus StW 418, Australopithecus afarensis A.L. 333w-39), with the aims of identifying shapes that may be correlated with human-like forceful opposition and determining if similar morphologies are present in fossil hominins. Results show that humans differ from extant great apes by having a distally flatter articular surface, larger epicondyle surface area, and a larger radial palmar condyle. We suggest that this suite of features is correlated with a lower range of motion at the MCP joint, which would enhance the thumbs ability to resist the elevated loads associated with the forceful precision grips typical of humans. Great ape genera are each differentiated by distinctive morphological features, each of which is consistently correlated with the predicted biomechanical demands of their particular locomotor and/or manipulatory habits. Neanderthals and U.W. 101-1282 fall within the modern human range of variation, StW 418, SK 84 and U.W. 88-119 fall in between humans and great apes, and A.L. 333w-39 falls within Pan variation. These results agree with those of traditional linear analyses while providing a more comprehensive quantitative basis from which to interpret the hand functional morphology of extinct hominins.