Stephanie M. Melillo

New species from Ethiopia further expands Middle Pliocene hominin diversity

Middle Pliocene hominin species diversity has been a subject of debate over the past two decades, particularly after the naming of Australopithecus bahrelghazali and Kenyanthropus platyops in addition to the well-known species Australopithecus afarensis. Further analyses continue to support the proposal that several hominin species co-existed during this time period. Here we recognize a new hominin species (Australopithecus deyiremeda sp. nov.) from 3.3–3.5-million-year-old deposits in the Woranso–Mille study area, central Afar, Ethiopia. The new species from Woranso–Mille shows that there were at least two contemporaneous hominin species living in the Afar region of Ethiopia between 3.3 and 3.5 million years ago, and further confirms early hominin taxonomic diversity in eastern Africa during the Middle Pliocene epoch. The morphology of Au. deyiremeda also reinforces concerns related to dentognathic (that is, jaws and teeth) homoplasy in Plio–Pleistocene hominins, and shows that some dentognathic features traditionally associated with Paranthropus and Homo appeared in the fossil record earlier than previously thought.

The Pliocene hominin diversity conundrum: Do more fossils mean less clarity?

Recent discoveries of multiple middle Pliocene hominins have raised the possibility that early hominins were as speciose as later hominins. However, debates continue to arise around the validity of most of these new taxa, largely based on poor preservation of holotype specimens, small sample size, or the lack of evidence for ecological diversity. A closer look at the currently available fossil evidence from Ethiopia, Kenya, and Chad indicate that Australopithecus afarensis was not the only hominin species during the middle Pliocene, and that there were other species clearly distinguishable from it by their locomotor adaptation and diet. Although there is no doubt that the presence of multiple species during the middle Pliocene opens new windows into our evolutionary past, it also complicates our understanding of early hominin taxonomy and phylogenetic relationships.

The shoulder girdle of KSD-VP-1/1

Extant humans and non-human apes differ markedly in shoulder girdle anatomy. Our understanding of the evolutionary history of this region was previously limited by poor fossil preservation, but over the past decade a number of impressively complete scapulae and clavicles have been described for the genus Australopithecus . However, independent analyses have reached different conclusions regarding the morphological affinity of each specimen and the degree of difference among specimens. This study provides a more detailed comparative description of the KSD-VP-1/1 scapula and clavicle, which constitute the oldest substantial evidence of hominin shoulder girdle anatomy currently known. The results suggest that the adult Australopithecus afarensis scapula is morphologically distinct, but more similar to that of modern humans than previously recognized. Some aspects of clavicle morphology are similar to non-human apes, but are also variably present in Pleistocene hominins. If comparable methodology is employed, no difference exists among Australopithecus specimens. When this morphology is considered with reference to a parsimony-based model of the chimpanzee–human last common ancestor, the adult Australopithecus shoulder girdle is derived toward morphology associated with emphasis on a manipulatory function of the pendant upper limb.

Conclusion: Implications of KSD-VP-1/1 for Early Hominin Paleobiology and Insights into the Chimpanzee/Human Last Common Ancestor

KSD-VP-1/1 is a 3.6 million years old (Ma) partial skeleton of Australopithecus afarensis recently discovered from the Woranso-Mille study area in the Afar region of Ethiopia. The recovered elements of this specimen, which include cervical vertebrae, a complete scapula, clavicle, numerous ribs, pelvis, and elements of the fore- and hindlimbs, greatly enhance our understanding of the paleobiology of early Australopithecus afarensis. Detailed analyses of the cervical vertebrae indicate that Australopithecus afarensis had a highly mobile neck, signaling human-like kinematics consistent with habitual upright posture and bipedalism. Elements of the shoulder girdle exhibit some primitive morphology but are overall more similar to humans than has been previously understood. This similarity is inconsistent with the notion that the Australopithecus afarensis shoulder retained primitive morphology from an African ape-like chimpanzee/human last common ancestor. Morphology of the thorax also indicates that while some individual traits may appear to superficially suggest arboreality, Australopithecus afarensis did not have an abundance of functionally significant morphological traits that would suggest high canopy arboreality as found today in large-bodied apes. Most of the inconsistencies in interpretations of early hominin paleobiology appear to stem from methodological differences, incorrect a priori assumptions, or incomplete information derived from fragmentary specimens.

Middle Pliocene hominin mandibular fourth premolars from Woranso-Mille (Central Afar, Ethiopia)

The Woranso-Mille study area has thus far yielded more than 120 early hominin fossil specimens dated to between 3.4 and 3.8 million years ago. Previous studies indicate that dentognathic fossil remains from the study area show a mosaic of features shared by both Australopithecus anamensis and Australopithecus afarensis. Here, we describe 12 isolated mandibular fourth premolars recovered from the Woranso-Mille study area and compare them with those of other early hominins using both traditional comparative methods and geometric morphometric methods. The results indicate that the Woranso-Mille sample is most similar to Au. afarensis among hominins. However, some specimens show distinctive features of crown shape, namely an extremely bulging distolingual corner associated with a relatively large talonid and a mesiodistally elongated crown. This unique morphology is accompanied by a root form that is different from those identified previously. The existence of distinctive P4 morphology is intriguing given the presence of more than one hominin species at Woranso-Mille, although support for taxonomic heterogeneity in this sample is equivocal. Further, the taxonomic significance of these features is unclear, as they fail to distinguish Pan from Gorilla and known hominin species from one another.

An alternative interpretation of the Australopithecus scapula

In PNAS, Young et al. (1) conclude that the scapula of the last common ancestor (LCA) of chimpanzees and humans was African ape-like, supporting what they call the “African ape” model. This model was favored over the “ape convergence” model, in which the ancestral condition was more primitive and some morphological similarities shared among modern apes would reflect convergent evolution. The authors suggest that less ape-like morphology in Australopithecus afarensis reflects an adaptive trade-off between arboreality and tool use (1).

The Geologic Context of Korsi Dora and the Partial Skeleton KSD-VP-1/1

KSD-VP-1/1, a partial skeleton of Australopithecus afarensis , was excavated from Pliocene strata at Korsi Dora, 3.3 km southeast of the confluence of the Waki and Mille rivers in the northwestern part of the Woranso-Mille paleoanthropological research site. A tuff collected from ~2.7 m below the fossil horizon, at the bottom of a trench dug 25 m to the east of the fossil excavation, yielded an 40Ar/39Ar age of 3.60 ± 0.03 Ma for anorthoclase feldspar. Strata in the trench and the fossil excavation site comprise a single normal magnetozone interpreted as part of the normal subchron C2An.3n, immediately above the Gauss/Gilbert paleomagnetic transition. Geologic mapping and tephrochemical analyses combined with paleomagnetic data place the fossil horizon and the trench section into local and regional stratigraphic context by constraining the partial skeleton to be younger than the Kilaytoli tuff (KT), a ~4 m thick vitric ash with an anorthoclase feldspar age of 3.570 ± 0.014 Ma. This unit is widely recognized at Korsi Dora, in collection areas north of the Waki-Mille confluence and outside the field area. The KT correlates with the Lokochot Tuff of the Omo-Turkana Basin in Kenya. Sedimentological features of the mudstone and sandstone in and near the excavation site are consistent with deposition in a floodplain or floodplain lake proximal to a stream channel.

The relationship between clavicle length and scapula position in living humans

Leprosy is one of the few specific infectious diseases that can be studied in bioarchaeology due to its characteristic debilitating and disfiguring skeletal changes. Leprosy has been, and continues to be, one of the most socially stigmatising diseases in history, over-riding all other aspects of social identity for the sufferers and frequently resulting in social exclusion. This study examines the stable isotopic evidence of mobility patterns of children, adolescents, and young adult individuals with the lepromatous form of leprosy in Medieval England (10 th –12 th centuries AD) to assess whether the individuals buried with the disease were non-locals, possibly from further afield. Enamel samples from 19 individuals from the St. Mary Magdalen Leprosy Hospital, Winchester (UK) were selected for strontium ( 87 Sr/ 86 6U DQG R[\JHQ į 18 O) stable isotope analysis based on age at death (<30 years), the presence of bone changes associated with lepromatous leprosy, and the underlying geology of their burial locations. The results from these data indicate that the St. Mary Magdalen Leprosy Hospital received an almost equal mixture of local and non-local individuals from further afield, including early pilgrims. At present, the St. Mary Magdalen Leprosy Hospital is the earliest dedicated leprosaria found within Britain and mobility studies such as these can help elucidate and test some of the broader historical notions and identities associated with the movements of those infected with the disease in Medieval England.

Anatomical configuration of the Australopithecus afarensis shoulder: evidence from a new clavicle (KSD-VP-1/1f)

Supplement: Program of the 81st Annual Meeting of the American Association of Physical Anthropologists (2012)The catacomb of Saints Peter and Marcellinus located in the south-east of Rome approximately contains 25 000 graves dated from the 3rd to the 5th century AD. Seven newly discovered rooms having an unusual organization in the heart of the catacomb were investigated in 2003. Excavations of these rooms revealed a mass grave, where 3000 corpses were laid together. These individuals were stacked in rows apparently following a common fatal incident. Presumably, this epidemic crisis occurred between the 1st and the 3rd century AD. The specific funerary treatment (textile wrapping and plaster) recalls mummification and might be related to exogenous practices, possibly connected to Early Christians. Moreover, the presence of rare and expensive materials (e.g., Baltic amber, resins and gold threads) may indicate a high social status. Stable isotope analyses (carbon, nitrogen and oxygen) of bone collagen, bone apatite and tooth hydroxyapatite were carried out on 111 individuals to obtain further information on their diet and residential mobility. Additionally, a study of dental nonmetric traits was conducted on 200 individuals to define the biological distance between the deceased and to assess their phenetic similarity. The combination of these two approaches will bring new insight into the homogeneity of the Early population of the catacomb of Saints Peter and Marcellinus and the relationship between funerary practices and geographical origin of buried individuals.

New data on enamel thickness in Homo sapiens

Understanding the relationship between bone strain and bone growth is critical for interpreting variations in skeletal robusticity. Recently we presented a model for interactions between estrogen, strain, and periosteal bone growth, in which high estrogen (E2) increases, and low (E2) decreases, osteogenic responses to strain. We compared cortical growth in expercised and sedentary sheep (Ovis aries) with higher vs. lower estrogen levels, and showed that exercised animals with high E2 added substantially more bone than those with lower E2. However, without normal controls, it was unclear whether exercise-induced cortical growth was stimulated by high E2, suppressed by lower E2, or both. Here we present a broader test of interactions between E2 levels (normal, low, high) and loading (exercised and sedentary). Low E2 animals were vaccinated against GnRH to suppress estrogen, while high E2 animals received estrogen implants. After 45 days, periosteal bone growth was measured at hindlimb midshafts. The results support the hypothesis that estrogen upregulates strain-induced cortical bone growth: exercised, high-E2 animals grew 6-27% more bone than exercised animals with lower E2 levels, or sedentary animals (p<.05). The effects of the anti-GnRH vaccine on bone growth are more complex. Assays showed that vaccinated animals had normal, not decreased, E2 levels, but grew 34-39% less bone in response to exercise than normal controls (p<.05). This suggests the vaccine affected strain-induced bone growth without changing circulating E2, an unexpected finding. These results demonstrate that variation in E2 levels may produce differential growth response to similar mechanical loading through complex mechanisms.