Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia

A partial skeleton from Hadar, Ethiopia (A.L. 438-1) attributed to Australopithecus afarensis is comprised of part of the mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus parts of the clavicle, humerus, radius, and right ulna. It is one of only a few early hominin specimens to preserve both cranial and postcranial elements. It also includes the first complete ulna from a large A. afarensis individual, and the first associated metacarpal and forelimb remains. This specimen, dated to approximately 3Ma, is among the geologically youngest A. afarensis fossils and is also one of the largest individuals known. Its ulnar to mandibular proportions are similar to those of the geologically older and much smaller A.L. 288-1, suggesting that body size increased without disproportional enlargement of the mandible. Overall, however, analysis of this large specimen and of the diminutive A.L. 288-1 demonstrates that the functional morphology of the A. afarensis upper limb was similar at all body sizes; there is no evidence to support the hypothesis that more than one hominin species is present at Hadar. Morphologically, all apparent apomorphic traits of the elbow, forearm, wrist, and hand of A.L. 438-1 are shared uniquely with humans. Compared to humans, A.L. 438-1 does have a more curved ulna, although A.L. 288-1 does not, and it appears to have had slightly less well-developed manipulatory capabilities of its hands, although still more derived than in apes. We conclude that selection for effective arboreality in the upper limb of Australopithecus afarensis was weaker than in non-hominins, and that manipulative ability was of greater selective advantage than in extant great apes.     

Mio-Pliocene mammals from the Middle Awash, Ethiopia

The Middle Awash paleontological study area, located in the Afar Rift of Ethiopia, has yielded fossils spanning the last six million years. The geology and geochronology of the Mio-Pliocene sites of the study area have been refined and a reliable chronostratigraphy has been established by ⁴⁰Ar/³⁹Ar radiometric dating. The latest Miocene Adu-Asa Formation is divided into four members distinguished from each other by silicic and basaltic tuff marker horizons, most of which are dated basaltic tuffs. Radiometric dating has constrained the age of the Adu-Asa Formation to between 5.2-5.8 Ma. These dates are also supported by paleomagnetic results and biochronology. More than 2,000 fossil specimens were collected from the Adu-Asa Formation between 1992 and 2000. These fossils document 64 mammalian species belonging to 32 genera, 23 families, and 8 orders. This assemblage includes a number of new taxa. Included in the assemblage are First and Last Appearance Datums (FADs and LADs) of some groups, including the earliest record of the hominid genus Ardipithecus. Most of the taxa indicate a predominance of mesic and wooded habitat during the deposition of the Adu-Asa Formation. In these deposits, colobines, viverrids, mustelids, bovines, boselaphines, and tragelaphines are abundant, whereas alcelaphines are absent. Quantitative analyses of biogeographic relationships of the Middle Awash Late Miocene (MALM) mammalian fauna indicate stronger relationships with other African sites than with faunas from Eurasian sites. The MALM deposits have generated a critical dataset for analytic work on past environments, biogeographic relationships, and African vertebrate evolution. Moreover, the geographic position of the Middle Awash, coupled with its precise calibration and chronological span, make it a key section for interpreting latest Miocene faunal interchanges between Africa and Eurasia.     

New postcranial fossils of Australopithecus afarensis from Hadar, Ethiopia (1990-2007)

Renewed fieldwork at Hadar, Ethiopia, from 1990 to 2007, by a team based at the Institute of Human Origins, Arizona State University, resulted in the recovery of 49 new postcranial fossils attributed to Australopithecus afarensis. These fossils include elements from both the upper and lower limbs as well as the axial skeleton, and increase the sample size of previously known elements for A. afarensis. The expanded Hadar sample provides evidence of multiple new individuals that are intermediate in size between the smallest and largest individuals previously documented, and so support the hypothesis that a single dimorphic species is represented. Consideration of the functional anatomy of the new fossils supports the hypothesis that no functional or behavioral differences need to be invoked to explain the morphological variation between large and small A. afarensis individuals. Several specimens provide important new data about this species, including new vertebrae supporting the hypothesis that A. afarensis may have had a more human-like thoracic form than previously appreciated, with an invaginated thoracic vertebral column. A distal pollical phalanx confirms the presence of a human-like flexor pollicis longus muscle in A. afarensis. The new fossils include the first complete fourth metatarsal known for A. afarensis. This specimen exhibits the dorsoplantarly expanded base, axial torsion and domed head typical of humans, revealing the presence of human-like permanent longitudinal and transverse arches and extension of the metatarsophalangeal joints as in human-like heel-off during gait. The new Hadar postcranial fossils provide a more complete picture of postcranial functional anatomy, and individual and temporal variation within this sample. They provide the basis for further in-depth analyses of the behavioral and evolutionary significance of A. afarensis anatomy, and greater insight into the biology and evolution of these early hominins.     

Limb-size proportions in Australopithecus afarensis and Australopithecus africanus

Previous analyses have suggested that Australopithecus africanus possessed more apelike limb proportions than Australopithecus afarensis. However, due to the errors involved in estimating limb length and body size, support for this conclusion has been limited. In this study, we use a new Monte Carlo method to (1) test the hypothesis that A. africanus had greater upper:lower limb-size proportions than A. afarensis and (2) assess the statistical significance of interspecific differences among these taxa, extant apes, and humans. Our Monte Carlo method imposes sampling constraints that reduce extant ape and human postcranial measurements to sample sizes comparable to the fossil samples. Next, composite ratios of fore- and hindlimb geometric means are calculated for resampled measurements from the fossils and comparative taxa. Mean composite ratios are statistically indistinguishable (alpha=0.05) from the actual ratios of extant individuals, indicating that this method conserves each sample's central tendency. When applied to the fossil samples, upper:lower limb-size proportions in A. afarensis are similar to those of humans (p=0.878) and are significantly different from all great ape proportions (p< or =0.034), while Australopithecus africanus is more similar to the apes (p> or =0.180) and significantly different from humans and A. afarensis (p< or =0.031). These results strongly support the hypothesis that A. africanus possessed more apelike limb-size proportions than A. afarensis, suggesting that A. africanus either evolved from a more postcranially primitive ancestor than A. afarensis or that the more apelike limb-size proportions of A. africanus were secondarily derived from an A. afarensis-like ancestor. Among the extant taxa, limb-size proportions correspond with observed levels of forelimb- and hindlimb-dominated positional behaviors. In conjunction with detailed anatomical features linked to arboreality, these results suggest that arboreal posture and locomotion may have been more important components of the A. africanus behavioral repertoire relative to that of A. afarensis.     

A new hominid parietal from Bodo, Middle Awash Valley, Ethiopia

A piece of left parietal of a Middle Pleistocene hominid, recovered from the Upper Bodo Sand Unit, in the Middle Awash, Ethiopia, is described anatomically and compared to Middle Pleistocene hominids and modern Homo sapiens. It bears several primitive features and has important implications for the original Bodo skull, found at the same stratigraphic level in the same area. The new fossil skull represents a different individual from the original Bodo skull.     

Acheulean Industries of the Early and Middle Pleistocene,Middle Awash,Ethiopia

The Middle Awash region of Ethiopia contains a rich record of Acheulean occupation spanning from Early Pleistocene times through much of the Middle Pleistocene. Here we will present an overview of some of the major reported features of the Acheulean archaeological record of the Middle Awash (Clark et al., 1994; de Heinzelin et al., 2000) and compare and contrast earlier and later biface technological patterns in this important study area. As an overall pattern, later Acheulean bifaces, here tend to differ from earlier ones in the following characteristics: later biface forms tend to be smaller, more ovate, wider relative to length, thinner (both relative to length and width and in absolute terms), more symmetrical, more heavily flaked, show greater use of soft hammer flaking and Kombewa technique, be straighter-edged or less sinuous, and often exhibit a remarkably high degree of standardization at a given site. These technological changes over perhaps half a million years (between approximately 1.0 and 0.5 million years ago) accompany the transition from Homo erectus to Homo heidelbergensis in this region. The later technological patterns thus correlate with the emergence of larger-brained, more intelligent hominids that exhibit greater technological finesse and also appear to develop and maintain stronger rules and traditions pertaining to their technological behaviors. It is likely that, relative to earlier hominids, these later hominid forms (which would evolve into early anatomically modern humans or Homo sapiens) had richer communicative abilities and cultural complexity, which we believe to be manifested in the technological finesse and standardization of their material culture.     

Metatarsophalangeal joints of Australopithecus afarensis

Metatarsophalangeal joints from African pongids, modern humans, and Australopithecus afarensis are compared to investigate the anatomical and mechanical changes that accompanied the transition to terrestrial bipedality. Features analyzed include the shape and orientation of the metatarsal heads, excursion of the metatarsophalangeal joints, and orientation of the basal articular surface of the proximal phalanges. These features unequivocally segregate quadrupedal pongids and bipedal hominids and demonstrate a clear adaptation to terrestrial bipedality in the Hadar pedal skeleton.     

Hominid cranial remains from upper Pleistocene deposits at Aduma, Middle Awash, Ethiopia

The Upper Pleistocene localities of Aduma and Bouri have yielded hominid fossils and extensive Middle Stone Age (MSA) archaeological assemblages. The vertebrate fossils recovered include parts of four hominid crania from Aduma and a complete right parietal from Bouri. Archaeological associations and radiometric techniques suggest an Upper Pleistocene age for these hominids. The more complete cranium from Aduma (ADU-VP-1/3) comprises most of the parietals, the occipital, and part of the frontal. This cranium is compared to late Middle and Upper Pleistocene hominid crania from Africa and the Middle East. The Aduma cranium shows a mosaic of cranial features shared with "premodern" and anatomically modern Homo sapiens. However, the posterior and lateral cranial dimensions, and most of its anatomy, are centered among modern humans and resemble specimens from Omo, Skhul, and Qafzeh. As a result, the Aduma and Bouri Upper Pleistocene hominids are assigned to anatomically modern Homo sapiens.     

Fossil Cercopithecidae from the Middle Pleistocene Dawaitoli Formation, Middle Awash Valley, Afar region, Ethiopia

A series of fossil cercopithecoids has been recovered from the Dawaitoli Formation, in the Middle Awash study area, Ethiopia. The Dawaitoli Fm. has been dated to approximately 600 Ka. This series includes several partial crania of Theropithecus oswaldi leakeyi as well as other cranial and postcranial fragments. This material is some of the most complete to date and adds considerably to what is known of this widespread and abundant taxon. There are also two isolated papionin molars here allocated to Papio cf. hamadryas based largely on size grounds and similarity to contemporary material from other sites in the Afar Depression. Cercopithecoids are rare in the Dawaitoli Fm., and among them only large papionins are known. This is unique among known Pleistocene sites in the Afar region, but is most similar to the Daka Member of the Bouri Formation. It is quite different from the other known Middle Pleistocene sites in the Afar: Asbole and the geologically younger Andalee where cercopithecoids are abundant and colobines predominate.     

The locomotor anatomy of Australopithecus afarensis

The postcranial skeleton of Australopithecus afarensis from the Hadar Formation, Ethiopia, and the footprints from the Laetoli Beds of northern Tanzania, are analyzed with the goal of determining (1) the extent to which this ancient hominid practiced forms of locomotion other than terrestrial bipedality, and (2) whether or not the terrestrial bipedalism of A. afarensis was notably different from that of modern humans. It is demonstrated that A. afarensis possessed anatomic characteristics that indicate a significant adaptation for movement in the trees. Other structural features point to a mode of terrestrial bipedality that involved less extension at the hip and knee than occurs in modern humans, and only limited transfer of weight onto the medial part of the ball of the foot, but such conclusions remain more tentative than that asserting substantive arboreality. A comparison of the specimens representing smaller individuals, presumably female, to those of larger individuals, presumably male, suggests sexual differences in locomotor behavior linked to marked size dimorphism. The males were probably less arboreal and engaged more frequently in terrestrial bipedalism. In our opinion, A. afarensis from Hadar is very close to what can be called a "missing link." We speculate that earlier representatives of the A. afarensis lineage will present not a combination of arboreal and bipedal traits, but rather the anatomy of a generalized ape.     

Australopithecus afarensis and the single species hypothesis

Ferguson (1989) has recently argued that the variability seen in the fossils assigned toA. afarensis is far more than expected for a single hominid species, and therefore proposes they represent multiple taxa. In particular, he utilizes data on variation in dental metrics and in premolar morphology in support of this hypothesis. A re-evaluation of these data finds the above conclusion to be unwarranted. Variation in dental metrics providesno basis for separating this sample into multiple taxa, regardless of the analog that is used (i.e. modern primate species or fossil hominid species). Additionally, data on P₃ morphology indicate that thepattern of variation seen in the Laetoli/Hadar sample is comparable to the sexual variation seenwithin a single hominoid species. Overall, the balance of the evidence at present indicates that the fossils from Laetoli and Hadar represent a single hominid species,A. afarensis.     

Molar microwear textures and the diets of Australopithecus anamensis and Australopithecus afarensis

Many researchers have suggested that Australopithecus anamensis and Australopithecus afarensis were among the earliest hominins to have diets that included hard, brittle items. Here we examine dental microwear textures of these hominins for evidence of this. The molars of three Au. anamensis and 19 Au. afarensis specimens examined preserve unobscured antemortem microwear. Microwear textures of these individuals closely resemble those of Paranthropus boisei, having lower complexity values than Australopithecus africanus and especially Paranthropus robustus. The microwear texture complexity values for Au. anamensis and Au. afarensis are similar to those of the grass-eating Theropithecus gelada and folivorous Alouatta palliata and Trachypithecus cristatus. This implies that these Au. anamensis and Au. afarensis individuals did not have diets dominated by hard, brittle foods shortly before their deaths. On the other hand, microwear texture anisotropy values for these taxa are lower on average than those of Theropithecus, Alouatta or Trachypithecus. This suggests that the fossil taxa did not have diets dominated by tough foods either, or if they did that directions of tooth–tooth movement were less constrained than in higher cusped and sharper crested extant primate grass eaters and folivores.     

Evolution of P3 morphology in Australopithecus afarensis

The Australopithecus afarensis dental sample exhibits a wide range of variation, which is most notable in the morphology of the lower third premolar (P3). P3 morphology in the A. afarensis sample ranges from the primitive sectorial extreme in AL 128-23 to the derived, bicuspid (molarized) extreme in AL 333w-1. In this paper, the degree and patterning of variation of the 20 known A. afarensis P3s are examined and the evolutionary implications are discussed. Initially, a series of dental and mandibular metric criteria are evaluated to determine whether this sample may be analyzed as a single species. From the metrics, it is clear that the single species hypothesis cannot be rejected. Next, a series of morphological criteria is devised to measure P3 molarization. Taken as a whole, the A. afarensis P3 sample displays more variation than a sample of modern hominoids (Pan troglodytes) and shows a slight trend toward increased molarization through time. When separated by sex, the A. afarensis sample still displays greater variation than the chimpanzee sample; however, only the male A. afarensis specimens show a trend toward increased molarization. Additionally, the male A. afarensis P3s are more molarized than the female, a pattern that is seen as well (though less markedly) in the chimpanzee sample. The trend toward increased molarization over time indicates selection for grinding in A. afarensis. The sexual differences parallel those seen in the postcrania (cf. Stern and Susman: Am. J. Phys. Anthropol. 60:279-318, 1983), as the females tend to retain the primitive condition, while the males display the derived morphology. Consequently, a model of sexual differences in niche exploitation, with the females exploiting a more arboreal environment, would seem to be supported by both the dental and postcranial evidence.     

The capitate of Australopithecus afarensis and A. africanus

The capitates of Australopithecus afarensis (AL 288-lw and AL 333–40) and A. africanus (TM 1526) have the identical combination of modern pongid, modern hominid, and unique characteristics. These traits include the combination of a length that is proximodistally shortened (Homo sapiens-like), a facet for the second metacarpal that is distolaterally facing (unique), the reduced styloid process on the third metacarpal (pongidlike), a dorsally placed trapezoid facet (pongidlike), mediolaterally constricted metacarpal III facet (pongidlike), a prominent palmar beak (pongidlike), a single elongated facet for the second metacarpal (H. sapiens-like), a waisted neck (pongidlike), and a reduced amount of “cupping” in the third metacarpal facet (H. sapiens-like). In overall shape the bones are more like H. sapiens than other extant hominids, although they are uniquely different. The two A. afarensis capitates provide no evidence that there are two postcranial morphotypes at Hadar. Available evidence shows that A. afarensis and A. africanus are strikingly similar postcranially. The morphological differences between the capitate of Australopithecus and H. sapiens may relate to the retention of climbing ability and an absence of certain grip capabilities in these early hominids.     

Metatarsophalangeal joint function and positional behavior in Australopithecus afarensis

Recent discussions of the pedal morphology of Australopithecus afarensis have led to conflicting interpretations of australopithecine locomotor behavior. We report the results of a study using computer aided design (CAD) software that provides a quantitative assessment of the functional morphology of australopithecine metatarsophalangeal joints. The sample includes A. afarensis, Homo sapiens, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus. Angular measurements of the articular surfaces relative to the long axes of the metatarsals and phalanges were taken to determine whether the articular surfaces are plantarly or dorsally oriented. Humans have the most dorsally oriented articular surfaces of the proximal pedal phalanges. This trait appears to be functionally associated with dorsiflexion during bipedal stride. Pongo has the most plantarly oriented articular surfaces of the proximal pedal phalanges, probably reflecting an emphasis on plantarflexion in arboreal positional behaviors, while the African hominoids are intermediate between Pongo and Homo for this characteristic. A. afarensis falls midway between the African apes and humans. Results from an analysis of metatarsal heads are inconclusive with regard to the functional morphology of A. afarensis. Overall, the results are consistent with other evidence indicating that A. afarensis was a capable climber. © 1994 Wiley-Liss, Inc.     

Size and shape variation in Australopithecus afarensis proximal femora

The degree of size and shape variation in the A. afarensis fossil sample has been interpreted in a variety of ways. Size variation has been described as exceeding that of extant hominoids, similar to that of strongly sexually dimorphic hominoids, and best matched to modern humans. The degree of shape variation has been characterized both as great and negligible. Recent fieldwork has increased the proximal femoral sample, providing new data with which to examine variation. The proximal femur of A. afarensis is analyzed in a comparative framework in order to gauge the magnitude of size and shape variation in this element.Seven of the best-preserved A. afarensis proximal femora contribute to the analysis (A.L. 128-1, A.L. 152-2, A.L. 211-1, A.L. 288-1ap, A.L. 333-3, A.L. 333-123, A.L. 827-1). Comparative samples from Pan, Pongo, Gorilla, and Homo provide context for interpreting variation among the fossils. The coefficient of variation (CV) of linear measurements is used to estimate size variation. Bootstrap resampling of CVs from extant hominoids provides distributions for comparison to A. afarensis CVs. Ratios of linear measurements provide scale-free shape variables that are used in pairwise comparisons. The Euclidean distance between pairs of A. afarensis are compared to the Euclidean distances between extant hominoid pairs.As found in some earlier analyses, size variation in A. afarensis is accommodated best in gorillas and orangutans. The magnitude of difference in shape between A. afarensis pairs is exceeded by most taxa, indicating that shape variation is not extreme. These general findings are contradicted by a few instances of excessive size and shape variation. These are uncharacteristic results and could point to temporal bias, although other alternatives are explored. The signal from the proximal femur is that size variation in A. afarensis is like that of the strongly sexually dimorphic apes, and shape variation is well within the range of most hominoids irrespective of their degree of size dimorphism.     

Dental topography and diets of Australopithecus afarensis and early Homo

Diet is key to understanding the paleoecology of early hominins. We know little about the diets of these fossil taxa, however, in part because of a limited fossil record, and in part because of limitations in methods available to infer their feeding adaptations. This paper applies a new method, dental topographic analysis, to the inference of diet from fossil hominin teeth. This approach uses laser scanning to generate digital 3D models of teeth and geographic information systems software to measure surface attributes, such as slope and occlusal relief. Because it does not rely on specific landmarks that change with wear, dental topographic analysis allows measurement and comparison of variably worn teeth, greatly increasing sample sizes compared with techniques that require unworn teeth. This study involved comparison of occlusal slope and relief of the lower second molars of Australopithecus afarensis (n=15) and early Homo (n=8) with those of Gorilla gorilla gorilla (n=47) and Pan troglodytes troglodytes (n=54). Results indicate that while all groups show reduced slope and relief in progressively more worn specimens, there are consistent differences at given wear stages among the taxa. Early Homo shows steeper slopes and more relief than chimpanzees, whereas A. afarensis shows less slope and relief than any of the other groups. The differences between the two hominin taxa are on the same order as those between the extant apes, suggesting similar degrees of difference in diet. Because these chimpanzees and gorillas differ mostly in fallback foods where they are sympatric, results suggest that the early hominins may likewise have differed mostly in fallback foods, with A. afarensis emphasizing harder, more brittle foods, and early Homo relying on tougher, more elastic foods.     

The cranial base of Australopithecus afarensis: new insights from the female skull

Cranial base morphology differs among hominoids in ways that are usually attributed to some combination of an enlarged brain, retracted face and upright locomotion in humans. The human foramen magnum is anteriorly inclined and, with the occipital condyles, is forwardly located on a broad, short and flexed basicranium; the petrous elements are coronally rotated; the glenoid region is topographically complex; the nuchal lines are low; and the nuchal plane is horizontal. Australopithecus afarensis (3.7–3.0 Ma) is the earliest known species of the australopith grade in which the adult cranial base can be assessed comprehensively. This region of the adult skull was known from fragments in the 1970s, but renewed fieldwork beginning in the 1990s at the Hadar site, Ethiopia (3.4–3.0 Ma), recovered two nearly complete crania and major portions of a third, each associated with a mandible. These new specimens confirm that in small-brained, bipedal Australopithecus the foramen magnum and occipital condyles were anteriorly sited, as in humans, but without the foramen''s forward inclination. In the large male A.L. 444-2 this is associated with a short basal axis, a bilateral expansion of the base, and an inferiorly rotated, flexed occipital squama—all derived characters shared by later australopiths and humans. However, in A.L. 822-1 (a female) a more primitive morphology is present: although the foramen and condyles reside anteriorly on a short base, the nuchal lines are very high, the nuchal plane is very steep, and the base is as relatively narrow centrally. A.L. 822-1 illuminates fragmentary specimens in the 1970s Hadar collection that hint at aspects of this primitive suite, suggesting that it is a common pattern in the A. afarensis hypodigm. We explore the implications of these specimens for sexual dimorphism and evolutionary scenarios of functional integration in the hominin cranial base.