SciCombinator

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Concept: Ardipithecus

290

Laetoli is a well-known palaeontological locality in northern Tanzania whose outstanding record includes the earliest hominin footprints in the world (3.66 million years old), discovered in 1978 at Site G and attributed to Australopithecus afarensis. Here, we report hominin tracks unearthed in the new Site S at Laetoli and referred to two bipedal individuals (S1 and S2) moving on the same palaeosurface and in the same direction as the three hominins documented at Site G. The stature estimates for S1 greatly exceed those previously reconstructed for Au. afarensis from both skeletal material and footprint data. In combination with a comparative reappraisal of the Site G footprints, the evidence collected here embodies very important additions to the Pliocene record of hominin behaviour and morphology. Our results are consistent with considerable body size variation and, probably, degree of sexual dimorphism within a single species of bipedal hominins as early as 3.66 million years ago.

Concepts: Human, Australopithecus afarensis, Hominina, Bipedalism, Australopithecus, Australopithecus africanus, Ardipithecus, Laetoli

270

While there is broad agreement that early hominins practiced some form of terrestrial bipedality, there is also evidence that arboreal behavior remained a part of the locomotor repertoire in some taxa, and that bipedal locomotion may not have been identical to that of modern humans. It has been difficult to evaluate such evidence, however, because of the possibility that early hominins retained primitive traits (such as relatively long upper limbs) of little contemporaneous adaptive significance. Here we examine bone structural properties of the femur and humerus in the Australopithecus afarensis A.L. 288-1 (“Lucy”, 3.2 Myr) that are known to be developmentally plastic, and compare them with other early hominins, modern humans, and modern chimpanzees. Cross-sectional images were obtained from micro-CT scans of the original specimens and used to derive section properties of the diaphyses, as well as superior and inferior cortical thicknesses of the femoral neck. A.L. 288-1 shows femoral/humeral diaphyseal strength proportions that are intermediate between those of modern humans and chimpanzees, indicating more mechanical loading of the forelimb than in modern humans, and by implication, a significant arboreal locomotor component. Several features of the proximal femur in A.L. 288-1 and other australopiths, including relative femoral head size, distribution of cortical bone in the femoral neck, and cross-sectional shape of the proximal shaft, support the inference of a bipedal gait pattern that differed slightly from that of modern humans, involving more lateral deviation of the body center of mass over the support limb, which would have entailed increased cost of terrestrial locomotion. There is also evidence consistent with increased muscular strength among australopiths in both the forelimb and hind limb, possibly reflecting metabolic trade-offs between muscle and brain development during hominin evolution. Together these findings imply significant differences in both locomotor behavior and ecology between australopiths and later Homo.

Concepts: Human, Locomotion, Australopithecus afarensis, Hominina, Hominid, Bipedalism, Australopithecus, Ardipithecus

61

The incorporation of C4 resources into hominin diet signifies increased dietary breadth within hominins and divergence from the dietary patterns of other great apes. Morphological evidence indicates that hominin diet became increasingly diverse by 4.2 million years ago but may not have included large proportions of C4 foods until 800 thousand years later, given the available isotopic evidence. Here we use carbon isotope data from early to mid Pliocene hominin and cercopithecid fossils from Woranso-Mille (central Afar, Ethiopia) to constrain the timing of this dietary change and its ecological context. We show that both hominins and some papionins expanded their diets to include C4 resources as early as 3.76 Ma. Among hominins, this dietary expansion postdates the major dentognathic morphological changes that distinguish Australopithecus from Ardipithecus, but it occurs amid a continuum of adaptations to diets of tougher, harder foods and to committed terrestrial bipedality. In contrast, carbon isotope data from cercopithecids indicate that C4-dominated diets of the earliest members of the Theropithecus oswaldi lineage preceded the dental specialization for grazing but occurred after they were fully terrestrial. The combined data indicate that the inclusion of C4 foods in hominin diet occurred as part of broader ecological changes in African primate communities.

Concepts: Human, Primate, Hominidae, Chimpanzee, Human evolution, Hominina, Hominini, Ardipithecus

22

Whether tree canopy habitats played a sustained role in the ecology of ancestral bipedal hominins is unresolved. Some argue that arboreal bipedalism was prohibitively risky for hominins whose increasingly modern anatomy prevented them from gripping branches with their feet. Balancing on two legs is indeed challenging for humans under optimal conditions let alone in forest canopy, which is physically and visually highly dynamic. Here we quantify the impact of forest canopy characteristics on postural stability in humans. Viewing a movie of swaying branches while standing on a branch-like bouncy springboard destabilised the participants as much as wearing a blindfold. However “light touch”, a sensorimotor strategy based on light fingertip support, significantly enhanced their balance and lowered their thigh muscle activity by up to 30%. This demonstrates how a light touch strategy could have been central to our ancestor’s ability to avoid falls and reduce the mechanical and metabolic cost of arboreal feeding and movement. Our results may also indicate that some adaptations in the hand that facilitated continued access to forest canopy may have complemented, rather than opposed, adaptations that facilitated precise manipulation and tool use.

Concepts: Forest ecology, Human anatomy, Canopy, Hominina, Hominid, Ardipithecus, Permaculture

11

A newly discovered partial hominin foot skeleton from eastern Africa indicates the presence of more than one hominin locomotor adaptation at the beginning of the Late Pliocene epoch. Here we show that new pedal elements, dated to about 3.4 million years ago, belong to a species that does not match the contemporaneous Australopithecus afarensis in its morphology and inferred locomotor adaptations, but instead are more similar to the earlier Ardipithecus ramidus in possessing an opposable great toe. This not only indicates the presence of more than one hominin species at the beginning of the Late Pliocene of eastern Africa, but also indicates the persistence of a species with Ar. ramidus-like locomotor adaptation into the Late Pliocene.

Concepts: Giraffe, Pliocene, Australopithecus afarensis, Hominina, Bipedalism, Australopithecus, Australopithecus africanus, Ardipithecus

10

Specimens of Australopithecus sediba from the site of Malapa, South Africa (dating from approximately 2 million years (Myr) ago) present a mix of primitive and derived traits that align the taxon with other Australopithecus species and with early Homo. Although much of the available cranial and postcranial material of Au. sediba has been described, its feeding ecology has not been investigated. Here we present results from the first extraction of plant phytoliths from dental calculus of an early hominin. We also consider stable carbon isotope and dental microwear texture data for Au. sediba in light of new palaeoenvironmental evidence. The two individuals examined consumed an almost exclusive C(3) diet that probably included harder foods, and both dicotyledons (for example, tree leaves, fruits, wood and bark) and monocotyledons (for example, grasses and sedges). Like Ardipithecus ramidus (approximately 4.4 Myr ago) and modern savanna chimpanzees, Au. sediba consumed C(3) foods in preference to widely available C(4) resources. The inferred consumption of C(3) monocotyledons, and wood or bark, increases the known variety of early hominin foods. The overall dietary pattern of these two individuals contrasts with available data for other hominins in the region and elsewhere.

Concepts: Human, Poaceae, Hominidae, Human evolution, Hominina, Australopithecus, Hominini, Ardipithecus

9

Despite discoveries of relatively complete hands from two early hominin species (Ardipithecus ramidus and Australopithecus sediba) and partial hands from another (Australopithecus afarensis), fundamental questions remain about the evolution of human-like hand anatomy and function. These questions are driven by the paucity of hand fossils in the hominin fossil record between 800,000 and 1.8 My old, a time interval well documented for the emergence and subsequent proliferation of Acheulian technology (shaped bifacial stone tools). Modern and Middle to Late Pleistocene humans share a suite of derived features in the thumb, wrist, and radial carpometacarpal joints that is noticeably absent in early hominins. Here we show that one of the most distinctive features of this suite in the Middle Pleistocene to recent human hand, the third metacarpal styloid process, was present ∼1.42 Mya in an East African hominin from Kaitio, West Turkana, Kenya. This fossil thus provides the earliest unambiguous evidence for the evolution of a key shared derived characteristic of modern human and Neandertal hand morphology and suggests that the distinctive complex of radial carpometacarpal joint features in the human hand arose early in the evolution of the genus Homo and probably in Homo erectus sensu lato.

Concepts: Human, Hand, Human evolution, Pleistocene, Hominina, Australopithecus, Homo erectus, Ardipithecus

7

Human bipedal locomotion is characterized by a habitual heel-strike (HS) plantigrade gait, yet the significance of walking foot-posture is not well understood. To date, researchers have not fully investigated the costs of non-heel-strike (NHS) walking. Therefore, we examined walking speed, walk-to-run transition speed, estimated locomotor costs (lower limb muscle volume activated during walking), impact transient (rapid increase in ground force at touchdown) and effective limb length (ELL) in subjects (n=14) who walked at self-selected speeds using HS and NHS gaits. HS walking increases ELL compared with NHS walking since the center of pressure translates anteriorly from heel touchdown to toe-off. NHS gaits led to decreased absolute walking speeds (P=0.012) and walk-to-run transition speeds (P=0.0025), and increased estimated locomotor energy costs (P<0.0001) compared with HS gaits. These differences lost significance after using the dynamic similarity hypothesis to account for the effects of foot landing posture on ELL. Thus, reduced locomotor costs and increased maximum walking speeds in HS gaits are linked to the increased ELL compared with NHS gaits. However, HS walking significantly increases impact transient values at all speeds (P<0.0001). These trade-offs may be key to understanding the functional benefits of HS walking. Given the current debate over the locomotor mechanics of early hominins and the range of foot landing postures used by nonhuman apes, we suggest the consistent use of HS gaits provides key locomotor advantages to striding bipeds and may have appeared early in hominin evolution.

Concepts: Running, Walking, Locomotion, Dinosaur, Hominina, Hominid, Bipedalism, Ardipithecus

5

The evolutionary pressures shaping humans' unique bipedal locomotion have been a focus of research since Darwin, but the origins of humans' economical walking gait and endurance running capabilities remain unclear. Here, I review the anatomical and physiological determinants of locomotor economy (e.g., limb length and posture) and endurance (e.g., muscle volume and fiber type) and investigate their development in the hominin fossil record. The earliest hominins were bipedal but retained ape-like features in the hind limb that would have limited their walking economy compared to living humans. Moreover, the evolution of bipedalism and the loss of the forelimbs in weight support and propulsion would have reduced locomotor endurance in the earliest hominins and likely restricted ranging. Australopithecus evinced longer hind limbs, extended limb posture, and a stiff midfoot, suggesting improved, human-like economy, but were likely still limited in their endurance compared to modern humans. The appearance of skeletal traits related to endurance (e.g., larger limb joints, spring-like plantar arch) in Homo was somewhat mosaic, with the full endurance suite apparent only ∼1 million years ago. The development of endurance capabilities in Homo appears to parallel the evolutionary increase in brain size, cognitive sophistication, and metabolic rate.

Concepts: Human, Evolution, Locomotion, Human evolution, Hominina, Hominid, Hominini, Ardipithecus

5

The early Pliocene African hominoid Ardipithecus ramidus was diagnosed as a having a unique phylogenetic relationship with the Australopithecus + Homo clade based on nonhoning canine teeth, a foreshortened cranial base, and postcranial characters related to facultative bipedality. However, pedal and pelvic traits indicating substantial arboreality have raised arguments that this taxon may instead be an example of parallel evolution of human-like traits among apes around the time of the chimpanzee-human split. Here we investigated the basicranial morphology of Ar. ramidus for additional clues to its phylogenetic position with reference to African apes, humans, and Australopithecus. Besides a relatively anterior foramen magnum, humans differ from apes in the lateral shift of the carotid foramina, mediolateral abbreviation of the lateral tympanic, and a shortened, trapezoidal basioccipital element. These traits reflect a relative broadening of the central basicranium, a derived condition associated with changes in tympanic shape and the extent of its contact with the petrous. Ar. ramidus shares with Australopithecus each of these human-like modifications. We used the preserved morphology of ARA-VP 1/500 to estimate the missing basicranial length, drawing on consistent proportional relationships in apes and humans. Ar. ramidus is confirmed to have a relatively short basicranium, as in Australopithecus and Homo. Reorganization of the central cranial base is among the earliest morphological markers of the Ardipithecus + Australopithecus + Homo clade.

Concepts: Hominidae, Hominina, Ardipithecus