Australopithecus sediba represents an extinct hominin species that inhabited what is now South Africa between 1.78 and 1.98 million years ago. This species belongs to the australopithecine genus, a group of early human relatives.
The Malapa Cave Discovery
The discovery of Australopithecus sediba occurred in August 2008 within the Malapa Nature Reserve, part of the Cradle of Humankind World Heritage site in South Africa. Paleoanthropologist Lee Berger and his nine-year-old son, Matthew Berger, were exploring the area when Matthew stumbled upon a fossilized clavicle. This initial find quickly led to the excavation of more remains, including a partial juvenile male skeleton (Malapa Hominin 1, or MH1) and a partial adult female skeleton (Malapa Hominin 2, or MH2).
The completeness of these fossil skeletons offered a detailed view of an early hominin’s skeletal structure near the time when the genus Homo evolved. The name sediba, meaning “fountain” or “wellspring” in the local Sesotho language, reflects the significance of this discovery as a source of new insights into human origins. It is thought that these individuals, along with an infant and other remains found later, may have accidentally fallen into a natural death trap within the cave, where their bodies were quickly buried and preserved.
A Mosaic of Ape and Human Traits
Australopithecus sediba presents a combination of features, showcasing mosaic evolution, where different parts of the body evolve at different rates. Its anatomy reveals a blend of primitive, ape-like characteristics alongside more advanced, human-like traits.
Among its primitive characteristics, A. sediba possessed a relatively small braincase, with a brain volume estimated between 350 and 440 cubic centimeters, comparable to other australopithecines. Its upper limbs, including long arms and primitive shoulders, suggest adaptations for climbing trees and suspending itself, indicating arboreal locomotion. The upper portion of its thorax was narrow and ape-like, differing from the broader chest seen in humans.
Conversely, A. sediba displayed several derived, human-like traits. Its hands featured a long thumb and relatively short fingers, indicating a strong precision grip, potentially useful for manipulating objects or early stone tool production. The teeth were smaller, particularly the premolars and molars, compared to other australopithecines. Facial features showed less prognathism (jutting out of the face) and a less pronounced brow ridge, resembling those found in Homo. The pelvis of A. sediba exhibited modern features, such as more vertically oriented and crescent-shaped iliac blades, similar to Homo, despite retaining some australopithecine characteristics. Its lower limbs also suggested a form of upright walking, though with a distinct “hyper-pronating gait” where the foot turned inward, focusing weight on the outer edge.
Diet and Environment
Reconstructing the lifestyle of Australopithecus sediba involves analyzing scientific evidence from its fossilized remains, particularly its teeth. Studies of dental plaque, known as calculus, allowed researchers to extract phytoliths, which are microscopic silica bodies from plants. These phytoliths offered direct evidence of the plants consumed by A. sediba almost two million years ago. The analysis revealed a diet rich in C3 plants, such as leaves, fruits, wood, and bark.
This C3-dominated diet, confirmed by stable carbon isotope analysis of tooth enamel, contrasts with many other early hominins in the region who consumed more C4 resources, like tropical grasses. The presence of bark and woody tissues in its diet was an unusual finding for early hominins. Dental microwear analysis, which examines microscopic scratches and pits on tooth surfaces, also indicated that A. sediba consumed harder food items. This dietary pattern suggests a preference for forest products, even in an environment where C4 grasses were abundant.
The environment inhabited by A. sediba was likely a mosaic of woodlands and savannas. Its anatomical features, such as relatively long arms for climbing and adaptations for bipedal walking, would have been advantageous in such a mixed habitat, allowing it to exploit both arboreal and terrestrial resources. This combination of climbing ability and a unique walking gait suggests a versatile form of locomotion suited to its varied environment.
Its Place in the Human Family Tree
The discovery of Australopithecus sediba led to significant scientific discussion regarding its position within the human family tree. The central question revolves around whether it represents a direct ancestor of our own genus, Homo, or if it was a specialized, late-surviving side branch of the Australopithecus lineage. Its combination of primitive and derived traits makes its evolutionary placement complex.
Arguments supporting A. sediba as a potential ancestor to Homo often highlight its human-like features, such as the precision grip of its hands, its evolved pelvis, and its smaller teeth and jaw. These features suggest an evolutionary trajectory towards traits observed in early Homo species. Some researchers initially proposed it as a progenitor to Homo ergaster or Homo erectus, given its date of approximately 1.98 million years ago.
However, counterarguments challenge this direct ancestral link. One significant point is the timing of A. sediba’s existence; older Homo fossils, such as a 2.8-million-year-old jawbone from Ethiopia, have been found, suggesting Homo may have emerged earlier than A. sediba. This chronological overlap means A. sediba could be a contemporary species rather than a direct ancestor. Its unique walking gait, described as a “hyper-pronating” stride, also differentiates it from locomotion patterns associated with the Homo lineage, suggesting an independent evolutionary path for upright walking. The debate continues, with some scientists suggesting A. sediba might be a sister species to the ancestor of Homo or a derived form of Australopithecus africanus that developed human-like traits independently.