Australopithecus sediba

Australopithecus sediba, extinct primate species that inhabited southern Africa beginning about 1.98 million years ago and that shares several morphological characteristics in common with the hominin genus Homo. The first specimens were found and identified by American-born South African paleoanthropologist Lee Berger in 2008 at Malapa Cave system in the Cradle of Humankind World Heritage site in northeastern South Africa. This discovery was viewed by scientists as a potential turning point in paleoanthropology, because the well-preserved remains of various structures (including major portions of the pelvis, vertebrae, foot, leg, hand, arm, and skull) revealed a form unique among known hominin species and appeared to be intermediate in terms of evolutionary development between the relatively primitive Australopithecus and the more-advanced Homo. The species takes its name from a word in the Sesotho language meaning “fountain” or “wellspring.”

The fossil evidence

On August 15, 2008, the first Australopithecus sediba remains, a fossilized jawbone and collarbone, were found outside Malapa Cave by Berger’s nine-year-old son, Matthew. The remains belonged to a juvenile male hominin that was labeled Malapa Hominin 1 (MH1) but was more commonly known as “Karabo,” a nickname, meaning “the answer,” given by the Malapa people of the region. Lee Berger identified the mix of primitive and modern characteristics in one of the specimen’s canine teeth before finding additional remains of MH1 inside the cave. He later discovered the partial skeleton of an adult female, labeled MH2, that possessed similar features. The hand and wrist of MH2 were the most complete of any extinct hominin known.

Dating

Although the fossil remains of MH1 and MH2 were too old to be dated directly, their ages were estimated from dating the uranium-rich flowstone matrix that surrounded them. This was done using a uranium-lead dating technique, as well as a process called paleomagnetic dating, which determines the age of a rock by comparing the magnetic orientation of its iron with that of surrounding rocks. The age of the fossils was estimated at approximately 1.977 million years old.

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Body structure

Using the size of the remains to estimate height, MH1 was thought to have stood approximately 1.3 metres (about 4.25 feet) tall. Although MH1 was estimated to be only about 10–13 years old at the time of his death, researchers had enough information to determine that the level of sexual dimorphism (the differences in appearance between males and females of the same species) between MH1 and MH2 was equivalent to that of modern humans. They also documented a number of similarities in facial structure and dentition between A. sediba and A. africanus, remains of which found in southern Africa show that it lived there between about 3.3 million and 2.0 million years ago. This evidence suggested that A. sediba could be a direct descendant of A. africanus.

Additional studies of the pelvis, hand, foot, and skull revealed that A. sediba not only shared a number of characteristics with apes and modern humans but also possessed a number of unique features.

Skull and dentition

Endocranial casts (three-dimensional projections) of the skull were made using a synchrotron scanner that estimated the size and shape of the brain of MH1. The examination revealed that the volume of the specimen’s head and braincase were similar to those of other australopiths—that is, smaller than those of genus Homo. However, the researchers also reported that a shift toward a more humanlike frontal lobe had taken place in the species. Some features, such as the shape of the braincase behind the eyes and the position of the olfactory bulbs, were similar to those of modern humans. These findings appeared to counter the idea that hominin brains began to increase in size during the changeover from Australopithecus to Homo between approximately 2 million and 1.5 million years ago. In addition, researchers noted that the teeth of both specimens were smaller than those of other australopiths, a development thought to suggest a major change in diet or social behaviour.

Pelvis

Many paleoanthropologists maintain that the evolution of the pelvis in hominins was driven in part by the increase in the size of the head; the pelvis needed to accommodate the birth of larger-brained offspring. The reconstruction and analysis of the pelvises of both specimens revealed that they had already developed certain modern features before brain size began to increase. Such features as more vertically oriented and crescent-shaped iliac blades, a characteristic of Homo, were present in A. sediba. The pelvis also displayed australopithecine characteristics, such as a large biacetabular diameter (the cup-shaped cavity that holds the top of the femur). In addition, the overall shape of the pelvises in A. sediba was short, curved, and broad (like those of Homo) rather than flat and broad (like those of the other australopiths). In light of this evidence, some paleoanthropologists argue that the evolution of the pelvis in the human lineage was driven not by the increase in brain size but by the need to facilitate bipedal locomotion.

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Ankle and foot

The foot and ankle associated with MH2 displayed a collection of primitive and derived features that suggest that the species was both bipedal and arboreal. The specimen possessed an apelike heel (calcaneous), which did not appear to be built to withstand the stresses of extended running, and a large medial malleolus (bony promontory on the inside of the ankle), which suggested that the specimen was adapted for life among the trees. Perhaps the most striking humanlike features present on the remains of the specimen’s lower leg were structures that suggest the presence of a foot arch and robust Achilles tendon.

Wrist and hand

A. sediba also displayed humanlike characteristics in its hand structure. Apes and earlier australopiths possessed long, robust fingers and reduced thumbs that facilitated quadrupedal locomotion as well as their movement between tree branches. The hand of MH2, in contrast, displayed shorter fingers and an elongated thumb. Some researchers maintained that these features would have enabled A. sediba to climb trees efficiently while also allowing the hand to manipulate small objects. This evidence caused some paleoanthropologists to speculate that such changes in the hand, combined with the reorganization of the brain, may have given the species the dexterity necessary to make and use simple tools, perhaps even stone tools. No tools of any kind, however, were found at the site.

Vertebrae

The structure of fossil lumbar vertebrae associated with MH2 reinforces the notion that A. sediba was both bipedal and arboreal. Lumbar lordosis (that is, the natural curve of the lower back) in the specimen, which suggests lower-back mobility consistent with bipedalism, is similar to that found in modern human beings. Other structures on the vertebrae suggest that A. sediba possessed strong trunk muscles, which enabled the species to hang from branches and move about in the trees.

Evolutionary implications

The mosaic of humanlike and apelike characteristics displayed by A. sediba was unlike any other known hominin. These features, combined with the completeness of the remains, especially that of the hand, called into question some of the established ideas in human evolution, such as the evolution of the human pelvis, as well as the stability of the hominin family tree. Until the discovery of MH1 and MH2, most paleoanthropologists maintained that H. habilis (a sub-Saharan hominin that lived between 2 million and 1.5 million years ago) and H. rudolfensis (a hominin whose remains were discovered at Koobi Fora in Kenya and dated to between 2.5 million and 1.5 million years ago) were the most likely direct ancestors of H. erectus, the earliest undisputed precursor to modern humans (H. sapiens). Some scientists argued that the specimens classified as H. rudolfensis may simply represent examples of sexual dimorphism in H. habilis.

Some paleoanthropologists, however, claimed that A. sediba may be a better candidate as a direct ancestor of H. erectus. They noted that there are more shared features between those two species than between H. erectus and H. habilis or H. rudolfensis and that the hand of A. sediba appears to be more advanced and more suited to early toolmaking than the hand of H. habilis, considered one of the earliest toolmaking species. Furthermore, dating has determined that A. sediba is older; the oldest known remains of H. habilis have been dated to approximately 1.85 million years ago. In contrast, other paleoanthropologists hypothesized that A. sediba may have been part of A. africanus or existed concurrently with the true direct ancestors of H. erectus.