It dominated science headlines when the news was first released. The discovery of the remains of a new species of ancient hominin (human ancestor) revealed a candidate that sported a mosaic of features both ape-like and human -- an unprecedented 2-million-year-old hybrid called Australopithecus sediba (Au. sediba). First stumbled upon in 2008 by Professor Lee Berger of the University of the Witwatersrand and his then 9-year-old son Matthew at the fossil bearing site of Malapa in South Africa, the finds, consisting of remarkably complete skeletal remains as well as other well-preserved fauna and flora, instantly became the subject of perhaps the most intense and thoroughly studied hominin fossils ever documented.
The team, led by Berger and composed of South African and international scientists from the Evolutionary Studies Institute (ESI) at the University of the Witwatersrand (Wits) and 16 other global institutions (totalling more than 100 researchers from around the world), recently examined the anatomy of Au. sediba based on its skeletons catalogued as "MH1" (a juvenile skeleton) and "MH2" (an adult female skeleton), as well as an adult isolated tibia catalogued as "MH4". The scientists have now completed what amounts to the second 2-year installment of a series of studies begun approximately four years ago, and the efforts of their research have resulted in additional new surprises. Dispersed among six separate studies, these latest examinations have determined in essence how the hominin walked, chewed and moved:
(1) DENTAL MORPHOLOHY AND THE PHYLOGENETIC "PLACE" OF AUSTRALOPETHICUS SEDIBA
The first study, led by Professor Joel Irish from the Research Centre for Evolutionary Anthropology and Palaeoecology at Liverpool John Moores University in the United Kingdom, examined dental traits in the subject fossils.
In this study, Irish, Debbie Guatelli-Steinberg of the Ohio State University and their colleagues examined the teeth from sediba and compared them to eight other African hominin species, which include modern humans from Africa, and extinct species of Homo, Australopithecus, and Paranthropus. In all, the researchers examined more than 340 fossils and 4,571 recent specimens. They also examined teeth from 44 gorillas for comparison.
Based on the examination, Irish and his colleagues suggest that the species is distinct from east African australopiths (a more ape-like hominin genus found in east Africa, such as at Olduvai Gorge and the Afar region of Ethiopia), but is close to Au. africanus (an ape-like hominin genus found in southern Africa), thus forming a southern African australopith "clade" (a group consisting of an ancestor and all its descendants).
The latter, in turn, shares a number of derived states or physical characteristics with a clade comprising four fossil samples of the genus Homo (the genus that includes modern humans and species closely related to them ). This surprising result has significant implications for our present understanding of hominin phylogeny (the evolution of the species), and alludes to the possibility that Au. sediba, and perhaps Au. africanus are not descendant from the Au. afarensis lineage, represented most prominently by the famous "Lucy" skeleton discovered by Donald Johanson in Ethiopia in 1974, as has been widely hypothesized.
"Our research on teeth can't definitively settle if either sediba or africanus is more closely related to humans than the other species," Guatelli-Steinberg said. "But our findings do suggest that both are closely related to each other and are more closely related to humans than afarensis."
Irish noted that even though the results of this study were surprising and were bound to be viewed as controversial given the long held hypotheses relating to the origins of the genus Homo (the genus more directly ancestral to humans), he would have come to the same conclusion.
"The extreme age and rarity of these fossils naturally draws enhanced interest in and scrutiny of any new findings", he says. "Based on the evidence, I would have come up with the same conclusions whether the samples were three million or 30 years old."
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Malapa Hominin 1 (MH1) left, Lucy (AL 288-1 (Centre), and Malapa Hominin 2 (MH2) right. Image compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand.
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(2) MANDIBULAR REMAINS
Professor Darryl de Ruiter of Texas A&M and the Evolutionary Studies Institute at the University of the Witwatersrand, and his colleagues, examined new mandibular (lower jaw) material from the MH2 individual.
The study concludes that the mandibular remains share similarities with other australopiths, but can be differentiated from the southern African ape-man Au. africanus in both size and shape, as well as in their growth trajectory.
"These results add further support to the claim that Au. sediba is taxonomically distinct from the temporally – and geographically – close species Au. africanus. Where the Au. sediba mandibles differ from those of Au. africanus, they appear most similar to representatives of early Homo," says De Ruiter.
(3) THE UPPER LIMB
Professor Steven Churchill of Duke University and the Evolutionary Studies Institute at University of the Witwatersrand and his examined new, remarkably well preserved upper limb elements of Au. sediba.
They announce the first complete (or nearly complete) and undistorted humerus, radius, ulna, scapula, clavicle and manubrium (a frontal chest bone) yet described from the early hominin record, all associated with one individual.
The researchers noted that with the exception of the hand skeleton (which exhibits a suite of derived features that may signal enhanced manipulative capabilities relative to earlier australopiths), the upper limbs of the Malapa hominins are largely primitive in their morphology (physical characteristics). Au. sediba thus shares with other australopiths an upper limb that was well-suited for arboreal or other forms of climbing and possibly suspension, though perhaps more so than has been previously suggested for any other member of the australopith genus.
Churchill adds that "it is possible that the climbing features in the skeleton of Australopithecus sediba and other australopiths are functionally unimportant primitive traits retained from a more arboreal ancestor. Even so, it is curious that these features persist unchanged for several million years, only to abruptly disappear with the emergence of the genus Homo."
(4) MORPHOLOGY OF THE THORAX
Dr. Peter Schmid and colleagues at the Evolutionary Studies Institute at the University of the Witwatersrand and the University of Zurich studied the remains of the rib cage of Au. sediba.
Their findings reveal a narrow upper thorax, much like that of the large-bodied apes, and unlike the broad, cylindrical chest characteristic of humans. In conjunction with the largely complete remains of the shoulder girdle, Schmid notes that "the morphological picture that emerges is one of a conical thorax with a high shoulder joint that produces in Au. sediba an ape-like "shrugged" shoulder appearance, and thus a configuration that is perhaps uniquely australopith, and that would not have been conducive to human-like swinging of the arms during bipedal striding and running".
The research however shows that the less well-preserved elements of the lower rib cage suggest some degree of human-like narrowing to the lower thorax, a surprising feature that is not like that of Homo erectus or H. sapiens (modern humans). Homo erectus is an extinct species of hominin that lived from around 1.8 million years ago to around 300,000 years ago. The species is thought to have originated in Africa and spread as far as England , Georgia , India , China and Java .
(5) THE VERTEBRAL COLUMN
Dr Scott Williams of the Center for the Study of Human Origins at New York University and colleagues examined the vertebral column of Au. sediba, including the cervical, thoracic, lumbar, and sacral regions of the vertebral column.
The researchers describe a remarkably articulated lumbar vertebral region that shows a human-like curvature of the lower back. Williams notes that "the adult female is the first early hominin skeleton that preserves an intact terminal thoracic region and this provides critical information on the transition in inter-vertebral joints, and, by inference, mobility of the lower back".
The study also demonstrates that Au. sediba had the same number of lumbar vertebrae as modern humans, but possessed a functionally longer and more flexible lower back. In addition, morphological indicators of strong lumbar curvature suggest that Au. sediba was more similar to the Nariokotome Homo erectus skeleton than to the australopiths.
(6) THE LOWER LIMB AND THE MECHANICS OF WALKING
Dr. Jeremy DeSilva and colleagues at Boston University and the Evolutionary Studies Institute at the University of the Witwatersrand examined the lower limb anatomy of Au. sediba. i
"The female Australopithecus sediba preserves a heel, ankle, knee, hip and lower back- all of the ingredients necessary to reconstruct how she walked with remarkable precision. Even the famous Lucy skeleton only preserves two of these five (ankle and hip)", says DeSilva.
In isolation, the anatomies of the heel, mid-foot, knee, hip, and back are unique and curious, but in combination, they are internally consistent for a biped walking with a hyper-pronating gait (relative to the modern human gait, feet that pronates or rotates too much, for too long at the wrong time during the gait cycle).
"The implications of this study are that multiple forms of bi-pedalism were once practiced by our early hominin ancestors," adds Berger.
“What these papers suggest is that sediba probably doesn’t come from the east African species that Lucy comes from, Australopithecus afarensis, and it may be considered the best candidate as an ancestor for the genus Homo", says project leader Berger......Such clear insight into the anatomy of an early hominin species will clearly have implications for interpreting the evolutionary processes that affected the mode and tempo of hominin evolution and the interpretation of the anatomy of less well preserved species."
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The U.W. 88-50 (MH 1) cranium. The cranium forms part of the holotype skeleton of Australopithecus sediba from the Malapa site, South Africa. This image relates to an article that appeared in the April 9, 2010, issue of Science, published by AAAS. The study, by Dr. L.R. Berger of University of the Witwatersrand in Johannesburg, South Africa, was titled, "Australopithecus sediba: A New Species of Homo-like Australopith from South Africa." Credit: Photo by Brett Eloff courtesy of Lee Berger and the University of the Witwatersrand.
OTHER FINDINGS FROM THE FIRST TWO YEARS
The Cranium Scan
The exceptionally well-preserved cranium of the MH-1 juvenile was scanned at the European Synchrotron Radiation Facility, the most powerful facility in the world for scanning fossils, under the direction of Dr. Kristian Carlson from the University of the Witwatersrand. By doing this, he and his colleagues were able to develop a precise map or image of the impressions on the interior surface of the cranium, or brain case, producing an endocast (or 3-dimensional image) of the area where the brain, long decayed into nothing, would have been.
Said Carlson, "the actual brain residing within a cranium does not fossilize. Rather, by studying the impressions on the inside of a cranium, palaeontologists have an opportunity to estimate what the surface of a brain may have looked like. By quantifying how much volume is contained within a cranium, palaeontologists can estimate the size of a brain."
The results revealed that the brain was human-like in shape, yet much smaller than brain volumes recorded in Homo species. In fact, the size was not significantly more than that of a modern chimpanzee. However, the orbito-frontal region of the brain, which is behind the eyes, showed characteristics of neural (nerve cell structure) reorganization. The researchers suggested that this indicated a “re-wiring” of the frontal lobe’s neural constitution to a pattern more human-like. This questioned the generally-accepted theory of brain enlargement during the transition from Australopithecus to Homo as the most essential consideration and supported the alternate theory that a neural reorganization in the orbitofrontal region made it possible for A. sediba to be more "human-like" with the smaller cranium. Thus, size may not matter more than brain tissue structure and organization.
The Hands and Toolmaking
Tracy Kivell of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a team of colleagues studied the more complete hand fossils of MH-2, the adult female A. sediba.
They determined that the hand exhibited a strong flexor capability, good for tree-climbing. However, it also featured a long thumb and short fingers, a clear requirement for precision gripping, or gripping that involves the more refined use of the thumb and fingers and not the palm. This suggested that A. sediba had the mechanical capability to make tools.
According to Kivell:
"The hand is one of the very special features of the human lineage, as it's very different from the hand of the apes. Apes have long fingers for grasping branches or for use in locomotion, and thus relatively short thumbs that make it very difficult for them to grasp like a human.
Au. sediba has, in contrast, a more human-like hand that has shortened fingers and a very long thumb. Although at the same time, it appears to have possessed very powerful muscles for grasping. Our team interpreted this as a hand capable of tool manufacture and use, but still in use for climbing and certainly capable of human-like precision grip."
These findings pair A. sediba with Homo habilis, the famous "handy man" Australopithecine discovered by Louis and Mary Leakey in the 1960's, as a hominin or early human candidate capable of making tools. It also suggests that there were more than one ancient hominin species that may have produced tools, either during different time periods, overlapping time periods, or concurrently in time.
To date, no stone tools have been found in association with the Malapa cave fossils. But full-scale excavations have not yet taken place, and it is anticipated that much more data will be forthcoming as excavations are carried out in the near future.
The Pelvis
Dr. Job Kibii of the University of the Witwatersrand and associates examined the partial pelvis of MH-2 and found that it also exhibited features that combined primitive elements more akin to that of earlier hominins and apes along with elements more characteristic of humans. They observed that the size of the joint that connects the sacrum with the vertebral column and the length of the front portion of the pelvis is like that of earlier hominins and apes, but the overall shape of the pelvis is short and broad, creating a bowl shape like that of humans, with an s-shape along the top of the blades, another human characteristic. Indeed, simply placing the reconstructed pelvis next to that of an ape and an earlier hominin is very telling. The pelvis clearly appears more human-like than ape-like.
Says Kibii, “It is surprising to discover such an advanced pelvis in such a small-brained creature because of previous ideas as to the origin of the shape of the human pelvis”
The generally-accepted theory is that broader pelvises evolved, at least in part, in response to the enlarging brains of hominins, on the assumption that the more human-like pelvis shape more easily accommodated the larger-brained hominin infants in childbirth. The new findings turn this on its head, suggesting that there was another, perhaps more important evolutionary reason why the pelvis changed:
The change in the pelvic morphology accommodates a more bipedal, or erect, gate – a salient hallmark of being human. Says Steven Churchill of Duke University, one of the co-authors of the paper detailing the pelvis study, "What's cool about sediba is their pelvises are already different from other australopiths [early hominins], and yet they're still small-brained… It's hard to imagine that there's no change in locomotion behind all this."
The Feet and Ankles
Dr. Bernhard Zipfel of the University of the Witwatersrand and colleagues examined the feet and ankle fossils of MH-1 and MH-2, finding them to consist of a mix of both primitive and modern characteristics unique to A. sediba as a species. As the ankle fossils represented a very rare and opportune find in that they constituted one of the most complete hominin ankles ever found, and in an articulated position or association, it was feasible to perform a study and reach reasonably sound conclusions about the characteristics of the fossils and their implications. The ankle joint and foot bones were constructed much like a human's, with some evidence for a human-like arch and a well-defined Achilles tendon. It was also clear that the distal tibia or leg bone had to contact the anklebone perpendicular to the vertical shaft of the leg bone. These are all requirements for habitual bipedal locomotion, or upright walking.
However, the heel and shin bone exhibited more ape-like qualities, suggesting that A. sediba was also a tree-climber, much like its ape cousins.
The overall analysis suggested that A. sediba likely practiced a unique form of upright walking, not quite like that of humans, along with some degree of tree climbing.
To date, investigations at the Malapa site have seen the discovery of more than 300 early human ancestor remains, including parts of skeletons still encased in rock. Included in the recent discoveries are a new species of fox, named by the team as Vulpes skinneri just three months ago.
The six papers detailing the most recent studies are published in the 12 April 2013 issue of the journal Science , a publication of the American Association for the Advancement of Science.
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Composite reconstruction of Au. sediba based on recovered material from MH1, MH2 and MH4 and based upon the research presented in the accompanying manuscripts. As all individuals recovered to date are approximately the same size, size correction was not necessary. Femoral length was established by digitally measuring a complete femur of MH1 still encased in rock. For comparison, small-bodied female modern H. sapiens on left, Male Pan troglodytes on right. Credit: Photo by Lee Berger, courtesy of the University of the Witwatersrand
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Lee Berger with a partial skeleton of Australopithecus sediba. Photo by Brett Eloff, courtesy Lee Berger and the University of the Witwatersrand.
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Cover Photo, Top Left: The unusually complete skeletons of Australopithecus sediba discovered at the two million year old site of Malapa have already yielded a world of data, allowing more complete skeletal restorations than are usually possible with fossil hominins. Credit: Painting by John Gurche, courtesy of Lee Berger and the University of the Witwatersrand
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