More flesh on Sediba
By: From Wits News, 12 April 2013
How Australopithecus sediba moved and chewed
A team of South African and international scientists from the Evolutionary Studies Institute (ESI) at the University of the Witwatersrand (Wits) and 15 other global institutions, have published six papers in the prestigious journal Science, on the 2-million-year-old Australopithecus sediba (Au. sediba) fossils. The fossil remains were discovered at the site of Malapa, in the Cradle of Humankind, in 2008 and provide an “unprecedented insight into the anatomy and phylogenetic position of an early human ancestor”, says Prof. Lee Berger, the lead author and project leader.
Malapa has yielded perhaps the richest assemblage of early hominin fossils on the continent of Africa and the latest reports describe the dentition, spine, upper and lower limbs and the thorax of three individuals: the holotype and paratype skeletons, commonly referred to as MH1 and MH2, and the adult isolated tibia referred to as MH4. The questions and answers below, from the Wits website, describe the research and implications of the finds.
Frequently Asked Questions
Section A – The Fossils
Prof. Lee Berger, Reader in Human Evolution at the University of the Witwatersrand, Johannesburg, and Sediba project leader, responds to some common questions pertaining to Au. sediba.
Since its discovery in August 2008, the site of Malapa has yielded well over 220 bones of early hominins representing more than five individuals, including the remains of babies, juveniles and adults. The evidence published in Science is based on two individuals from the site – MH1 and MH2. The fossils from the site date to 1.977 to 1.98 million years in age.
How were the individuals preserved?
The site where the fossils were discovered is technically the infill of a de-roofed cave that was about 30 to 50 metres underground just under two million years ago. The individuals appear to have fallen, along with other animals, into a deep cave, landing up on the floor for a few days or weeks. The bodies were then washed into an underground lake or pool probably pushed there by a large rainstorm. They did not travel far, maybe a few metres, where they were solidified into the rock, as if thrown into quick setting concrete. The rock they are preserved in is called calcified clastic sediment. Over the past 2 million years the land has eroded to expose the fossil bearing sediments.
Did they die at the same time, or was it a catastrophe?
The hominin skeletons were found with the bones either in partial articulation or in close anatomical association, which suggests that the bodies were only partially decomposed at the time of deposition in the lower chamber. This further suggests that they died very close in time to each other, either at the same time, or hours, days or weeks apart. Other animals have been found with them - equally complete - including sabre-toothed cats, hyenas, antelopes, mice, birds and even snails. There is also plant material that has been found.
Is there organic preservation like plant remains or skin?
The preservation at Malapa is excellent and there are certainly organic remains preserved like plant remains. There are some indications that even the soft tissues of animals are preserved, including possibly skin. This material is presently under study by a worldwide based team of experts, who are attempting to prove or disprove the existence of such important material, and to develop methods to study specimens that have never been found before in the early hominin record.
How old are the children you have found?
The juvenile MH-1 is around 10 – 13 years old in human developmental terms. He was probably a bit younger in actual age (perhaps as young as eight or nine) as he is likely to have matured faster than humans. The age estimate is based on modern human standards by which the eruption stages of the teeth are evaluated and the degree of development of the growth centres of the bones. Studies are presently underway to attempt to precisely determine the age of this child at death. The other young hominins found at the site are still under study and no exact, or even good estimates of their age have yet been made.
How old is the female skeleton?
Based on the extreme wear of her teeth, MH-2 is probably at least in her late twenties or early thirties but it is very difficult to determine the age of an adult at death because her bones would have completed growing.
Did she have children?
It is sometimes the case that females develop small pits on the back side of the pubic bone when they deliver a baby (caused by stress on the ligaments crossing the front of the pelvis). These pits are known as “scars of parturition,” and MH-2 may have one such scar. However, these pits can also be produced by other factors, and thus they are not always indicative that a female has given birth. It is likely that a female Australopith of her age would have had children.
If she did have children, would the child be large-brained or small-brained?
The estimated adult brain size based on the MH-1 juvenile is approximately 440 cm3, which is slightly below the average for Au. afarensis (Lucy’s species) and Au. africanus (Mrs. Ples’ species). This suggests that, like australopiths, sediba gave birth to small-brained babies (based on the relationship of adult to neonatal brain size in chimpanzees, australopiths are thought to have given birth to babies with brains on the order of ca. 153 – 201 cm3).
How do you know the child is a male?
There are features of the face that help us determine that the child is a male. The muscles of the child are larger than that of the MH-2 skeleton, even though it is a child. We can now directly compare the male and female pelvises.
Are they related to each other?
We are not sure at this stage, but given the very short time of accumulation and the varying age of the individuals, it is likely that they are related. Detailed studies are being designed to address this important question.
Section B – The Dentition
Prof.’s Joel Irish and Darryl de Ruiter provide an overview of the papers pertaining to an analysis of the dentition of Au. sediba.
What did your research into the mandibles of Australopithecus sediba tell you?
Several of our colleagues were not convinced that Australopithecus sediba was different enough from the better known Australopithecus africanus to warrant naming a new species. They thought that perhaps we were only looking at a smaller variant of Australopithecus africanus. Our study showed that the mandibles of Australopithecus sediba differ significantly in both size and shape from those of Australopithecus africanus, providing strong support for our assertion that Australopithecus sediba is something new and unique. We also showed that the ontogenetic growth trajectory of Australopithecus sediba was unique, meaning that their growth pattern was different from Australopithecus africanus.
What is morphometrics?
Morphometrics is a mathematical technique for looking at several different measurements at once to decide how different objects, in this case mandibles, are from each other. It allows us to investigate shape in complex, three-dimensional mandibles from a variety of different measurements, and decide if they are different enough from each other to be called separate species.
What is an ontogenetic growth trajectory?
This refers to the way individual animals change as they grow from juveniles to adults. In this study, we were fortunate to have a young individual that we could compare to a fully adult individual. What we saw was that the differences between the juvenile and the adult, and the magnitude of change in those differences, was unlike what we see in any other hominin species. In other words, the way that young Australopithecus sediba individuals grew into adults differed from other hominins, supporting their status as a new species.
Do your studies prove that Australopithecus sediba was ancestral to the genus Homo?
Not directly, though it does support that argument. What we can demonstrate is that Australopithecus sediba differs significantly from Australopithecus africanus in several features, and where they do differ, Australopithecus sediba appears most similar to early Homo.
What are the broader impacts of your research? What does it tell us beyond Australopithecus sediba?
As is seen elsewhere in the skull and skeleton of Australopithecus sediba, the mandibles share some characters with australopiths, and some with early Homo. This marks Australopithecus sediba as a transitional form, which is exactly what would be predicted on Darwin’s theory of evolution by natural selection. The changes taking place in the skull and skeleton of these individuals are exactly those changes that ultimately lead to us humans. Understanding the ecological, environmental, and evolutionary pressures that brought about these changes will help us to understand not only where we come from, but what makes us so unique in this world.
How does the growth of Australopithecus sediba’s mandible compare to other hominids?
The magnitude of growth (the amount of growth) represented by mandibular shape change from juvenile to adult in Australopithecus sediba differs from Australopithecus africanus (Mrs Ples’s species),Homo erectus, humans and chimpanzees, with Au. sediba displaying a greater amount of shape change than any of these other species. The pattern of growth (the way in which growth occurs) in the Au. sediba mandible is most similar to Homo erectus which may indicate a closer connection between Au. sediba and Homo.
How do we know that the growth pattern of Australopithecus sediba’s mandible is unique?
When the growth patterns of Homo erectus and Au. africanus are applied to Au. sediba to hypothetically “grow down” the adult Au. sediba to a juvenile, we get a completely different juvenile form compared to the actual Au. sediba juvenile, MH1. This means that the pattern of mandibular growth in Au. sediba to get from juvenile to adult is unlike any of the other two fossil species in this study.
Why compare the teeth of Australopithecus sediba to gorillas?
As one of humans' closest living relatives among the primates (along with chimpanzees), we can use comparison to gorilla teeth as a way to help understand the fossil's evolutionary relationship to other hominins.
Why might teeth be preserved in the fossil record for millions of years?
They are very hard, fossilise easier than the rest of the skeleton, and so outlast bones
What is 'dental morphology'? Give an example.
The various highly heritable discrete traits that are visible on the crowns and roots, such as Carabelli’s cusp, incisor shoveling, etc.
Why did the authors of the papers compare Australopithecus sediba to other species of human ancestors?
To determine how the species is related to these hominins, including members of the genus Homo (to which modern humans also belong).
Which human ancestors and close relatives do the authors find that Australopitehcus sediba is most similar to in its dental morphology?
Au. Africanus
What relationship do the authors think Australopithecus sediba has to people alive today?
Possible candidate ancestor, though other scenarios are possible.
What is the Arizona State University Dental Anthropology System?
Standardised system for recording expression of non-metric traits in modern and, in this case, fossil hominin teeth.
What unique molar states do some East African australopiths and Paranthropus sp. share, that are not found in the other African hominin samples?
Trace-small cuspule for cusp 5 UM1, six-cusped LM1, absent-pit protostylid LM1, absent cusp 7, and 6-cusped LM2.
What is the difference between a dendrogram and a cladogram?
The two methods of illustration relate to different methods for estimating species’ relationships. The former involves simply comparisons of all traits (a.k.a. phenetics), and those samples that have the most in common are plotted nearer to one another. The latter method (cladistics) is based only on those traits that are shared-derived or recent in derivation (i.e., not primitive or ancestral).
Has the phylogenetic position of Au. sediba be explicitly determined? Why/why not?
No. The new dental data help to further define among-hominin relationships, but more data are required for a more exact determination.
Why are definitive results so difficult to obtain?
All fossil hominin samples, especially Au. sediba, are at present very small. As such they: 1) may not be entirely representative of overall species, and as a result 2) make quantitative analyses an statistical inference difficult.
Section C – The Upper Limb
Prof. Steve Churchill provides an overview of the paper pertaining to an analysis of the upper limb of Au. sediba.
What upper limb remains has been recovered from Australopithecus sediba?
The upper limb skeleton is remarkably well-represented in Australopithecus sediba. One individual, the adult female MH2, preserves a nearly complete right upper limb. This includes a complete clavicle (collar bone), a nearly complete scapula (shoulder blade), and a complete humerus (upper arm bone) and radius and ulna (forearm bones).
The MH2 skeleton also preserves 20 of the 27 bones of the wrist and hand (these wrist and hand bones were described in a 2011 paper). MH2’s left arm is less complete, and is represented only by a partial clavicle, three fragments of the scapula, a fragment of the humerus, and six bones of the wrist and hand. The upper limb is also fairly well-represented in the juvenile male skeleton MH1, although not nearly to the extent seen in MH2. From the right side of MH1 we have recovered half
of the clavicle, a nearly complete humerus (lacking only two portions that, because of MH1’s young age at death, had not yet fused onto the main part of the bone), about half of the ulna, a fragment of the radius, and one hand bone. As with MH2, MH1’s left upper limb is not well-represented, preserving only a fragment of the humerus.
What can we learn from these upper limb remains?
There is a long-standing debate in human evolutionary studies about the extent to which australopiths were relying on trees as a part of their behavioral ecology. In Australopithecus sediba we see upper limbs that were relatively long and that had elongated forearms, a shoulder joint that was somewhat up-turned, an obliquely oriented scapular spine, and pronounced attachment markings for some of the upper limb muscles. In combination these features reflect an upper limb that was well adapted to the arm movements involved in climbing and suspension (hanging below branches). Furthermore, Australopithecus sediba displays a few details of upper limb anatomy, such as curved finger bones, that in apes do not appear developmentally until youngsters start climbing (and would not develop at all if no climbing occurred). These features all suggest that Australopithecus sediba regularly climbed trees.
Does this mean that all australopiths were tree climbers?
No. It is quite possible that there was variation in the amount of tree climbing between the different species of australopith. The evidence for or against climbing must be examined independently for each species.
Did Australopithecus sediba climb like a chimpanzee?
Probably not. Chimpanzees have very long arms for their body size, elongated forearms, and short, powerful lower limbs. Chimpanzees also have a divergent big toe on their foot that improves their ability to grasp branches and tree trunks with their feet. The arms of Australopithecus sediba were somewhat shorter and the legs somewhat longer. Furthermore, Australopithecus sediba had a foot that was adapted to bipedal locomotion, and that did not have a grasping, divergent big toe. Thus while climbing and suspensory behaviors in Australopithecus sediba may have superficially looked like those used by chimpanzees, the underlying kinematics were likely quite different.
Australopithecus sedibawas a terrestrial biped, so why would it still climb trees?
The evidence from the vertebral column, pelvis, and lower limb does show that Australopithecus sediba was a terrestrial biped, as were all other australopiths. However, it is possible that Australopithecus sediba still regularly climbed trees to forage for fruits or other food items that could only be found there. It is also possible that Australopithecus sediba climbed trees to escape predators, or slept in trees to reduce the risk of nighttime predation from terrestrial carnivores.
Was Australopithecus sediba a good thrower?
The shoulder joint of Australopithecus sediba faced forwards, which would have limited their ability to cock the arm for an over-hand throw. Thus if they did throw objects (to scare off predators, for example), they probably did so under-handed, as do chimpanzees.
Section D – The Spine
Dr Scott Williams responds to some common questions pertaining to Au. sediba.
Was the spinal column of Australopithecus sediba similar or different to our own?
Both. In some ways, like regional and total numbers of vertebrae, it was just like that of modern humans; but in other ways - its flexibility and mobility, for example - it was different. Australopithecus sediba had a highly flexible lower back compared to modern humans. This is likely related to locomotor and postural differences between our two species.
What parts of the spine have been recovered from Australopithecus sediba?
All major regions of the bony spine, or vertebral column, are represented in the Australopithecus sediba fossil assemblage. The skeleton of the young male (MH1) preserves twelve vertebrae, including parts of four cervical (neck) vertebrae, six thoracic (upper back) vertebrae, and two lumbar (lower back) vertebrae. The adult female (MH2) preserves eleven vertebrae as well as a partially complete sacrum (the set of fused vertebrae comprising the back of the pelvis). These include two cervical vertebrae, seven thoracic vertebrae, and the last two lumbar vertebrae. While MH1 and MH2 do not represent the most complete vertebral columns known for early fossil hominins, they are among a small number of skeletons that preserve multiple vertebrae from multiple vertebral regions.
What can the spine tell us about upright walking in Australopithecus sediba?
Human-like bipedal walking involves balancing the torso in upright posture over two legs. This unusual way of getting around leaves its mark on the anatomy of the legs, hips, and feet, and also on the vertebral column. An important feature of the vertebral column related to upright walking is an S-shaped curve of the spine, including a marked curvature of the lower back called lumbar lordosis. The lumbar vertebrae of sediba are wedged and help create this curve. In addition, there are other bony indicators of postural lordosis.
Was the vertebral column of Australopithecus sediba similar to or different from our own?
Both. Our findings suggest that sediba probably had the same regional and total numbers of vertebrae as we do, but had more vertebrae that are shaped to allow forward and backward bending of the spine. To us, this indicates that Australopithecus sediba had a more mobile and flexible back than the average modern human. Additionally, the degree of wedging observed in the last lumbar vertebra of MH2 is extremely high and nearly outside the range of modern human variation. This is consistent with findings by our colleagues working on the lower limb (hip, legs, and feet) and suggests that Australopithecus sediba might have carried itself and walked differently than modern humans normally do.
How do we know that all Australopithecus sediba individuals had these characteristics?
We cannot be certain that the flexible and lordotic lower back of MH2 is characteristic of the species in general; we need to find more fossil vertebrae from other individuals to be sure of that! However, MH2 does not appear to be pathological and the material preserved for MH1 does not contradict the findings for MH2. Again, more discoveries and research are needed.
Did Australopithecus sediba have a tail?
The fifth region of the vertebral column is the caudal (in animals with an external tail) or coccygeal (in tailless animals) region. Although vertebrae belonging to this region of Australopithecus sediba were not recovered, we can tell from the shape of the bottom end of MH2’s sacrum that an external tail would not have been present in this species. Indeed, a tail would be very surprising considering humans and our closest relatives, the living apes, all lack tails. Instead, like us, sediba most certainly possessed a coccyx – a segment of fused vertebrae representing the vestigial tail.
Section E – The Thorax
Dr Peter Schmid, lead author on the paper pertaining to the thorax, responds to some common questions pertaining to Au. sediba.
How much of the rib cage of Australopithecus sediba is preserved?
Ribs are very fragile and in general do not preserve well in the fossil record. In the case of the Malapa skeletons, the ribs of the upper part of the rib cage are better represented than those of the lower thorax. While most of the rib remains are fragmentary, we have recovered four complete (or nearly complete) ribs, including the right first rib of the juvenile male individual MH1, and the left and right first ribs and right fourth rib of the adult female MH2. Ribs one through seven or eight are represented by at least fragments from one of the individuals. A single fragment likely representing the ninth rib (or possibly rib 10) was also recovered from MH2.
The upper rib cage of Australopithecus sediba is said to be “ape-like,” what does this mean exactly?
Humans and apes both have rib cages that are somewhat flattened from front to back, but in the large-bodied apes (orangutans, chimpanzees, and gorillas) the side-to-side width of the rib cage also narrows as one moves up the rib cage, while in humans it does not. This gives apes a conical shape to their thorax (which some people refer to as an “inverted funnel”) when seen from the front, whereas humans have a more uniform, cylindrical rib cage. The preserved rib remains of Australopithecus sediba reveal a thorax that narrowed towards the top, as seen in the larger apes.
What is the lower thorax like in Australopithecus sediba?
The lower thorax is poorly represented in the fossils from Malapa. In apes, the lower thorax continues to flare laterally (towards the side) as one moves from top to bottom, and the lowermost ribs are the longest and straightest of the series. This flaring, combined with a narrow upper thorax, contributes to the conical shape seen in the ape rib cage. In addition, the very wide lower rib cage of apes matches in width their very wide hip blades. In humans the lower ribs become smaller, and do not flare laterally beyond what is seen in the upper ribs, such that the lower rib cage matches in width the narrower, curved hip blades. A fragmentary lower rib from MH2 (probably representing the right ninth rib, but possibly the right 10th rib) has a shaft that is smaller than the ribs above and that is curved and twisted like the lower ribs in humans. Given that the shape of the lower rib cage closely corresponds with the shape of the pelvis in apes and humans, the narrower, more curved hip blades of Australopithecus sediba also suggest that the lower rib cage of this species was somewhat more human-like. Thus the overall thoracic morphology of Australopithecus sediba combined a narrow, ape-like upper rib cage with a narrow, human-like lower rib cage.
What is the significance of an ape-like, conical upper thorax?
The narrow upper rib cage of apes appears to be important in allowing a wide range of motion of the scapula (shoulder blade) needed for climbing and suspension (hanging below branches). Thus the conical shape to Australopithecus sediba’s thorax could be seen as consistent with the evidence from the upper limb that suggests that this species still engaged in a significant amount of arboreal locomotion. It is also important to note that a conical rib cage makes it difficult to engage in arm swinging during bipedal walking and running. In humans, our trunk naturally twists towards the opposite side as our swinging leg moves forward with each step (or running stride). While we could use the muscles on the sides of our abdomen (the abdominal obliques) to counter this rotation, it is energetically more efficient to counter swing our arms (that is, to swing the opposite side arm forward, which produces a countering twist on the upper thorax). Because humans have a thorax that is broad at the top, the arms are positioned well out towards the sides, and better able to swing freely without striking the sides of the body. Thus the conical rib cage in Australopithecus sediba suggests that they were not as proficient at the kind of striding bipedalism and running that characterizes modern human locomotion.
Could Australopithecus sediba run?
Most all animals, no matter what their form of locomotion, have some sort of running gait that can be used when greater speed is needed (as when capturing prey or avoiding predators), and Australopithecus sediba was likely no exception. The conical thorax of Australopithecus sediba would not have prevented them from running, but it does suggest that they were not capable of long-distance, endurance running (in which arm swinging is an important energy saving device).
Section F – The Lower Limb
Dr Jeremy deSilva, lead author on the paper pertaining to the lower limb, responds to some common questions pertaining to Au. sediba.
What parts of the leg have been recovered from Australopithecus sediba?
The skeleton of the young male (MH1) preserves a hip joint, and fragmentary parts of a shin bone, fibula and a few foot bones. The adult female (MH2) is more complete- she preserves a hip joint, knee, ankle, and heel. In fact, she is the first skeleton of an early human (hominin) that preserves all of these parts in a single skeleton, allowing us to reconstruct how all of these joints worked together in a single individual. Not even Lucy preserves all of these parts. There is a shin bone from another adult individual (MH4) as well.
How do we know that Australopithecus sediba walked on two legs?
Human leg bones differ from the bones of other primates in ways that are related to upright walking. For instance, humans are knock-kneed, meaning our knees are directly under our bodies and almost touch. But, we are not born with knees like this. Instead, the end of our femur begins to angle as we begin to walk on two legs as toddlers. Because this angulation only happens in individuals who walk on two legs, finding it in a fossil is strong evidence for upright walking. Australopithecus sediba possesses this angled femur. Other anatomies that we can see in the back, pelvis, hip joint, and ankle of Australopithecus sediba are found only in upright walking humans as well.
Does that mean Australopithecus sediba walked like a human?
No. The heel of Australopithecus sediba is chimpanzee-like, meaning that this species did not heel-strike during upright walking the way that most modern humans do. In fact, the heel of Australopithecus sediba is even more ape-like than the heel of earlier hominins attributed to Australopithecus afarensis (Lucy’s species).
So, how did Australopithecus sediba walk?
We propose that Australopithecus sediba was a “hyper-pronator.” The heel bone indicates that Australopithecus sediba contacted the ground on the outside of an inverted, or twisted in, foot. Contacting the ground on the outside edge of a twisted in foot causes the foot to rapidly and excessively rotate so that the inside (big-toe side) of the foot is driven into the ground (pronation). This begins a chain reaction in which the shin bone rotates inwards, the femur rotates inwards, and the torso pitches forward a bit.
OK. But, that is a weird way to walk. How do we know they walked this way?
There are some (often pathological) humans today who hyperpronate. Hyperpronators often have problems with their feet, knees, hips, and back. Interestingly, the Australopithecus sediba skeleton possesses anatomical solutions to the very problems faced by modern hyperpronating humans. We regard this as strong evidence that this species was adapted to walk in this manner. For example, because of the internal rotation of the shin bone and femur in hyperpronators, humans who walk this way have a higher likelihood of dislocating their knee-cap. The Australopithecus sediba knee possesses an exceptionally high retaining wall of bone (called the lateral lip), that would prevent the knee-cap from dislocating. Humans today have this too, but not nearly as high as that found in Australopithecus sediba.
But, maybe just MH2 walked that way. Maybe she was pathological. How do we know the species moved in this manner?
The bones of the heel and foot of MH1, and the shin bone from MH4 display the same anatomies that are found in the MH2 skeleton, indicating that they all probably walked in a similar manner. However, we will need more complete skeletons from additional individuals to confirm these initial findings.
Why would they walk this way?
There is good evidence from the arm, hand, torso, and even reconstructions of the diet, that tree climbing remained an important part of the lives of this particular species of Australopithecus. We suspect that hyperpronation might be a compromise locomotion for a hominin that had features of the foot that are adaptive for both upright walking and tree climbing.
Does this mean that Lucy, Little Foot, and other Australopithecus individuals walked this way?
No. The anatomy of the heel, parts of the midfoot and ankle, and knee are far different in Australopithecus sediba than in any other species of Australopithecus. The differences are in areas that are functionally important, and the differences are quite pronounced. We regard this as evidence that Australopithecus sediba moved differently than other species of Australopithecus, and that there were different ways to be an upright walking hominin in the past.
Can I walk like a sediba?
Yes! Please try it. Do not heel strike. Instead, land on the outside of your foot in a “flatfooted” manner. The ground will push back on your foot and roll it inwards (so that the inside of your foot is driven into the ground), your shin will twist inwards, as will your femur. You may feel your upper body pitching forward over your hips. If you do this enough, your foot, knee, hip and lower back may start to hurt a bit. But, remember, you are not a sediba! The anatomy of your bones differ from Australopithecus sediba in precisely those areas that start to hurt because while you are not adapted to walk this way, but we think that they were.
Section G – The Discovery
Prof. Lee Berger, Reader in Human Evolution at the University of the Witwatersrand, Johannesburg, responds to some common questions pertaining to Au. sediba.
How was the site and the fossils discovered?
In the mid-1990s, I had conducted an expedition across southern Africa, funded by the National Geographic Society designed to map fossil sites using the then relatively new technology of GPS and to discover new sites. While the expedition discovered many new caves and fossil bearing localities (over 100 caves and four new fossil sites in the Cradle of Humankind World Heritage Site), it did not yield any major discoveries.
In early 2008, using Google Earth to spot caves in the Cradle of Humankind World Heritage Site, Berger renewed the exploration programme in the area. With the assistance of new technology available, Berger discovered, over a few months, more than 600 caves and more than three dozen new fossil sites in one of the most explored areas on the planet.
On the 1st of August 2008, while mapping with my dog Tau, I discovered the fossil site of Malapa. On the 15th of August he returned to the site with Dr Job Kibii, Tau and his then 9- year-old son, Matthew. Within minutes, Matthew had discovered the first piece of hominid, belonging to the MH-1 skeleton. Two weeks later, I discovered the remains of the adult female skeleton MH-2 and since then, the site has yielded one of the most remarkable records of early human origins of any site on the planet.
What does Australopithecus sediba mean?
Australopithecusmeans “southern ape”, after the genus of the Taung child, named by Prof. Raymond Dart, also from the University of the Witwatersrand, Johannesburg. Sediba means natural spring, fountain or wellspring in Sotho, an appropriate name for a species that might be the point from which the genus Homo arises. As the hominids were also found preserved in an ancient underground lake or spring, the name also relates to their place of discovery.
What is a hominid/hominin?
A hominid is a member of the taxonomic family that includes humans, chimpanzees, gorillas and their extinct ancestors. Hominins are members of the human branch after the human lineage split from that of chimpanzees, and thus include living humans and extinct human ancestors, such as the Australopiths. Hominins are characterised by bipedal locomotion, although this may not have been the case for the very earliest members of the group, and relatively small canine teeth. Later members of this group (those in the genus, Homo) are characterised by larger brains than those of living apes like chimpanzees, bonobos, gorillas, orangutans and gibbons.
How does this find relate to Lucy?
Australopithecus sedibais approximately a million years younger than Lucy. Some scientists feel that Lucy’s species, Au. afarensis, gave rise to Au. africanus and in these papers, the team is suggesting that Au. africanus or something earlier than Au. africanus, gave rise to Au. sediba. There is some evidence, based upon the both primitive and advanced foot and ankle of sediba, that it does not descend from lucy’s species or africanus, but comes from some as yet unidentified lineage of early hominin. Additionally, the very advanced nature of sediba’s hand suggests it may not give rise to Homo habilis, which although later in time, has a more primitive hand, even though Homo habilis is some 200,000 to 300,000 years younger than sediba.
Has the new species named last year been accepted by the scientific community?
There is broad acceptance of the species Au. sediba among scientists as something previously unknown to science. Very little debate has occurred around whether these bones represent a new species. The debate has centered, largely, around whether the species should be placed in the genus Homo.
So why is this not the genus Homo?
The fossils have an overall body plan that is like that of other Australopiths – they have small brains, relatively small bodies and long and seemingly powerful arms. They do, however, have some features in the skull, hand and pelvis that are found in later definitive members of the genus Homo but not in other Australopiths. However, given the small brains and Australopith-like upper limbs, and features of the foot and ankle, the team has felt that keeping this species in the genus Australopithecus was the conservative thing to do. Nevertheless, sediba is turning out to be one of the most intriguing hominins yet discovered, and it certainly shows a mosaic of features shared by both earlier and later hominins.
What about Homo habilis?
Our study indicates that Australopithecus sediba may be a better ancestor of Homo erectus and it may certainly help to clear up some of this “muddle in the middle”.
Even after a year, why is there still rock attached to the child’s skull?
Due to the fragility of the base of the cranium of the specimen and to preserve part of the adhering matrix for future research as technologies improve, the team has decided
to leave the specimen partially in rock. The team has been able to visualise this hidden part using some of the most sophisticated scanning technology available.
Will there be more discoveries from Malapa?
Malapa is already one of the richest early hominin sites ever discovered but excavations have not commenced yet. When they do, later this year, we expect to make even more remarkable finds at the site.
How Australopithecus sediba moved and chewed
A team of South African and international scientists from the Evolutionary Studies Institute (ESI) at the University of the Witwatersrand (Wits) and 15 other global institutions, have published six papers in the prestigious journal Science, on the 2-million-year-old Australopithecus sediba (Au. sediba) fossils. The fossil remains were discovered at the site of Malapa, in the Cradle of Humankind, in 2008 and provide an “unprecedented insight into the anatomy and phylogenetic position of an early human ancestor”, says Prof. Lee Berger, the lead author and project leader.
Malapa has yielded perhaps the richest assemblage of early hominin fossils on the continent of Africa and the latest reports describe the dentition, spine, upper and lower limbs and the thorax of three individuals: the holotype and paratype skeletons, commonly referred to as MH1 and MH2, and the adult isolated tibia referred to as MH4. The questions and answers below, from the Wits website, describe the research and implications of the finds.
Frequently Asked Questions
Section A – The Fossils
Prof. Lee Berger, Reader in Human Evolution at the University of the Witwatersrand, Johannesburg, and Sediba project leader, responds to some common questions pertaining to Au. sediba.
Since its discovery in August 2008, the site of Malapa has yielded well over 220 bones of early hominins representing more than five individuals, including the remains of babies, juveniles and adults. The evidence published in Science is based on two individuals from the site – MH1 and MH2. The fossils from the site date to 1.977 to 1.98 million years in age.
How were the individuals preserved?
The site where the fossils were discovered is technically the infill of a de-roofed cave that was about 30 to 50 metres underground just under two million years ago. The individuals appear to have fallen, along with other animals, into a deep cave, landing up on the floor for a few days or weeks. The bodies were then washed into an underground lake or pool probably pushed there by a large rainstorm. They did not travel far, maybe a few metres, where they were solidified into the rock, as if thrown into quick setting concrete. The rock they are preserved in is called calcified clastic sediment. Over the past 2 million years the land has eroded to expose the fossil bearing sediments.
Did they die at the same time, or was it a catastrophe?
The hominin skeletons were found with the bones either in partial articulation or in close anatomical association, which suggests that the bodies were only partially decomposed at the time of deposition in the lower chamber. This further suggests that they died very close in time to each other, either at the same time, or hours, days or weeks apart. Other animals have been found with them - equally complete - including sabre-toothed cats, hyenas, antelopes, mice, birds and even snails. There is also plant material that has been found.
Is there organic preservation like plant remains or skin?
The preservation at Malapa is excellent and there are certainly organic remains preserved like plant remains. There are some indications that even the soft tissues of animals are preserved, including possibly skin. This material is presently under study by a worldwide based team of experts, who are attempting to prove or disprove the existence of such important material, and to develop methods to study specimens that have never been found before in the early hominin record.
How old are the children you have found?
The juvenile MH-1 is around 10 – 13 years old in human developmental terms. He was probably a bit younger in actual age (perhaps as young as eight or nine) as he is likely to have matured faster than humans. The age estimate is based on modern human standards by which the eruption stages of the teeth are evaluated and the degree of development of the growth centres of the bones. Studies are presently underway to attempt to precisely determine the age of this child at death. The other young hominins found at the site are still under study and no exact, or even good estimates of their age have yet been made.
How old is the female skeleton?
Based on the extreme wear of her teeth, MH-2 is probably at least in her late twenties or early thirties but it is very difficult to determine the age of an adult at death because her bones would have completed growing.
Did she have children?
It is sometimes the case that females develop small pits on the back side of the pubic bone when they deliver a baby (caused by stress on the ligaments crossing the front of the pelvis). These pits are known as “scars of parturition,” and MH-2 may have one such scar. However, these pits can also be produced by other factors, and thus they are not always indicative that a female has given birth. It is likely that a female Australopith of her age would have had children.
If she did have children, would the child be large-brained or small-brained?
The estimated adult brain size based on the MH-1 juvenile is approximately 440 cm3, which is slightly below the average for Au. afarensis (Lucy’s species) and Au. africanus (Mrs. Ples’ species). This suggests that, like australopiths, sediba gave birth to small-brained babies (based on the relationship of adult to neonatal brain size in chimpanzees, australopiths are thought to have given birth to babies with brains on the order of ca. 153 – 201 cm3).
How do you know the child is a male?
There are features of the face that help us determine that the child is a male. The muscles of the child are larger than that of the MH-2 skeleton, even though it is a child. We can now directly compare the male and female pelvises.
Are they related to each other?
We are not sure at this stage, but given the very short time of accumulation and the varying age of the individuals, it is likely that they are related. Detailed studies are being designed to address this important question.
Section B – The Dentition
Prof.’s Joel Irish and Darryl de Ruiter provide an overview of the papers pertaining to an analysis of the dentition of Au. sediba.
What did your research into the mandibles of Australopithecus sediba tell you?
Several of our colleagues were not convinced that Australopithecus sediba was different enough from the better known Australopithecus africanus to warrant naming a new species. They thought that perhaps we were only looking at a smaller variant of Australopithecus africanus. Our study showed that the mandibles of Australopithecus sediba differ significantly in both size and shape from those of Australopithecus africanus, providing strong support for our assertion that Australopithecus sediba is something new and unique. We also showed that the ontogenetic growth trajectory of Australopithecus sediba was unique, meaning that their growth pattern was different from Australopithecus africanus.
What is morphometrics?
Morphometrics is a mathematical technique for looking at several different measurements at once to decide how different objects, in this case mandibles, are from each other. It allows us to investigate shape in complex, three-dimensional mandibles from a variety of different measurements, and decide if they are different enough from each other to be called separate species.
What is an ontogenetic growth trajectory?
This refers to the way individual animals change as they grow from juveniles to adults. In this study, we were fortunate to have a young individual that we could compare to a fully adult individual. What we saw was that the differences between the juvenile and the adult, and the magnitude of change in those differences, was unlike what we see in any other hominin species. In other words, the way that young Australopithecus sediba individuals grew into adults differed from other hominins, supporting their status as a new species.
Do your studies prove that Australopithecus sediba was ancestral to the genus Homo?
Not directly, though it does support that argument. What we can demonstrate is that Australopithecus sediba differs significantly from Australopithecus africanus in several features, and where they do differ, Australopithecus sediba appears most similar to early Homo.
What are the broader impacts of your research? What does it tell us beyond Australopithecus sediba?
As is seen elsewhere in the skull and skeleton of Australopithecus sediba, the mandibles share some characters with australopiths, and some with early Homo. This marks Australopithecus sediba as a transitional form, which is exactly what would be predicted on Darwin’s theory of evolution by natural selection. The changes taking place in the skull and skeleton of these individuals are exactly those changes that ultimately lead to us humans. Understanding the ecological, environmental, and evolutionary pressures that brought about these changes will help us to understand not only where we come from, but what makes us so unique in this world.
How does the growth of Australopithecus sediba’s mandible compare to other hominids?
The magnitude of growth (the amount of growth) represented by mandibular shape change from juvenile to adult in Australopithecus sediba differs from Australopithecus africanus (Mrs Ples’s species),Homo erectus, humans and chimpanzees, with Au. sediba displaying a greater amount of shape change than any of these other species. The pattern of growth (the way in which growth occurs) in the Au. sediba mandible is most similar to Homo erectus which may indicate a closer connection between Au. sediba and Homo.
How do we know that the growth pattern of Australopithecus sediba’s mandible is unique?
When the growth patterns of Homo erectus and Au. africanus are applied to Au. sediba to hypothetically “grow down” the adult Au. sediba to a juvenile, we get a completely different juvenile form compared to the actual Au. sediba juvenile, MH1. This means that the pattern of mandibular growth in Au. sediba to get from juvenile to adult is unlike any of the other two fossil species in this study.
Why compare the teeth of Australopithecus sediba to gorillas?
As one of humans' closest living relatives among the primates (along with chimpanzees), we can use comparison to gorilla teeth as a way to help understand the fossil's evolutionary relationship to other hominins.
Why might teeth be preserved in the fossil record for millions of years?
They are very hard, fossilise easier than the rest of the skeleton, and so outlast bones
What is 'dental morphology'? Give an example.
The various highly heritable discrete traits that are visible on the crowns and roots, such as Carabelli’s cusp, incisor shoveling, etc.
Why did the authors of the papers compare Australopithecus sediba to other species of human ancestors?
To determine how the species is related to these hominins, including members of the genus Homo (to which modern humans also belong).
Which human ancestors and close relatives do the authors find that Australopitehcus sediba is most similar to in its dental morphology?
Au. Africanus
What relationship do the authors think Australopithecus sediba has to people alive today?
Possible candidate ancestor, though other scenarios are possible.
What is the Arizona State University Dental Anthropology System?
Standardised system for recording expression of non-metric traits in modern and, in this case, fossil hominin teeth.
What unique molar states do some East African australopiths and Paranthropus sp. share, that are not found in the other African hominin samples?
Trace-small cuspule for cusp 5 UM1, six-cusped LM1, absent-pit protostylid LM1, absent cusp 7, and 6-cusped LM2.
What is the difference between a dendrogram and a cladogram?
The two methods of illustration relate to different methods for estimating species’ relationships. The former involves simply comparisons of all traits (a.k.a. phenetics), and those samples that have the most in common are plotted nearer to one another. The latter method (cladistics) is based only on those traits that are shared-derived or recent in derivation (i.e., not primitive or ancestral).
Has the phylogenetic position of Au. sediba be explicitly determined? Why/why not?
No. The new dental data help to further define among-hominin relationships, but more data are required for a more exact determination.
Why are definitive results so difficult to obtain?
All fossil hominin samples, especially Au. sediba, are at present very small. As such they: 1) may not be entirely representative of overall species, and as a result 2) make quantitative analyses an statistical inference difficult.
Section C – The Upper Limb
Prof. Steve Churchill provides an overview of the paper pertaining to an analysis of the upper limb of Au. sediba.
What upper limb remains has been recovered from Australopithecus sediba?
The upper limb skeleton is remarkably well-represented in Australopithecus sediba. One individual, the adult female MH2, preserves a nearly complete right upper limb. This includes a complete clavicle (collar bone), a nearly complete scapula (shoulder blade), and a complete humerus (upper arm bone) and radius and ulna (forearm bones).
The MH2 skeleton also preserves 20 of the 27 bones of the wrist and hand (these wrist and hand bones were described in a 2011 paper). MH2’s left arm is less complete, and is represented only by a partial clavicle, three fragments of the scapula, a fragment of the humerus, and six bones of the wrist and hand. The upper limb is also fairly well-represented in the juvenile male skeleton MH1, although not nearly to the extent seen in MH2. From the right side of MH1 we have recovered half
of the clavicle, a nearly complete humerus (lacking only two portions that, because of MH1’s young age at death, had not yet fused onto the main part of the bone), about half of the ulna, a fragment of the radius, and one hand bone. As with MH2, MH1’s left upper limb is not well-represented, preserving only a fragment of the humerus.
What can we learn from these upper limb remains?
There is a long-standing debate in human evolutionary studies about the extent to which australopiths were relying on trees as a part of their behavioral ecology. In Australopithecus sediba we see upper limbs that were relatively long and that had elongated forearms, a shoulder joint that was somewhat up-turned, an obliquely oriented scapular spine, and pronounced attachment markings for some of the upper limb muscles. In combination these features reflect an upper limb that was well adapted to the arm movements involved in climbing and suspension (hanging below branches). Furthermore, Australopithecus sediba displays a few details of upper limb anatomy, such as curved finger bones, that in apes do not appear developmentally until youngsters start climbing (and would not develop at all if no climbing occurred). These features all suggest that Australopithecus sediba regularly climbed trees.
Does this mean that all australopiths were tree climbers?
No. It is quite possible that there was variation in the amount of tree climbing between the different species of australopith. The evidence for or against climbing must be examined independently for each species.
Did Australopithecus sediba climb like a chimpanzee?
Probably not. Chimpanzees have very long arms for their body size, elongated forearms, and short, powerful lower limbs. Chimpanzees also have a divergent big toe on their foot that improves their ability to grasp branches and tree trunks with their feet. The arms of Australopithecus sediba were somewhat shorter and the legs somewhat longer. Furthermore, Australopithecus sediba had a foot that was adapted to bipedal locomotion, and that did not have a grasping, divergent big toe. Thus while climbing and suspensory behaviors in Australopithecus sediba may have superficially looked like those used by chimpanzees, the underlying kinematics were likely quite different.
Australopithecus sedibawas a terrestrial biped, so why would it still climb trees?
The evidence from the vertebral column, pelvis, and lower limb does show that Australopithecus sediba was a terrestrial biped, as were all other australopiths. However, it is possible that Australopithecus sediba still regularly climbed trees to forage for fruits or other food items that could only be found there. It is also possible that Australopithecus sediba climbed trees to escape predators, or slept in trees to reduce the risk of nighttime predation from terrestrial carnivores.
Was Australopithecus sediba a good thrower?
The shoulder joint of Australopithecus sediba faced forwards, which would have limited their ability to cock the arm for an over-hand throw. Thus if they did throw objects (to scare off predators, for example), they probably did so under-handed, as do chimpanzees.
Section D – The Spine
Dr Scott Williams responds to some common questions pertaining to Au. sediba.
Was the spinal column of Australopithecus sediba similar or different to our own?
Both. In some ways, like regional and total numbers of vertebrae, it was just like that of modern humans; but in other ways - its flexibility and mobility, for example - it was different. Australopithecus sediba had a highly flexible lower back compared to modern humans. This is likely related to locomotor and postural differences between our two species.
What parts of the spine have been recovered from Australopithecus sediba?
All major regions of the bony spine, or vertebral column, are represented in the Australopithecus sediba fossil assemblage. The skeleton of the young male (MH1) preserves twelve vertebrae, including parts of four cervical (neck) vertebrae, six thoracic (upper back) vertebrae, and two lumbar (lower back) vertebrae. The adult female (MH2) preserves eleven vertebrae as well as a partially complete sacrum (the set of fused vertebrae comprising the back of the pelvis). These include two cervical vertebrae, seven thoracic vertebrae, and the last two lumbar vertebrae. While MH1 and MH2 do not represent the most complete vertebral columns known for early fossil hominins, they are among a small number of skeletons that preserve multiple vertebrae from multiple vertebral regions.
What can the spine tell us about upright walking in Australopithecus sediba?
Human-like bipedal walking involves balancing the torso in upright posture over two legs. This unusual way of getting around leaves its mark on the anatomy of the legs, hips, and feet, and also on the vertebral column. An important feature of the vertebral column related to upright walking is an S-shaped curve of the spine, including a marked curvature of the lower back called lumbar lordosis. The lumbar vertebrae of sediba are wedged and help create this curve. In addition, there are other bony indicators of postural lordosis.
Was the vertebral column of Australopithecus sediba similar to or different from our own?
Both. Our findings suggest that sediba probably had the same regional and total numbers of vertebrae as we do, but had more vertebrae that are shaped to allow forward and backward bending of the spine. To us, this indicates that Australopithecus sediba had a more mobile and flexible back than the average modern human. Additionally, the degree of wedging observed in the last lumbar vertebra of MH2 is extremely high and nearly outside the range of modern human variation. This is consistent with findings by our colleagues working on the lower limb (hip, legs, and feet) and suggests that Australopithecus sediba might have carried itself and walked differently than modern humans normally do.
How do we know that all Australopithecus sediba individuals had these characteristics?
We cannot be certain that the flexible and lordotic lower back of MH2 is characteristic of the species in general; we need to find more fossil vertebrae from other individuals to be sure of that! However, MH2 does not appear to be pathological and the material preserved for MH1 does not contradict the findings for MH2. Again, more discoveries and research are needed.
Did Australopithecus sediba have a tail?
The fifth region of the vertebral column is the caudal (in animals with an external tail) or coccygeal (in tailless animals) region. Although vertebrae belonging to this region of Australopithecus sediba were not recovered, we can tell from the shape of the bottom end of MH2’s sacrum that an external tail would not have been present in this species. Indeed, a tail would be very surprising considering humans and our closest relatives, the living apes, all lack tails. Instead, like us, sediba most certainly possessed a coccyx – a segment of fused vertebrae representing the vestigial tail.
Section E – The Thorax
Dr Peter Schmid, lead author on the paper pertaining to the thorax, responds to some common questions pertaining to Au. sediba.
How much of the rib cage of Australopithecus sediba is preserved?
Ribs are very fragile and in general do not preserve well in the fossil record. In the case of the Malapa skeletons, the ribs of the upper part of the rib cage are better represented than those of the lower thorax. While most of the rib remains are fragmentary, we have recovered four complete (or nearly complete) ribs, including the right first rib of the juvenile male individual MH1, and the left and right first ribs and right fourth rib of the adult female MH2. Ribs one through seven or eight are represented by at least fragments from one of the individuals. A single fragment likely representing the ninth rib (or possibly rib 10) was also recovered from MH2.
The upper rib cage of Australopithecus sediba is said to be “ape-like,” what does this mean exactly?
Humans and apes both have rib cages that are somewhat flattened from front to back, but in the large-bodied apes (orangutans, chimpanzees, and gorillas) the side-to-side width of the rib cage also narrows as one moves up the rib cage, while in humans it does not. This gives apes a conical shape to their thorax (which some people refer to as an “inverted funnel”) when seen from the front, whereas humans have a more uniform, cylindrical rib cage. The preserved rib remains of Australopithecus sediba reveal a thorax that narrowed towards the top, as seen in the larger apes.
What is the lower thorax like in Australopithecus sediba?
The lower thorax is poorly represented in the fossils from Malapa. In apes, the lower thorax continues to flare laterally (towards the side) as one moves from top to bottom, and the lowermost ribs are the longest and straightest of the series. This flaring, combined with a narrow upper thorax, contributes to the conical shape seen in the ape rib cage. In addition, the very wide lower rib cage of apes matches in width their very wide hip blades. In humans the lower ribs become smaller, and do not flare laterally beyond what is seen in the upper ribs, such that the lower rib cage matches in width the narrower, curved hip blades. A fragmentary lower rib from MH2 (probably representing the right ninth rib, but possibly the right 10th rib) has a shaft that is smaller than the ribs above and that is curved and twisted like the lower ribs in humans. Given that the shape of the lower rib cage closely corresponds with the shape of the pelvis in apes and humans, the narrower, more curved hip blades of Australopithecus sediba also suggest that the lower rib cage of this species was somewhat more human-like. Thus the overall thoracic morphology of Australopithecus sediba combined a narrow, ape-like upper rib cage with a narrow, human-like lower rib cage.
What is the significance of an ape-like, conical upper thorax?
The narrow upper rib cage of apes appears to be important in allowing a wide range of motion of the scapula (shoulder blade) needed for climbing and suspension (hanging below branches). Thus the conical shape to Australopithecus sediba’s thorax could be seen as consistent with the evidence from the upper limb that suggests that this species still engaged in a significant amount of arboreal locomotion. It is also important to note that a conical rib cage makes it difficult to engage in arm swinging during bipedal walking and running. In humans, our trunk naturally twists towards the opposite side as our swinging leg moves forward with each step (or running stride). While we could use the muscles on the sides of our abdomen (the abdominal obliques) to counter this rotation, it is energetically more efficient to counter swing our arms (that is, to swing the opposite side arm forward, which produces a countering twist on the upper thorax). Because humans have a thorax that is broad at the top, the arms are positioned well out towards the sides, and better able to swing freely without striking the sides of the body. Thus the conical rib cage in Australopithecus sediba suggests that they were not as proficient at the kind of striding bipedalism and running that characterizes modern human locomotion.
Could Australopithecus sediba run?
Most all animals, no matter what their form of locomotion, have some sort of running gait that can be used when greater speed is needed (as when capturing prey or avoiding predators), and Australopithecus sediba was likely no exception. The conical thorax of Australopithecus sediba would not have prevented them from running, but it does suggest that they were not capable of long-distance, endurance running (in which arm swinging is an important energy saving device).
Section F – The Lower Limb
Dr Jeremy deSilva, lead author on the paper pertaining to the lower limb, responds to some common questions pertaining to Au. sediba.
What parts of the leg have been recovered from Australopithecus sediba?
The skeleton of the young male (MH1) preserves a hip joint, and fragmentary parts of a shin bone, fibula and a few foot bones. The adult female (MH2) is more complete- she preserves a hip joint, knee, ankle, and heel. In fact, she is the first skeleton of an early human (hominin) that preserves all of these parts in a single skeleton, allowing us to reconstruct how all of these joints worked together in a single individual. Not even Lucy preserves all of these parts. There is a shin bone from another adult individual (MH4) as well.
How do we know that Australopithecus sediba walked on two legs?
Human leg bones differ from the bones of other primates in ways that are related to upright walking. For instance, humans are knock-kneed, meaning our knees are directly under our bodies and almost touch. But, we are not born with knees like this. Instead, the end of our femur begins to angle as we begin to walk on two legs as toddlers. Because this angulation only happens in individuals who walk on two legs, finding it in a fossil is strong evidence for upright walking. Australopithecus sediba possesses this angled femur. Other anatomies that we can see in the back, pelvis, hip joint, and ankle of Australopithecus sediba are found only in upright walking humans as well.
Does that mean Australopithecus sediba walked like a human?
No. The heel of Australopithecus sediba is chimpanzee-like, meaning that this species did not heel-strike during upright walking the way that most modern humans do. In fact, the heel of Australopithecus sediba is even more ape-like than the heel of earlier hominins attributed to Australopithecus afarensis (Lucy’s species).
So, how did Australopithecus sediba walk?
We propose that Australopithecus sediba was a “hyper-pronator.” The heel bone indicates that Australopithecus sediba contacted the ground on the outside of an inverted, or twisted in, foot. Contacting the ground on the outside edge of a twisted in foot causes the foot to rapidly and excessively rotate so that the inside (big-toe side) of the foot is driven into the ground (pronation). This begins a chain reaction in which the shin bone rotates inwards, the femur rotates inwards, and the torso pitches forward a bit.
OK. But, that is a weird way to walk. How do we know they walked this way?
There are some (often pathological) humans today who hyperpronate. Hyperpronators often have problems with their feet, knees, hips, and back. Interestingly, the Australopithecus sediba skeleton possesses anatomical solutions to the very problems faced by modern hyperpronating humans. We regard this as strong evidence that this species was adapted to walk in this manner. For example, because of the internal rotation of the shin bone and femur in hyperpronators, humans who walk this way have a higher likelihood of dislocating their knee-cap. The Australopithecus sediba knee possesses an exceptionally high retaining wall of bone (called the lateral lip), that would prevent the knee-cap from dislocating. Humans today have this too, but not nearly as high as that found in Australopithecus sediba.
But, maybe just MH2 walked that way. Maybe she was pathological. How do we know the species moved in this manner?
The bones of the heel and foot of MH1, and the shin bone from MH4 display the same anatomies that are found in the MH2 skeleton, indicating that they all probably walked in a similar manner. However, we will need more complete skeletons from additional individuals to confirm these initial findings.
Why would they walk this way?
There is good evidence from the arm, hand, torso, and even reconstructions of the diet, that tree climbing remained an important part of the lives of this particular species of Australopithecus. We suspect that hyperpronation might be a compromise locomotion for a hominin that had features of the foot that are adaptive for both upright walking and tree climbing.
Does this mean that Lucy, Little Foot, and other Australopithecus individuals walked this way?
No. The anatomy of the heel, parts of the midfoot and ankle, and knee are far different in Australopithecus sediba than in any other species of Australopithecus. The differences are in areas that are functionally important, and the differences are quite pronounced. We regard this as evidence that Australopithecus sediba moved differently than other species of Australopithecus, and that there were different ways to be an upright walking hominin in the past.
Can I walk like a sediba?
Yes! Please try it. Do not heel strike. Instead, land on the outside of your foot in a “flatfooted” manner. The ground will push back on your foot and roll it inwards (so that the inside of your foot is driven into the ground), your shin will twist inwards, as will your femur. You may feel your upper body pitching forward over your hips. If you do this enough, your foot, knee, hip and lower back may start to hurt a bit. But, remember, you are not a sediba! The anatomy of your bones differ from Australopithecus sediba in precisely those areas that start to hurt because while you are not adapted to walk this way, but we think that they were.
Section G – The Discovery
Prof. Lee Berger, Reader in Human Evolution at the University of the Witwatersrand, Johannesburg, responds to some common questions pertaining to Au. sediba.
How was the site and the fossils discovered?
In the mid-1990s, I had conducted an expedition across southern Africa, funded by the National Geographic Society designed to map fossil sites using the then relatively new technology of GPS and to discover new sites. While the expedition discovered many new caves and fossil bearing localities (over 100 caves and four new fossil sites in the Cradle of Humankind World Heritage Site), it did not yield any major discoveries.
In early 2008, using Google Earth to spot caves in the Cradle of Humankind World Heritage Site, Berger renewed the exploration programme in the area. With the assistance of new technology available, Berger discovered, over a few months, more than 600 caves and more than three dozen new fossil sites in one of the most explored areas on the planet.
On the 1st of August 2008, while mapping with my dog Tau, I discovered the fossil site of Malapa. On the 15th of August he returned to the site with Dr Job Kibii, Tau and his then 9- year-old son, Matthew. Within minutes, Matthew had discovered the first piece of hominid, belonging to the MH-1 skeleton. Two weeks later, I discovered the remains of the adult female skeleton MH-2 and since then, the site has yielded one of the most remarkable records of early human origins of any site on the planet.
What does Australopithecus sediba mean?
Australopithecusmeans “southern ape”, after the genus of the Taung child, named by Prof. Raymond Dart, also from the University of the Witwatersrand, Johannesburg. Sediba means natural spring, fountain or wellspring in Sotho, an appropriate name for a species that might be the point from which the genus Homo arises. As the hominids were also found preserved in an ancient underground lake or spring, the name also relates to their place of discovery.
What is a hominid/hominin?
A hominid is a member of the taxonomic family that includes humans, chimpanzees, gorillas and their extinct ancestors. Hominins are members of the human branch after the human lineage split from that of chimpanzees, and thus include living humans and extinct human ancestors, such as the Australopiths. Hominins are characterised by bipedal locomotion, although this may not have been the case for the very earliest members of the group, and relatively small canine teeth. Later members of this group (those in the genus, Homo) are characterised by larger brains than those of living apes like chimpanzees, bonobos, gorillas, orangutans and gibbons.
How does this find relate to Lucy?
Australopithecus sedibais approximately a million years younger than Lucy. Some scientists feel that Lucy’s species, Au. afarensis, gave rise to Au. africanus and in these papers, the team is suggesting that Au. africanus or something earlier than Au. africanus, gave rise to Au. sediba. There is some evidence, based upon the both primitive and advanced foot and ankle of sediba, that it does not descend from lucy’s species or africanus, but comes from some as yet unidentified lineage of early hominin. Additionally, the very advanced nature of sediba’s hand suggests it may not give rise to Homo habilis, which although later in time, has a more primitive hand, even though Homo habilis is some 200,000 to 300,000 years younger than sediba.
Has the new species named last year been accepted by the scientific community?
There is broad acceptance of the species Au. sediba among scientists as something previously unknown to science. Very little debate has occurred around whether these bones represent a new species. The debate has centered, largely, around whether the species should be placed in the genus Homo.
So why is this not the genus Homo?
The fossils have an overall body plan that is like that of other Australopiths – they have small brains, relatively small bodies and long and seemingly powerful arms. They do, however, have some features in the skull, hand and pelvis that are found in later definitive members of the genus Homo but not in other Australopiths. However, given the small brains and Australopith-like upper limbs, and features of the foot and ankle, the team has felt that keeping this species in the genus Australopithecus was the conservative thing to do. Nevertheless, sediba is turning out to be one of the most intriguing hominins yet discovered, and it certainly shows a mosaic of features shared by both earlier and later hominins.
What about Homo habilis?
Our study indicates that Australopithecus sediba may be a better ancestor of Homo erectus and it may certainly help to clear up some of this “muddle in the middle”.
Even after a year, why is there still rock attached to the child’s skull?
Due to the fragility of the base of the cranium of the specimen and to preserve part of the adhering matrix for future research as technologies improve, the team has decided
to leave the specimen partially in rock. The team has been able to visualise this hidden part using some of the most sophisticated scanning technology available.
Will there be more discoveries from Malapa?
Malapa is already one of the richest early hominin sites ever discovered but excavations have not commenced yet. When they do, later this year, we expect to make even more remarkable finds at the site.