New evidence suggests that Au.sediba is the best candidate for the genus Homo

A series of five papers based on new evidence pertaining to various aspects of the anatomy of the species Australopithecus sediba (announced in April 2010 by Berger et al), will appear in the prestigious journal Science on Friday, 9 September 2011.

The papers will reveal new, important elements attributed to the two type skeletons, which will include an analysis of the most complete hand ever described in an early hominin, the most complete undistorted pelvis (hip bone) ever discovered, the highest resolution and most accurate scan of an early human ancestors brain ever made, new pieces of the foot and ankle skeleton, and one of the most accurate, if not the most accurate dates ever achieved for an early hominin site in Africa.

According to Prof. Lee Berger, a Reader in Evolution at the Institute for Human Evolution at the University of the Witwatersrand in Johannesburg, South Africa, Au. sediba demonstrates a surprisingly unique combination of features, never before seen in an early human ancestor.

"The fossils demonstrate a surprisingly advanced but small brain, a very evolved hand with a long thumb like a humans, a very modern pelvis, but a foot and ankle shape never seen in any hominin species that combines features of both apes and humans in one anatomical package. The many very advanced features found in the brain and body, and the earlier date make it possibly the best candidate ancestor for our genus, the genus Homo, more so than previous discoveries such as Homo habilis."

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.

Given the open access policy of the team, sediba is already one of the best studied hominin species yet discovered.

The team studying these fossils is one of the largest ever assembled in the history of archaeology or palaeontology. With more than 80 scientists, students and technicians from across the globe involved in the study, the teams expertise range from geologists, computer specialists, functional morphologists and anatomists, to physicists.

This is one of the largest collection of scientific papers ever produced by an African based team or University, on a single subject to be published in a journal of this impact level.

View a video at www.wits.ac.za/ihe

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.

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 Brain

Dr Kristian Carlson at the University of the Witwatersrand, Johannesburg, responds to some common questions pertaining to Au. sediba.

What does the study of the brain show?
This study of the brain shows a surprising mix of characters. The overall shape of the MH-1 endocast groups closely with all humans. Given how small the brain of sediba is (around 420cc's or the size of a medium-sized grapefruit), these results are consistent with a model of gradual neural (brain) reorganisation in the front part of the brain. However, one of the major discoveries announced in the Science papers is that the shape and form of sediba's brain is not consistent with a model of gradual brain enlargement, which is what has been hypothesised previously for the transition from Australopithecus to Homo.

What is so special about the brain scan?
The synchrotron scan of the brain used in this study is the most accurate ever produced for an early human ancestor. At a resolution of around 90 microns (scan widths just below the size of a human hair) incredible details of the anatomy of sediba's brain are revealed that give the team an unprecedented look at the brain of the MH-1 individual.

Where is the synchrotron scanner that was used?
The brain of sediba was scanned by a multi-billion Euro synchrotron scanner located in Grenoble, France called the European Synchrotron Radiation Facility (ESRF).

How does a brain endocast form?
With every heartbeat, the brain pounds out its shape on the developing skull of a child, eventually leaving a beautiful impression of the external shape and form of the brain on the inside of the skull. By mapping the contours of this surface, a clear image of the original brain located in the skull can be produced. This resolution changes through time and the endocasts of children thus tend to be clearer than those of adults.

Section C – The Hand

Dr Tracy Kivell is a Researcher in the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology in Germany. She provides an overview of the paper pertaining to an analysis of the hand of Au. sediba.

Overview

This paper describes the earliest, most complete fossil hominin hand post-dating the appearance of stone tools in the archaeological record. Almost all other fossil hominin hand bones prior to Neandertals are isolated bones that are not anatomically associated (i.e., do not belong to the same individual) and are not clearly affiliated with specific hominin species.

The Au. sediba hand thus allows us for the first time prior to Neandertals to evaluate the functional morphology of the hand overall, rather than just isolated bones. In our paper, we investigate the presence of several features that have been associated with human-like precision grip and the ability to make stone tools. Au. sediba has many of these features, including a relatively long thumb compared to the fingers – longer than even that of modern humans – that would facilitate thumb-to-finger precision grips. Importantly, Au. sediba has more features related to tool-making than the OH 7 hand that was used to originally define the "handy man" species, Homo habilis.

However, Au. sediba also retains morphology that suggests the hand was still capable of powerful flexion needed for climbing in trees. Taken together, we conclude that mosaic morphology of Au. sediba had a hand still used for arboreal locomotion but was also capable of human-like precision grips. In comparision with the hand of Homo habilis, Au. sediba makes a better candidate for an early tool-making hominin hand and the condition from which the later Homo hand evolved.

Is this the first, and most complete hand of an early hominin ever described?
The right hand of the female MH-2 published in these papers is the most complete hand of an early hominin ever described. It is missing only a few bones (the tips of four fingers and a wrist bone). It is probable that these bones may be found in the near future.

Why is the hand unique?
The hand is one of the very special features of human lineage, as it is 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 making 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. The team has interpreted this as being a hand capable of tool manufacture and use, but still in use for climbing and certainly capable of a human-like precision grip.

However, the Au. sediba hand is still primitive in many ways compared to modern humans, and the team does not suggest that Au. sediba was the only hominin around 2 million years ago capable of making tools. For example, the Au. sediba hand morphology is very different from the hand bones that were used to define the first tool-maker, Homo habilis or the "handy man". This may suggest that there were many different hominins making tools with different types of hand morphology around the same time period.

Compared to the hand of Au. afarensis – Lucy's species – Au. sediba has fingers that are shorter (thus, a proportionately longer thumb) and less curved, suggesting that Au. sediba was not moving around in the trees as much as Lucy. Au. sediba has broader finger tips, stronger muscles of the thumb and a more human-like wrist that also suggest it had better manipulative abilities than Lucy and her kin.

What is a precision grip?
A precision grip is a grip that involves the thumb and one or more fingers, but the palm is not actively involved. There are several different types of precision grips, but the most common one people think of is the ability to touch the end of your thumb to the tip of your index finger. Other precision grips include a pad-to-side grip between the thumb and the side of the index finger, such as when turning a key in a lock, or a pad-to-pad "baseball" grip, like when gripping a ball with just your fingertips. Other primates are capable of some precision grips but humans are unique in their ability to use these grips forcefully and for fine manoeuvring of objects within the hand. Au. sediba would have, without a doubt, had an excellent precision grip.

Does Au. sediba imply that Homo habilis was not a tool-maker?
No, but the answer is a bit more complicated than that. The OH 7 hand bones were found in the early 1960s in direct association with stone tools. Thus the OH 7 hand was assumed to be capable of making stone tools, which was key to including these fossils within the genus Homo and naming of the species Homo habilis or the "handy man". Since this time, few have questioned the tool-making ability of Homo habilis.

When a direct comparison between the bones preserved in Au. sediba and the OH 7 "handy man" hand bones is made, their anatomy is very different. The OH 7 hand appears to have a derived large, robust thumb and very broad finger tips, while the Au. sediba thumb is gracile and the fingers tips are not quite as broad.

In contrast, OH 7 still has fingers that are strongly curved and the wrist bones are more like those of African apes, while Au. sediba is more derived or human-like in these features. This difference in morphology implies two (though not necessarily mutually exclusive) scenarios: (1) that both species are capable of making tools but that they do this with different anatomy or (2) that the OH 7 hand does not in fact belong to Homo habilis, but is instead the hand of another early hominin species. Either way, it is likely that many hominin species were capable of making stone tools given that stone tools appear in the archaeological record long before either Au. sediba or Homo habilis are known to have lived.

However, the paucity of complete hand bones in the fossil record and our poor understanding of how the human hand functions and what morphology is necessary to make tools has limited our ability to determine exactly which species made tools and when and how tool-making first evolved. Au. sediba has shed new light on these questions.

Have you found tools on sites?
Excavations have not yet been conducted, so it is premature to speculate whether any tools in direct association with Au. sediba will be found. The hand and brain morphology suggest that Au. sediba may have had the capacity to manufacture and use complex tools.

Section D – The Pelvis

Dr Job Kibii at the University of the Witwatersrand, Johannesburg, responds to some common questions pertaining to Au. sediba.

What is special about the pelvis?
The pelvis (hip bone) of Au. sediba is a combination of earlier hominin shape and form and later human shape and form. It is short and broad like a human pelvis, creating more of a bowl shape than in an australopiths like Lucy. It still retains some features of earlier hominins, particularly in the size of the joint that links the sacrum with the vertebral column and the length of the front part of the pelvis. Parts of the pelvis are indistinguishable from that of humans, and it has a sigmoid shape (s-shape) along the top of the blades. 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.

How does the pelvis of Au. sediba relate to the Obstetric Hypothesis?
The Obstetric Hypothesis is a scientific theory developed more than three decades ago that suggested that the evolution of the larger brains of early Homo were the reason the human pelvis is shaped differently to early hominins like Lucy, which are broader, flatter and more flaring. Au. sediba, with its small brain, proves that at least in this lineage, the Obstetric Hypothesis for the origins of the human shaped pelvis is wrong, and it is therefore probably a hypothesis that has been refuted in the entire human lineage. The shape and form of the Au. sediba pelvis suggests that there is a need to look at other explanations for the origins of modern pelvic shape.

Section E – The Foot & Ankle

Dr Bernhard Zipfel at the University of the Witwatersrand, Johannesburg, responds to some common questions pertaining to Au. sediba.

What is so important about the foot?
The foot is very important in the evolution of the human lineage. Of all the evolutionary specialisations that define the human species, the foot is thought to be one of the most important, and is pivotal in allowing the evolution of arguably the most critical defining character of the hominins – habitual upright walking or bipedalism.

What is special about the ankle of Au. sediba?
Parts of both the MH-1 and MH-2's ankles were found. The female MH-2 ankle is one of the most complete ankles ever found with only Little Foot's ankle nearly as complete (currently being prepared by a Wits scientist), and it was found in articulation (the bones connected, nearly in living anatomical position).

The ankle joint is mostly human-like in form and inferred function, and there is some evidence for a human-like arch and Achilles tendon. However, a surprising find is that Au. sediba is ape-like in possessing a more gracile calcaneal (heel) body and a more robust medial malleolus (the lowest part of the tibia, or shin bone) than expected. This suggests that Au. sediba may have practiced a unique form of bipedalism, and would have almost certainly climbed trees.

It is also surprising that with parts of the heel being more primitive than earlier hominins like Lucy, it may mean that Au. sediba did not descend from this lineage. No ankle has ever been described with so many primitive and advanced features in one complex, and if the bones had not been found stuck together, the team may have described them as belonging to different species.

With these ape like features, how do you know that Au. sediba was a habitual biped?
The distal tibia or leg bone contacts the ankle bone perpendicular to the shaft of the leg bone, like that of modern humans. This is indirectly related to the position of the knee required for upright walking. In addition, there is evidence of a longitudinal arch and strong Achilles tendon insertion at the back of the heel, both being requirements of bipedalism. Au. sediba, however, practiced a unique form of upright walking, not exactly like that of humans, with some degree of tree climbing.

How did you take the ankle bones apart?
The ankle bones were taken apart 'virtually' using 3D scanners, like CT scanners, separating the bones. These digital bones were then printed out so that they could be studied by scientists. The team did thus not damage the articulated ankle.

Section F - Dating of the fossils

Dr Robyn Pickering, lead author on the paper pertaining to dating, responds to some common questions pertaining to Au. sediba.

The new date of the fossils is (1.977 to 1.98 million years old) is one of the most accurate dates ever achieved in the early hominin record.

How were the fossils dated?
When the fossils were announced in 2010, the age of the fossils was announced as being between 1.78 and 1.95 million years old. At the time, the team had not discovered the top of the deposit, which was only discovered in late 2010. The deposit is capped by a flowstone (a limestone layer), that was dated using the Uranium-Lead dating method and found to be the same age as the flowstone at the base of the deposit. This allowed the team, using palaeomagnetic dating, to identify a very specific, well dated reversal of the Earth's magnetic poles that occurred between 1.977 and 1.98 million years ago. This is a remarkable 3,000 years in 2 million years, or an incredible 0.15 % error in the date. The work of the Au. sediba team suggests that as the fossil record for early human ancestors increases, the need for more accurate dates is becoming paramount.

Cosmogenic dating was also used to interpret the landscape formation and to determine the depth of the cave at the time of deposition.

Why can't you date the fossil themselves?
The fossils themselves are too old to be dated directly with something like radio-carbon dating. Radio-carbon dating methods can only be used to date fossils around 50,000 years. The fossils themselves do not contain materials suitable for dating and the team therefore has to rely on dating the rocks that contain them to achieve accurate dates.

What is a flowstone/speleothem?
A specific type of rock forms within caves - the most common variations of this are stalagmites and stalactites. A flowstone is the same type of material as a stalagmite (calcium carbonate or lime) but instead of growing upwards and forming pillar like structures, the flowstone flows out across the floor of the cave to form a layer of calcium carbonate. These layers are usually a few centimetres thick but can be up to several metres thick.

A speleothem is a technical term used to describe any type of rock forming in a cave, so stalagmites, stalactites and flowstones are all types of speleothems.

What is uranium-lead dating?
When the flowstone rocks are formed, uranium gets trapped in them and decays by natural processes to form lead. Members of the team know the rate at which uranium decays and by measuring the amount of uranium and lead present in the rocks today, one can calculate the age of the rocks. In this way the uranium acts as a clock, which starts ticking once the rocks are formed. The amounts of uranium and lead are extremely small - in the parts per million and even the parts per billion range. In order to measure such small abundances of these elements, powerful machines called mass-spectrometers are used. The samples are prepared in a laboratory in Australia, the uranium and lead isotopes are isolated and concentrated from the rocks and collected for measurements.

What is Palaeo-Magnetic dating?
The Earth's magnetic field currently runs from north to south, which is known as a NORMAL field state. However, at various times throughout the Earth's history, the field has reversed by 180 degrees in what is known as a REVERSED field state. The timing of such major field changes is recognised for the last 7 million years (Ma) based on studies of sea floor spreading zones, where these changes are recorded like a bar code, as well as the radiometric dating of volcanic rocks. This bar code of field change is known as the Geomagnetic Polarity Time-Scale (GPTS). The direction of the Earth's field is preserved in cave sediments when water flushes these sediments into pools of water. When the sediment settles out of suspension in water the magnetic minerals orient themselves to the contemporary magnetic field. The sediments then become compacted and cemented, preserving the magnetic field direction. By measuring sediments in caves and comparing the polarity with the GPTS, the age of the sediments, and therefore the fossils within them, can be determined.

What happens during a palaeo-magnetic reversal event like the one seen in the rocks of Malapa?
We are not sure, but there may be severe changes in weather patterns as well as exposure to increased solar radiation as the Earth's magnetic field fluctuates.

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, Prof. Lee Berger 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 his dog Tau, Berger 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, Berger 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?
Australopithecus means "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 sediba is 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.

CONTACT DETAILS

For more information or to download the media pack, including images, podcasts and video material, visit www.wits.ac.za/sediba once the embargo is lifted.

For interviews with any of the authors from the University of the Witwatersrand, Johannesburg (aka Wits University) including: Prof. Lee Berger (project leader), Dr Kris Carlson (brain), Dr Bernhard Zipfel (foot and ankle) and Dr Job Kibii (pelvis) contact Shirona Patel, Head: Communications, on +27 11 717 1019 or +27 83 362 1995 or email Shirona.Patel@wits.ac.za

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