SA’s ‘best candidate for human ancestor’

9 September 2011

The latest edition of the prestigious journal Science contains no less than five articles detailing new evidence that Australopithecus sediba, whose fossils were uncovered at a site in South Africa’s Cradle of Humankind, is the best candidate for ancestor of the genus Homo.

The delicate skeleton of the hand of the Australopithecus sediba lay protectively in the hand of Wits University professor Lee Berger, who first discovered the site in August 2008, at a press conference in Johannesburg on Thursday.

It is the most complete hand skeleton of an early hominin ever discovered. Besides the hand, a complete undistorted pelvis, along with pieces of the foot and ankle skeleton, were unveiled this week.

First-time achievements

Five papers detailing the findings and analysis of the find were published in Science on Friday. This is the largest number of scientific papers every produced by an African-based team or university to be published in a major science journal. An image of the hand appears on the cover of the journal.

Other first-time achievements in palaeontological science are the highest resolution and most accurate scan of an early human ancesters’ brain, and the most accurate dates to be obtained for an early hominin site in Africa. Australopithecus sediba dates back 1.9-million years.

“The fossils demonstrate a surprisingly advanced but small brain, a very evolved hand with a long thumb like a human, 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,” said Berger.

Last year the skeleton of a boy, nicknamed Karabo, was unveiled. This year a second female skeleton was unveiled, along with the other fossils.

‘Best candidate ancestor for the genus Homo’

“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,” he said. This means that the status of Homo habilis is now going to be re-evaluated.

The site – now called Malapa, in the Cradle of Humankind northwest of Johannesburg – has yielded over 220 bones, collectively making up more than five individuals. The skeleton of an adult female and adolescent male have so far been assembled.

The find was first announced at Maropeng, the intrepretation centre attached to the Sterkfontein Cave area, in April 2010.

Over 80 scientists from across the world – geologists, computer specialists, morphologists, anatomists and physicists, among others – have come together to study the fossils.

It has been estimated that the boy was between 11 and 13 years, while the woman was in her late 20s or early 30s.

It is believed that they fell into a deep cave, where they were solidified into rock, and over 2-million years of erosion the fossils have been exposed. Other animal fossils have been found with them – sabre-toothed cats, hyenas, antelopes, mice, birds and snails.

Sediba is believed to be the ancestor of the Homo genus. It had long arms, like an ape and short powerful hands, meaning that although it was bipedal, it retained its ability to climb. The advanced pelvis and long legs suggest it was able to stride and possibly even run like a human.

Significance of the complete hand

The significance of the complete hand is extraordinary. Almost all other hominin hand bone finds do not belong to the same individual, and therefore cannot be affiliated with a specific hominin species.

“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,” said Dr Tracy Kivell, a palaeoanthropologist in the department of human evolution at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The hand is unique because it has shortened fingers and a very long thumb, but at the same time very powerful muscles for grasping, making it a hand capable of tool manufacture and use, but still able to climb.

“However, the Au. Sebida 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,” said Kivell.

A comparison has been between Homo habilis and Au. Sebida, both capable of tool making, although with different hands. What this indicates is that there were many hominin species capable of making stone tools, long before either of them are known to have roamed the earth.

“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,” said Kivell.

“Au. Sediba has shed new light on these questions.”

The pelvis

The pelvis is short and broad like a human pelvis, but it retains features of earlier hominins.

“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,” said Dr Job Kibii, a palaeoanthropologist in the Institute for Human Evolution at Wits.

For this reason, the linking of larger brains to a more advanced pelvis will be revisited. “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,’ adds Kibii.

The foot

The foot is particularly important in the evolution of humans, and indicates what defines humans – the ability to walk upright, or bipedalism.

The ankle of the female is one of the most complete ankles ever found, with bones connected. It shows evidence of an arch and Achilles tendon, but at the same time is ape-like in its heel and shin bone.

“This suggests that Au. sediba may have practised a unique form of bipedalism, and would have almost certainly climbed trees,” said Dr Bernhard Zipfel, a palaeoanthropologist at the Institute for Human Evolution at Wits.

No ankle has been described with so many primitive and advanced features in one complex, he says. This means that if the bones had not been found as a whole, it may have been assumed that they belonged to different species.

Interestingly, the bones were not taken apart physically but rather separated virtually with 3D scanners. These digital bones were then printed out and studied, with no damage caused to the ankle.

The brain

The brain pounds out its shape against the developing skull of a child with every heartbeat, and leaves an impression of the shape and form of the brain on the inside of the skull. Scientists have mapped the contours of this surface, thus producing a clear image of the original brain.

The boy’s brain was around 450 cubic centimetres, compared to a human brain of around 1 600 cubic centimetres.

“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,” said Dr Kris Carlson from the Institute for Human Evolution at Wits.

The fossils were too old to be dated by radio-carbon dating, a method that can only date fossils back 50 000 years. Instead, they were dated by means of the Uranium-Lead dating method. When the rocks were formed, uranium got trapped in them and decayed, forming lead. Scientists are able to measure the amount of uranium and lead in the limestone rocks, which capped the find.

Sediba is Sotho for “well” or “spring”; Australopithecus sediba was so named because it was hoped that “a great source of information will spring from the fossils”.

Excavation of the site is at a very early stage, and holds the promise of many more fossil discoveries.

Source: City of Johannesburg