The Australopithecus sediba discovered in 2008 could be the direct ancestor of the Homo genus. That is the conclusion of a team from the University of Witwatersrand, with participation by anthropologist Peter Schmid of the University of Zurich. The researchers describe in five publications in "Science" why their finding is more likely to come into consideration than earlier discoveries, like Homo habilis.
The fossil bones of the pre-human, excavated north of Johannesburg in Malapa, are 1.98 million years old. That figure was yielded by the latest analyses on the two skeletons MH-1 and MH-2, originating from a boy of approximately 10-13 years and a woman of approximately 30 years.
It was discovered that Australopithecus sediba unites various properties that were not yet seen in early ancestors of humans. The fossils show a surprisingly modern, yet small brain; a very modern, developed hand with long thumbs, like in humans; a very human-like pelvis; but a form in the foot and ankle shape that is both ape- and human-like. In light of these findings, Prof. Lee Berger, University of Witwatersrand, is of the opinion that Australopithecus sediba is the best candidate ancestor for our genus, the genus Homo.
One of the partial skeletons consists of a skull measured to have a brain volume of 420 cubic centimeters. Considering the growth zones on the long bones and the tooth formation, it must be the skull of a young individual who is 10-13 years old. The brain of an adult would therefore have a volume of ca. 440 cubic centimeters. "This very small volume is irritating, especially when one observes the thoroughly advanced facial features and the very human-like particularities in the locomotion system," observes Peter Schmid from the University of Zurich, co-author of the publications.
Beginnings of a Modern Brain
The extraordinarily well preserved skull made it possible to clarify whether the windings and grooves correspond to those of the earlier Australopithecus, or if it shows the properties of a modern brain. It was revealed that the frontal pole region and the olfactory bulb area are already similar to those in humans. This has led the researchers to postulate that neuronal reorganization of the forebrain must have occurred before the actual size increase in the brain.
New information was also facilitated by the oldest nearly complete hand of an early hominin ever found and described. This is significant because the hand is seen as the trademark of humanity. Throughout the course of human evolution, the hand was no longer used for locomotion, as is the case with apes, but rather to manipulate objects.
A Hand for Making Tools
Compared to earlier forms, Australopithecus sediba has shorter fingers, very long thumbs, and more robust metacarpal bones. Astonishingly, the Sediba's hand shows more modern properties than the hand fragments from an earlier find considered to be the origin of the tool-making human (Homo habilis, "handy man") and therefore to be the first representative of the Homo genus. "Australopithecus sediba should therefore have been even more capable of making tools," says Schmid. Since this hand is different from that of the Homo habilis, there must have been various hominins with various types of hands producing tools during the same time period.
The hand found is more complete than that of the Homo habilis, and therefore allows for more conclusions to be drawn. It is more modern, although Homo habilis is 200,000 to 300,000 years younger. The researchers are therefore of the opinion that Australopithecus sediba is an earlier toolmaker than Homo habilis, and therefore also better suited to be the morphotype of a basal hand.
In contrast to later forms of the Homo genus and several australopithecines, the hand of Australopithecus sediba conservedseveral modifications for tree life. While Sediba does appear to already be capable of using and making tools with his hands, those hands are also well suited for climbing.
Locomotion Changed the Pelvic Form
The pelvis is a mix of original, Australopithecus-like, and later, Homo-like properties. The modern features of the pelvis are surprising for the researchers, as Sediba has such a small brain. Previously, the assumption was that a size increase in the brain changed the demands on the pelvis to facilitate the birth of babies with larger brains. The researchers surmise that the pelvis adapted in at least one of the lines of earlier hominins before brain volume increased. The most likely scenario, according to Peter Schmid, is that locomotion on two legs changed the pelvis. That would mean that the model pelvic form can be traced to the requirements of the locomotion system and not those of the birthing process.
What is confusing, according to Schmid, is the foot. In contrast to the pelvis, the hand, and the skull, it is very ape-like. Compared to older ancestors, it is also much less modern. There are various properties that indicate erect, bipedal walking, while others are suitable for climbing.
Australopithecus sediba: Finding and Name
In August 2008, Matthew Berger, the son of paleoanthropogist Lee Berger, found the fragment of a human-like collar bone. The first excavations at the discovery site, Malapa, north of Johannesburg, were performed by a team from the Swiss Field School of the Anthropological Institute at the University of Zurich, under the direction of Peter Schmid.
The fossils did not match any previously known hominin species -- that is why they constitute a new milestone in humanity's history. Based on age and morphology, researchers carefully allocated the new hominin species to the Australopithecus genus and not to the Homo genus. They gave it the name Australopithecus sediba, which in the Sesetho language means "fountain" or "source."
- Tracy L. Kivell, Job M. Kibii, Steven E. Churchill, Peter Schmid, Lee R. Berger. Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities. Science, 2011; DOI: 10.1126/science.1202625
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