September 8, 2011. Copyright 2011, Graphic News. All rights reserved

Fossil discovery sheds light on human ancestors

LONDON, September 8, Graphic News: Professor Lee Berger heard his nine-year-old son, Mathew, shout “Dad, I found a fossil!” Berger glanced over at the rock his son was holding and immediately realised the significance of the find.

On that day in August 2008, Mathew had found the collar bone of an ancient hominin. When Berger turned the rock around there was a lower jaw jutting out. “I couldn’t believe it,” he says.

Berger, a paleoanthropologist at the University of the Witwatersrand, Johannesburg, soon had the partial skeleton of a young boy and the skeleton of an older female, which he believes could have been related to the boy.

He claims that these specimens, along with numerous other fossils found since 2008 in Malapa cave north of Johannesburg, are those of a new species dubbed Australopithecus sediba. Sediba means “wellspring” in the Sesotho language. Berger’s team argues that the fossils have a mix of primitive features typical of australopithecines and more advanced characteristics typical of later humans. Thus, the team says, the new species may be the best candidate yet for the immediate ancestor of our genus, Homo.

Research teams in South Africa, Australia, France and Germany have now published new details about the brain, pelvis, hands and feet of A. sediba, in the September 9 issue of the journal Science.

Using advanced uranium-lead dating techniques and palaeo-magnetic dating -- which measures how many times the Earth’s magnetic field has reversed -- Robyn Pickering and colleagues at Melbourne University established the date of the fossils at 1.977 million years (plus or minus 2,000 years). This predates the earliest appearances of Homo-specific traits in the fossil record. Until now, fossils dated to 1.90 million years ago -- and mostly attributed to Homo habilis and Homo rudolfensis -- have been considered ancestral to Homo erectus, the earliest undisputed human ancestor.

The cranium of the juvenile A. sediba, known as MH-1, was scanned at the European Synchrotron Radiation Facility in Grenoble, France. Kristian Carlson and colleagues examined the resulting endocast -- the detailed scan of the space where the juvenile’s brain would have been -- and found that the brain was human-like in shape but still much smaller than the brains seen in Homo species. By studying the impressions of the brain on the inside of a cranium, the researchers say that the orbitofrontal region of MH-1’s brain, located directly behind the eyes, shows some signs of neural reorganization, which perhaps indicates a rewiring toward a more human-like frontal lobe, the area of the brain associated with cognition and social responses.

Researchers also analyzed the partial pelvis of the adult female A. sediba, known as MH-2, and found a surprising combination of both primitive and modern features shared with the genus Homo. In light of their findings, Job Kibii and colleagues at Witwatersrand posit that Homo pelvises could not have evolved specifically to accommodate larger brains, which would have had to pass through the pelvis during childbirth -- A. sediba’s pelvis was already developing modern, Homo-like features while their brains and craniums were still small.

The wrist and hand of MH-2 are only missing a few bones, making them the most complete hand fossils for an early hominin on record. Tracy Kivell and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig found the hand had a long thumb and short fingers, which is a sign of precision gripping -- a grip that just involves the thumb and fingers, but not the palm -- and say that it’s even possible that A. sediba was capable of simple tool-making. 

In a separate study, Bernhard Zipfel and colleagues at Witwatersrand analyzed the feet and ankles of MH-1 and MH-2 and found evidence for a human-like arch and Achilles tendon. A. sediba probably practised a unique form of bipedal walking, the researchers conclude.

Berger takes great joy in his find, and in his luck. “Most scientists who do what I do, don’t find anything like this in their entire lives. To find this sort of record, that might contribute to the understanding about where our genus comes from: that’s just extraordinary.”
/ENDS