The hominin fossil, Australopithecus sediba may be the oldest known ancestor of the Homo genus - the earliest relatives of modern humans. This endocast of the skull was created using synchotron radiation.
Credit: Kristian Carlson/University of the Witwatersrand.
BRISTOL: With a combination of ape and human-like features, the hominin fossil, Australopithecus sediba, may be the oldest known ancestor of the Homo genus, a new study suggests.
Just over a year after the initial description of the fossils, a detailed account of the key anatomical features has been released in a series of articles in the journal Science.
The anatomy described - the brain, hand, pelvis, foot and ankle - represents a true mosaic of primitive Australopithecus-like features as well as more advanced Homo-like features, which has the scientific community both excited and puzzled.
Discovered in 2008 at the Malapa caves, 45 kilometres northwest of Johannesburg, South Africa, the find sparked considerable debate as to whether the fossils should be classified as belonging to the genus Australopithecus or to the genus Homo.
Handy evolution
They were classified as Australopithecus sediba due to an overall body plan that resembled other Australopithecus fossils, including a small brain and long upper arms.
"A. sediba has long ape-like arms, like a chimpanzee, so we expected to see a primitive hand, " said Lee Berger, a physical anthropologist from the University of the Witwatersrand in Johannesburg and senior author on all five articles.
"But it's not primitive. In fact, it's the most human-like hand we've ever seen - more advanced than the hand of Homo habilis. The thumb is elongated relative to the fingers, allowing precision gripping, but has features that suggest it was powerful enough to still allow tree climbing."
Caught between two genera?
So with this unexpected combination of primitive and advanced features, does A. Sediba lie between two genera? "This is where the date becomes quite critical," said Berger, "because it places it right at the moment of the transition from Australopithecus to the earliest ancestors of Homo."
The fossils, a young male 10 to 13 years old and a female thought to be in her late twenties or early thirties, were originally thought to be between 1.78 and 1.95 million years old. This is younger than the earliest Homo species on record and therefore it was thought to be an unlikely ancestor.
New excavations shed light on date
However, recent excavation away from the first pit at the Malapa caves revealed a second shallow pit containing a limestone layer formed after the hominins became trapped in the caves.
By dating the flowstone layer capping the fossil-bearing sediment, an international group of scientists has been able to put an upper age limit on the Malapa hominin fossils and provide strong evidence that A. sediba is a likely ancestor in our lineage.
"The original paper describing the fossils had an age range of 170,000 years, but in this paper we've been able to narrow it down to 3,000 years," said geochemist Robyn Pickering from the University of Melbourne and lead author of one of the Science studies. "Results from this study are the most precise dates ever for early hominins."
Magnetism key for dating
Pickering and her colleagues used a combination of uranium-lead dating and paleomagnetic dating to hone in on the age of the fossils, analysing the surrounding rocks.
As magnetic minerals come out of suspension, they orient themselves with the magnetic field and are trapped there when the rock forms. The result is a barcode of normal (north to south) and reverse (south to north) orientations that can be matched to the known Geomagnetic Polarity Time Scale.
These methods enabled Pickering and her colleagues to match the fossil sediments to a well-dated reversal of the Earth's magnetic poles that occurred between 1.977 and 1.98 million years ago.
"The new and detailed descriptions of the skeletons of two individuals from the Malapa site return the spotlight to South Africa as the possible location for the postulated transition from Australopithecus to Homo," commented Chris Stringer, research leader in human origins at the Natural History Museum in London, who was not involved in the study.
