Fossil may be earliest human ancestor

By AAP with AG staff 9 September 2011
Reading Time: 3 Minutes Print this page
Work by Australian scientists has found that fossils of one of our oldest human ancestors are two million years old.

AUSTRALIAN RESEARCHERS HAVE played a major role in confirming the age of what could be our oldest ancestor. The scientists from the University of Melbourne and La Trobe University have pinned down the age of two fossils found in South Africa  to around 1.98 million years old.

The fossils made headlines when they were discovered and were hailed as among the most complete early human fossils ever found. But initial efforts to date them could only estimate their age age to within a 200,000 year range.

Australopithecus sediba, discovered in a South African cave in 2009, had a dextrous hand that might have fashioned tools but may still have been used to climb trees. Its brain was small – the size of a grapefruit – but shaped more like that of a human than a chimpanzee, and while A.sediba had modern-looking ankles, its heel and shin-bone were mostly ape-like.

Australia’s oldest ancestors walked upright

Scientists believe the creature walked upright, but not in the same way as living humans. The combination of features, gleaned from the fossil remains of an adult female and male child, suggest the species may have been a direct ancestor of our own species.

Lead researcher behind the dating work, Dr Robyn Pickering of the University of Melbourne, says that arriving at a more precise age was critical to working out where the species fitted into human evolution. “It’s because of the improved dating that we know the fossils are from a narrow time range and a little bit older than we originally thought, which makes them the possible candidate to be our most distant ancestor,” she says.

Robyn, who used uranium-lead dating of the rocks above and below the fossils, said the results were arguably the most precise dates ever achieved for any early human fossils.

Magnetic field reversal helps date fossils

The research also shows the fossils were deposited in the Malapa Cave site in South Africa during a 3000-year period when the Earth’s magnetic field reversed itself by 180 degrees and flipped back again.

Dr Andy Herries, from the Archaeology Program at La Trobe University, discovered the change in the magnetic field through palaeomagnetic analysis of sediments surrounding the fossils. This was combined with Robyn’s research to confirm their age.

Andy says that being able to date and identify such magnetic reversal events will also help scientists understand the possible effects they have on climate, plants and animals. “Our ability to date and correctly identify these short geomagnetic field events is crucial as it will enable us in the future to both better date fossil and archaeological sites as well as understand the physical workings of our own planet and its core,” he says.

Candidate for early human genus

The findings are published today in the journal Science, along with other major research conducted on the fossils by scientists around the world.

The collection of work reveals that Australopithecus sediba has a mixture of characteristics typical of older australopthecines and the younger species Homo ergaster, which eventually led to Homo sapiens.

Professor Lee Berger, from the Institute for Human Evolution at the University of the Witwatersrand in Johannesburg, South Africa, who led the team that found A.sediba, says: “The fossils demonstrate a surprisingly advanced small brain, a very evolved hand with a long thumb like 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,” he adds.

The caves of Malapa, where the fossils were discovered, lie nearly 30 miles north-west of Johannesburg.