Fossils’ age set in stone

Researchers have confirmed the age of possibly our oldest direct human ancestor at 1.98 million years old.

The discovery was made after researchers conducted further dating of the early human fossils, Australopithecus sediba, found in South Africa last year.

A series of studies carried out on newly exposed cave sediments at the Malapa Cave site in South Africa, where the fossils were found, has helped researchers to determine their more precise age at 1.98 million years old, making the Malapa site one of the best dated early human sites in the world.

A series of papers published today in a special issue of the prestigious international journal Science provide a new, more precise age for the fossils, as well as more detailed studies of the hands, feet, pelvis and brain.

Professor Paul Dirks from James Cook University and Professor Lee Berger of the University of the Witwatersrand in Johannesburg led the team that originally discovered the fossils in 2008.

“It is through the new exposures and our understanding of the stratigraphy of the site, together with the advances we’ve made in the dating techniques, that we can be even more precise now,” Professor Dirk said.

Uranium-lead dating of the flowstone conducted by the University of Melbourne, combined with palaeomagnetic analysis of sediments surrounding the fossils conducted by La Trobe University, provide the tightly constrained new age.

The team was able to pin down the age of the fossils to within 3000 years of 1.98 million years, a massive advance on the age range of around 200,000 years from the 2010 estimate.

Dr Robyn Pickering of the University of Melbourne’s School of Earth Sciences, a lead researcher involved in the dating of the flowstone surrounding the fossils, said researchers had long been searching for fossils from this time period to answer questions about the beginnings of our own genus, Homo.

“Knowing the age of the fossils is critical to placing them in our family tree, and this new age means that Australopithecus sediba is the current best candidate for our most distant human ancestor.”

“The results of these studies present arguably the most precise dates ever achieved for any early human fossils," she said.

It appears the fossils were deposited in the Malapa Cave during a 3,000-year period around 1.98 million years ago, when the Earth’s magnetic field reversed itself by 180 degrees and back again.

Dr Andy Herries from the Archaeology Program at La Trobe University, who undertook the palaeomagnetic analysis, said our ability to date and correctly identify these magnetic reversal events was crucial.

“They enable us to better date fossil and archaeological sites, as well as to understand the possible effects they have on climate, plants and animals.

“It’s very rare to find short geomagnetic field events such as this and be able to accurately date them, so this was an exciting find and helped to tightly confine the age of the fossils,” Dr Herries said.

"The strong collaboration between South African and Australian universities has allowed us to push the boundaries of what was once thought possible in dating critical moments in early human origins in Africa,” Professor Berger said.

Issued: September 9, 2011

JCU Media, Jim O’Brien 07 4781 4822