A new calculation of the age of many of South Africa’s best-known fossils by a team at the University of Witwatersrand has turned the clock of human evolution forward by a million years.
But not everyone in scientific circles is happy about the way the team rearranged our evolutionary tree.
The researchers found that the Cradle of Humankind heritage site at Sterkfontein near Krugersdorp might be a million years younger than thought. Their recalculation puts Australopithecus africanus, the Southern African apeman who was thought to be a progenitor of humankind, living alongside early members of our own genus Homo.
Lee Berger, director of the Wits Palaeoanthropology Unit for Research and Exploration (Pure), Darryl de Ruiter and Rodrigo Lacruz have revised the estimated age of the australopithecus-bearing deposits at Sterkfontein. Their findings are published in the October issue of the American Journal of Physical Anthropology.
But Tim Partridge, dating expert at the Sterkfontein caves and a geologist with the Climatology Research Group at Wits, slams the claim.
“How can they make this claim when none of them works at the Sterkfontein caves? Their article certainly can’t carry much weight. “I want to know where and when they conducted their research,” he said.
Partridge says that he and a team of international scientists have also been working on new dating techniques. “Our findings will also shortly be published, but all I can say is that it confirms the results of dating methods used up to now.”
The Partridge team’s absolute dating techniques allow scientists to calculate dates accurately without comparing the material they find with material found elsewhere.
Phillip Tobias, the world-renowned palaeoanthropologist who popularised South Africa’s rich fossil history, wasn’t aware of any changes in dating calculations.
“I’ll have to read the article first before I can comment on the revised age estimate,” he said. “The dating methods that we currently use are based on research of the world’s top scientists.”
The dating of the remains of early man and his forebears at Sterkfontein has long evoked debate. Unlike in East Africa, where volcanic bedrock allows accurate dating by means of physical and chemical tests, in South Africa, scientists must rely on less exact relative methods.
There are no volcanic rocks at Sterkfontein. Remains are found in dolomite, or limestone, caves. Water melts the limestone and the solution mixes with the sand or bones on the cave floor, forming a concrete-like mixture called brecchia that contains the fossils.
Berger says that biochronogical dating has been the most common method used to date South African sites. This relative method of dating uses index fossils of a known age to date other fossils in the same deposit, including hominins.
“No absolute dating method has come forward and we are reliant on the use of multiple methods to establish relative and absolute ages of the South Africa fossils,” the article says.
Partridge devised a geological model for interpreting Sterkfontein’s complicated layers of brecchia. He divided the Sterkfontein deposits into age classes called members, labelling them in sequence from oldest to youngest. Member 1 at the bottom would be the oldest and member 6 the youngest.
Paleomagnetic dating, where the magnetic properties of artefacts are studied, estimated that some of South Africa’s oldest hominin fossils were up to 3,5-million years old.
Berger’s team found that the fauna of Sterkfontein did not tally with those ages and he determined that the fossils could not be older than 3,04-million years.
They now estimate that Sterkfontein member 2, where the Little Foot fossil is being unearthed, is younger than three million years and that Sterkfontein member 4, previously dated at between 2,4-million and 2,8-million years, is between 1,5-million and 2,5-million years old.
“The new facts shake up 30 to 40 years of palaeoanthropology where hominins at Sterkfontein were compared with earlier hominins, when they should’ve been compared to later hominins,” said Berger. “The whole middle branch of human evolution will have to be re-evaluated.”
Inconsistencies with the physical characteristics of the hominins found at the Cradle first alerted Berger and his team that something was amiss. Their form showed a more developed hominin than expected from the time.
“Us placing Australopithecus africanus closer to two million years ago resolves much of this debate,” De Ruiter said. “We had been comparing the early South African ape-men to the wrong early hominin species elsewhere in Africa.”
The new dating technique has specific implications for Little Foot, the most complete early hominin yet discovered.
“There is at present no good evidence that the Little Foot skeleton is as old as has been suggested and it may in fact be younger than fossils like Mrs Ples,” Berger said.
Berger’s new dates may indicate that Australopithecus africanus, long thought to be an ancestor of the genus Homo, is more likely to be a side branch of the human family tree.