For the first time palaeoanthropologists working at the Cradle of Humankind near Krugersdorp have an accurate method of dating the hominids they unearth.
A team of South African and United States scientists has discovered a revolutionary dating method — the cosmogenic burial technique — that establishes absolute dates for many of the older hominid fossils found in South Africa.
Tim Partridge and Ron Clarke of the University of the Witwatersrand and Darryl Granger and Marc Caffee of Purdue University in the US have shaken the very bones of the palaeoanthropological world with a research article that appeared in this week’s issue of the prestigious US publication Science.
Their paper also describes several specimens of an early hominid discovered recently in a new locality within the Sterkfontein cave system.
”The [dating] method is of major importance because, in the past, estimates of the age of the remains from South Africa have been based mainly on comparisons of animal fossils that accompanied the hominids,” said Partridge.
The age of ”fossils of similar species recovered at sites in East Africa was determined radiometrically from associated volcanic material”, Partridge said. ”Such comparisons are often problematical and can usually provide ages within broad limits only.”
The researchers believe their new technique, though restricted to the oldest deposits at Sterkfontein, will help to eliminate the uncertainties inherent in the comparative approach.
Little Foot, one of the most complete hominid skeletons yet found, was discovered by Ron Clarke and his team in 1997 and was the first fossil to undergo the new dating technique.
The team’s research shows that Little Foot is about 4,17-million years old, considerably older than the estimate of 3,3-million years that Clarke and Partridge reported in 1999.
But the new date fuels one of the most strident disagreements raging in South African science. Lee Berger, director of the University of the Witwatersrand’s palaeoanthropology unit for research and exploration, and his team estimate that Little Foot is not even 3-million years old. The team used a range of techniques to calculate their date, which was published in the October issue of the American Journal of Physical Anthropology.
”Little Foot may in fact be younger than fossils like Mrs Ples,” Berger said at the time.
He said Sterkfontein Member 4, a layer rich in fossils that was previously dated at between 2,4-million and 2,8-million years, is only between
1,5-million and 2,5-million years old.
”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,” Berger said in his article.
His trio of scientists used three different techniques to calculate that Sterkfontein’s fossils could not be older than 3,04-million years.
Partridge and Clarke criticised Berger’s analysis at the time and hinted that they were working on a technique that would blow his research to pieces.
Now they have revealed their procedure, which indeed calls Berger’s dates into question. In 1999 Partridge and Clarke measured the magnetic reversals in the deposits surrounding Little Foot. They combined these results with the known ages of associated animal remains to estimate the fossil’s age.
The two scientists say that the new cosmogenic dating method is the technique that South African palaeoanthropologists have been waiting for.
The cosmogenic burial technique is based on the known rate of radioactive decay of unstable isotopes, or nuclides. Granger used the elements aluminium-26 and beryllium-10.
Grains of quartz in the surface soil accumulate radionuclides of these elements over time. If the quartz grains are buried deeper than 20m — for example, by falling into a cave such as that at Sterkfontein — they accumulate little more alumunium-26 and beryllium-10. The alumunium nuclide decays faster than the beryllium, so the ratio of the two changes exponentially with burial time, providing a means of dating the collapse of the sediment into the cave.
The Science paper also describes a previously unexplored part of the Sterkfontein cave system, known as Jacovec cavern, where Clarke and his team have discovered other fossil hominid and animal remains.
The Little Foot skeleton and the new Jacovec specimens are the oldest hominids yet discovered in South Africa.
”In the Jacovec the deposits are less heavily consolidated than those around the [Little Foot] skeleton, and a number of important specimens — including part of a skull, several teeth, parts of limb bones and a collar bone — have been retrieved over the past few years,” said Partridge. The cosmogenic burial technique dated these fossils at 4,02-million years.
The Jacovec specimens persuaded Clarke and Partridge to look for other dating techniques because conventional techniques were not holding up.
”The Jacovec deposits are not in continuity with those elsewhere at Sterkfontein and their relationship to the well-documented sequence that was host to the remainder of its hominid specimens remains unknown,” Partridge said.
The problem was compounded because many of the fossil animals from Jacovec belong to species that survived over long periods and were of little use as age indicators.
Help arrived from Darryl Granger, who wanted to explore the potential of cosmogenic isotopes for dating the Little Foot skeleton and the Jacovec deposits. He took delicately chosen samples, handpicked by Partridge, to Purdue University in 1999 to begin the slow process of dating the specimens.
”The first results, obtained late in 2001, showed that not only was Granger’s technique applicable to the early Sterkfontein deposits, but that it was able to give reproducible results,” said Partridge. ”Moreover, nothing in the anatomical features of the hominid specimens or in the range of animal remains recovered with them appeared to be incompatible with these results.”
Berger’s younger dates implied that Australopithecus africanus, the South African species long thought to be an ancestor of our genus Homo, represented a side-branch of the hominid family tree. His research placed Little Foot firmly outside our direct lineage.
But Partridge and Clarke’s new technique makes Little Foot and the Jacovec specimens contenders for membership of the lineage that gave rise to our earliest ancestors.
”The new dates tell us that the genus Australopithecus was widely dispersed across the African continent for a period of at least 2-million years. During that time several different species of Australopithecus evolved,” said Partridge.
”Australopithecus africanus, which existed for about 1-million years in the South African fossil record, is a less likely contender, with late representatives of that species, such as Mrs Ples, apparently surviving after the first appearance of Homo in East Africa.”
Partridge said the early Sterkfontein specimens are unusually complete and therefore provide important new insights into the functional anatomy of creatures near the beginning of the road to humanity.
According to Partridge, the Sterkfontein Member 2 Australopithecines are not the oldest hominids yet discovered.
”That distinction probably belongs to the 6-million-year-old fragmentary remains of Orrorin tugenensis, discovered by Martin Pickford and Brigitte Senut in the Tugen Hills of Kenya in 2000.”