Post by Craig on May 30, 2003 1:06:39 GMT -5
WASHINGTON -- A new dating technique suggests that a human-like fossil skeleton found in South Africa was buried about 4 million years ago, which makes it one of the oldest known hominid discoveries.
That's 1 million years earlier than previously thought.
The nearly complete skeleton came from the Sterk-fontein caves that contain rich deposits of remains from the pre-human branches of the ancestral tree that led to modern humans. The bones are identified as a type of Australo-pithecus, an extinct form of pre-humans.
Skeleton fossils unearthed in 1997 were age dated at about 3 million years using a technique that measured the changes in the Earth's magnetic field geochemically recorded within rocks found with the fossils. The results had been disputed by experts who said the method was not precise.
In the new effort, researchers, led by Darryl E. Granger and M.W. Caffee of Purdue University, measured the decay of isotopes in cave sediments to establish the older ages. A report on their study appears Friday in the journal Science.
The new technique is based on chemical changes in elements caused by cosmic rays. The bombardment of cosmic rays creates unstable isotopes of beryllium and aluminum, both of which decay at a known rate. The older a sediment is, the more isotopes it acquires. When the sediments are buried, the cosmic ray bombardment stops, but the isotopic decay continues.
Granger said that the cave is carved by water out of dolamite and cherte rock. Over time, both the fossils and fragments of quartz crystals that were on the surface were washed into the cave and deposited together. Once the quartz is inside the cave, it is no longer being hit by cosmic rays.
"That sets the clock," said Granger.
He said by measuring the isotopic decay in the quartz, the researchers could establish when both the fossils and the quartz were deposited.
The other co-authors of the study are T.C. Partidge and R.J. Clarke, both of the University of the Witwatersrand in Johannesburg, South Africa. Partridge and Clark did the earlier studies on the Sterkfontein fossils.
Australopithecus is an extinct hominid species that lived from about 4 to 2.5 million years ago. Lucy, a famed pre-human fossil discovery found in Ethiopia, was identified as a type of Australopithecus. The new age dating technique suggests that the fossils from Sterkfontein Cave are even older than Lucy.
Granger said only an Australopithecus fossil found in Kenya is older than the remains from the South African cave.
However, some researchers said in Science that the specimens used by Granger and Caffee may have been contaminated by debris that was not deposited at the same time as the fossils.
Henry Schwarez of McMaster University in Hamilton, Canada, said in Science that the collapsed cave ceilings may have dropped sediments into the area where the fossils were found and thus distorted the age-dating results. Schwarez said that the Purdue researchers need to test the technique using sediments from South African caves of a known age.
Granger said the age-dating technique is not affected by sediments from inside the cave. He said the cave minerals are distinctly different from the quartz washed in from the surface. The material from the cave walls and ceiling is separated from the quartz before the isotopic measurements are taken, he said.