Purdue's Darryl Granger and Marc Caffee have determined the age of a fossilized skeleton thought to be an Australopithecus � a genus of African hominids from which humanity is thought to have developed � by measuring the radioactivity of the cave sediments in which the skeleton was buried millions of years ago.

Their measurement technique, generally used to estimate the age of geological formations such as glaciated valleys and river terraces, has never before been used to date biological fossils.

"By dating the sediments surrounding the fossil skeleton, we have determined that this species reached southern Africa approximately 4 million years ago," said Granger, associate professor of earth and atmospheric sciences in Purdue's School of Science.

"If the skeleton is indeed an Australopithecus, as we believe, the findings could mean that these hominids were present in the area far earlier than is generally accepted."

The research was published in the April 25 issue of Science.

Tracing the development and spread of the hominid species that may have been mankind's ancestor is an arduous process, and it is difficult to determine what happened because precisely dated fossil records are hard to come by. Many such fossils have been found in eastern Africa's Rift Valley, a region that was geologically active when Australopithecus walked the Earth.

The abundance of lake sediments and volcanic ash that often surrounds Rift Valley hominid fossils provide good clues as to their age. But there is no such luck with similar fossils from South Africa, a region that also is rich in hominid remains but lacks the definitive geological clues that are present in the Rift Valley.

T. C. Partridge and R.J. Clarke, researchers from the University of the Witwatersrand in South Africa, were thus confronted with a mixed blessing when, in 1997, they discovered a nearly complete skeleton of what appeared to be an Australopithecus buried in the sediments on the floor of the Sterkfontein cave in central South Africa.

The fossil was well preserved, but its age was uncertain. It was more than 2 million years old, but how much more? The answer to that question would affect theories of how and when Australopithecus spread through Africa.

"Their initial estimate of 3.3 million years provoked a lot of controversy," Granger said. "Few thought that Australopithecus had traveled so far so long ago, and scientists wanted more proof. If the estimate was accurate, it might require a rethinking of human prehistory."

A firmer answer would require the innovative use of a particle accelerator half a world away at Purdue in Indiana, where Granger was using radioactive isotopes in sediment to determine the age of rivers and caves.

Dating fossils by examining the minute quantities of radioactive elements they contain is not a new technique in archaeology. Carbon-14, a radioactive isotope that slowly decays as centuries pass, has been a common benchmark for dating many human fossils; the more carbon-14 has decayed in a sample, the older it must be.

The speed at which carbon-14 decays, called its half-life, is only 5,730 years. This means that after a few millennia, the isotope is no longer useful to mark a fossil's age.

"Once your fossil is older than 50,000 years, its carbon-14 is nearly gone," Granger said. "We knew the South African fossils were at least 2 million years old, so it was clear we needed another way to establish their age."

Granger, a physics major as an undergraduate, eventually became an earth scientist when he developed an interest in studying the age and changes in mountains and river beds, geological formations that are often many millions of years old.

Rather than use carbon-14 to date his landscapes, he looked for isotopes with far longer half-lives � and found them in aluminum-26 and beryllium-10. These elements often form in common quartz when it is on the Earth's surface and exposed to cosmic rays.

"When radiation from outer space strikes silicon and oxygen atoms in a quartz crystal, they split into aluminum-26 and beryllium-10, both of which have half-lives of around a million years," Granger said. "Because they decay so slowly, they allow you to reach back much further in history than you can using carbon-14."

The use of aluminum and beryllium for radiometric dating had only been around for a few years when Granger began to use it on cave sediments. The technique was not widely known outside geological circles, so when he heard about the Australopithecus discovery, he contacted the South African scientists who found the skeleton and asked if he could be of assistance.

"It was the first time this method had been used to determine the age of something that had been alive," Granger said. "But based on the evidence, we found that the fossil was even older than the initial estimate."

Granger and Caffee, of Purdue's physics department, analyzed samples of the skeleton in Purdue's accelerator mass spectrometer, a device capable of detecting the infinitesimal quantities of radioactive aluminum and beryllium in the samples.

"We found that the skeleton was between 3.5 and 4.5 million years old," Granger said. "That's admittedly quite a large window of possibility, but even if it's on the young side, it still puts Australopithecus in southern Africa far earlier than expected."

The significance for anthropologists would be the possibility that mankind's earliest ancestors were a different species than scientists generally believe.

"Early hominid finds in eastern Africa have traditionally been regarded as the base populations from which all later hominids sprung," said Melissa Remis, associate professor of anthropology in Purdue's School of Liberal Arts.

"An earlier hominid find in southern Africa, whatever the species might be, could force a rethinking of what that base population might have been."

Granger said he hesitated to speculate on the broader significance of the findings, but he hopes the technique will prove useful in other investigations into prehuman history.

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