[cdn-nucl-l] Pre-human fossil four million year old

["Adam McLean" <adam.mclean@utoronto.ca>]

Posted in the Globe and Mail on April 24, 2003 and at:
http://www.globeandmail.com/servlet/story/RTGAM.20030424.wbone424/BNStory/In
ternational
Results from Purdue but a bit of a healthy critique by McMaster.

Adam

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Pre-human fossil four million year old 
Associated Press 

A new dating technique suggests that a human-like fossil skeleton found in
South Africa was buried about four million years ago, which makes it one of
the oldest known hominid discoveries.

That's a million years earlier than previously thought.

The nearly complete skeleton came from the Sterkfontein 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
Australopithecus, an extinct form of pre-humans.

Skeleton fossils unearthed in 1997 were age dated at about three 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 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 by an earth
movement, the cosmic ray bombardment stops, but the isotopic decay
continues.

Dr. Granger and Dr. Caffee measured the decay of the isotopes in sediments
collected from the caves where the hominid fossils were found to establish
the new age dates.

Australopithecus is an extinct hominid species that lived from about four to
2.5 million years ago. Lucy, a famed pre-human fossil find 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.

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, Ont., said in Science
that the collapsed cave ceilings may have dropped surface sediments into the
area where the fossils were found and thus distorted the age-dating results.
Dr. Schwarez said that the Purdue researchers need to test the technique
using sediments from South African caves of a known age.