We strongly suspect that the DNA that we have is that of a hominid, but we still want to conduct more research to verify our claim

Tom Loy, University of Queensland

This is the claim that is dividing the archaeological community. Two researchers say they have extracted the DNA of a 1.8-million-year-old hominid from stone tools excavated at the Sterkfontein Caves near Johannesburg.

"The DNA we have found is something between a chimpanzee and a human, which suggests a hominid," Bonnie Williamson of Wits University, South Africa, told a Johannesburg newspaper this week.

"We strongly suspect that the DNA that we have is that of a hominid, but we still want to conduct more research to verify our claim," added co-researcher Tom Loy of the University of Queensland, Australia.

'Highly improbable'

The scientists believe that the DNA is that of either Homo habilis, thought to be a direct human ancestor, or Paranthropus robustus, a flat-faced hominid. If confirmed, it would be the oldest DNA yet extracted. But the news has been greeted with scepticism by some experts.

Sterkfontein Caves

One of the richest sites of early hominid fossils in Africa Upright-walking creatures roamed the landscape at the site over three million years ago In 1936, the first adult skull of Australopithecus africanus (2.6-3-million-year-old human-like creature) was discovered Subsequent finds include stone tools and many hominid skulls and bones

"It seems very unlikely," Chris Stringer of London's Natural History Museum told BBC News Online. "It would be astonishing if such material could survive under these conditions for two million years."

It is a view shared by Alan Cooper of the Ancient Biomolecules Centre at Oxford University, UK. The likelihood that DNA could survive the heat of Africa for more than 10,000 years was, he said, "highly improbable, in fact almost impossible".

"Sadly, it is part of continuing claims of ancient human DNA finds lacking experimental and intellectual rigor," said Dr Cooper.

Contamination concerns

The issue at stake is the technical difficulty of piecing together stretches of old DNA that have decayed over the passage of time. Some researchers believe that it is chemically impossible to isolate DNA that is older than 100,000 years. The possibility of contamination from extraneous sources of DNA during the handling of specimens has raised further concerns.

Critics point out that even a sneeze, a speck of dust containing skin or hair, or a drop of sweat, could leave traces of human DNA on a sample.

Ancient DNA controversies

DNA extracted from an 80-million-year-old dinosaur bone turned out to be that of a mammal Australian scientists announced in January that they had extracted a tiny amount of DNA from the country's most ancient human skeleton, the 60,000-year-old Mungo Man - some scientists have cast doubts

The Wits/Queensland researchers say they have taken rigorous precautions to minimise contamination. Ken Dusza, a graduate student in Professor Loy's laboratory, says methods have advanced so much in recent years that it is quite feasible that hominid DNA could survive for more than a million years.

"We have improved our techniques beyond what initially gave ancient DNA analysis a bad name," he told BBC News Online.

"Now, it's a very well-developed and almost foolproof science in that we know when we have contamination and we take great precautions to ensure that we don't get contamination."

The Queensland researchers say they plan to publish the findings in a leading scientific journal soon.