By Paul Rincon BBC News Online science staff

Our ancestors may have used fire to cook food

US and South African experts analysed burnt bones from Swartkrans, just north of Johannesburg, using the technique of electron spin resonance.

It showed the bones had been heated to high temperatures usually only achieved in hearths, possibly making it the first evidence of fire use by humans.

The results will be presented at the 2004 Paleoanthropology Society Annual Meeting in Montreal, Canada, in March.

These bones could have been burnt in a forest fire or brush fire but that's generally a low temperature flame.These had been heated to a very high temperature Dr Anne Skinner, Williams College

The research is a collaboration between South African researchers Dr Bob Brain and Dr Francis Thackeray of the Transvaal Museum in Pretoria, and researchers at Williams College in Williamstown, US.

"These bones could have been burnt in a forest fire or brush fire but that's generally a low temperature flame. These had been heated to a very high temperature," Dr Anne Skinner, of Williams College, told BBC News Online.

Electron Spin Resonance (ESR) looks at free radicals, fragments of molecules produced by a variety of processes, such as radiation damage or fire.

Studying the light signature, or spectra, produced by these free radicals can give scientists information on the nature of the damage.

As organic material, such as bone and collagen, is broken down by heating, the particles get smaller and smaller until only the carbon is left.

Given the brush

"What I was doing was taking these bones and seeing whether in fact I could see electron spin resonance spectra getting progressively smaller and ending up with carbon," said Dr Skinner.

Forest or brush fires usually only reach temperatures of around 300 degrees Celsius. But hearths or campfires can reach temperatures of 600 degrees Celsius or more.

The burnt bones were first described by Dr Bob Brain and Dr Andrew Sillen of the University of Cape Town in 1988. Dr Brain found that the burnt bones from Swartkrans could be sorted into types that had been burnt at low and high temperatures.

He also found that if modern bones were heated at low temperatures for long periods of time they began to look like bones that had been heated to high temperatures in a camp fire.

However, the electron spin resonance data would seem to confirm original suggestions about the bones.

This is because the degree of carbonisation of organic material as measured with electron spin resonance is dependent only upon the amount of carbon and not on the time material has been heated for.

It is not known which hominid species made the fires at Swartkrans. There seem to have been two hominid species present at Swartkrans around two million years ago.

These were Australopithecus (or Paranthropus) robustus and an early species of Homo, possibly Homo erectus.

The next oldest evidence for controlled use of fire may come from Zhoukoudian in China, dating to between 400,000 and 250,000 years ago.