LONDON (Reuters) - Ancient bacteria are able to survive nearly half a million years in harsh, frozen conditions, researchers said on Monday in a study that adds to arguments that permafrost environments on Mars could harbour life.

The findings also represent the oldest independently authenticated DNA to date obtained from living cells and could offer clues to better understand ageing, said Eske Willerslev, a researcher at the University of Copenhagen who led the study.

"When it can live half a million years on Earth it makes it very promising it could survive on Mars for a very long time," Willerslev said. "Permafrost would be an excellent place to look for life on Mars."

The international team, which also included researchers from the United States, Canada, Russia and Sweden, tested the microbes living up to 10 metres deep in permafrost collected from Northern Canada, the Yukon, Siberia and Antarctica.

When a cell dies, its DNA fragments into pieces but the samples the researchers studied were all very long strands -- evidence the cells were able to continuously repair genetic material and remain alive, said Willerslev, whose findings were published in the Proceedings of the Academy of Sciences.

"These cells are active cells repairing DNA to deal with continuous degradation of the genomes, which is the genetic material that is key to life," he said in a telephone interview. "It is the same thing with humans."

The scientists do not yet know the mechanism driving the continuous repair but Willerslev said the cells survived by eating nutrients like nitrogen and phosphate lodged in the permafrost.

This is interesting because the temperature on Mars is much colder with more stable temperatures, representing an even better environment to sustain this kind of life, he added.

While most scientists think our neighbour in the solar system is lifeless, the discovery of microbes on Earth that can exist in environments previously thought too hostile has fuelled debate over extraterrestrial life.

Researchers had known these microbes could survive for a long time without food but until now there was little agreement on how long they could live, Willerslev said.

Knowing this, and eventually pinpointing the key to this longevity, may also help scientists better understand the ageing process, he added.

"It is interesting to see why some cells can survive for a very long time," he said. "That can be a key for understanding ageing."