Microbes locked in Antarctic ice for as much as eight million years have been "resuscitated" in a laboratory.
The samples came from ice in cold, secluded valleys
Researchers melted five samples of ice from the debris-covered glaciers of Antarctica which range in age from 100,000 years to eight million years.
When given nutrients and warmth, the microbes resumed their activity - although younger microorganisms grew more successfully than the older ones.
Details appear in Proceedings of the National Academy of Sciences.
The findings raise the possibility that ancient bugs, long frozen in ice, will return to life as climate change causes the glaciers to melt, flushing their genetic material into the oceans.
However, experts say this process has been going on for billions of years, and is unlikely to cause human disease.
Kay Bidle of Rutgers University in New Jersey, US, and colleagues extracted bacteria from ice found between three and five metres beneath the surface of a glacier in the Beacon and Mullins valleys of Antarctica.
"The ice sheets are continually undergoing accumulation, so they are flowing outward and the ice is lost through sublimation or calving into the ocean," explained co-author David Marchant of Boston University, US.
"What you have to do to get very old ice is go to secluded, very cold areas, where small alpine glaciers are covered by debris."
The combination of slow-moving glaciers with a debris covering that prevents ice from subliming - or evaporating - means that very ancient ice is preserved in these regions.
The ice gets older as it flows away from the headwall, where the snow and ice of the glacier first accumulates.
The researchers took five samples that were between 100,000 and eight million years old and were able to extract DNA and microbes from them. More organisms were found in the young samples than in the old.
"We tried to grow them in media, and the young stuff grew really fast," Dr Bidle explained.
"We recovered them easily; we could plate them and isolate colonies."
The cultures grown from organisms in the 100,000-year-old ice doubled in size every seven days on average.
By contrast, microbes from the eight million-year-old ice grew much more slowly, doubling every 30-70 days.
This suggested some microorganisms in this old ice were alive, but only just. Their DNA had been severely damaged by long exposure to cosmic radiation. This radiation is stronger at the poles, where the Earth's protective magnetic field is weakest.
A scanning electron microscope revealed bacteria-like cells (arrows)
The researchers were unable to identify them as they grew, because their DNA had degraded so much.
The researchers found that DNA in the five samples examined showed an exponential decline in quality after 1.1 million years.
In the younger samples, the team found evidence of some of the most common bacteria around today, including the firmicutes, proteobacteria and actinobacteria.
But the team also compared genetic sequences extracted from the ice to known genes of modern bacteria. Curiously, there were few matches, meaning the ancient microbes may have had genes that were new to science.
Dr Bidle and his colleagues describe the glaciers as "gene popsicles" containing DNA that can be acquired by existing organisms when it is thawed.
Eske Willerslev, of the University of Copenhagen, Denmark, who was not involved in the research, described the work as "very significant".
But he cautioned that, as with most claims of ancient microbes being revived, contamination of samples with genetic material from modern microbes was always a possibility.
"These results show patterns that you can't easily explain by contamination," he told ScienceNow, "But I would feel more comfortable with the results if they had been replicated in two independent labs."
Dr Marchant said temperatures in the Beacon and Mullins valleys were so cold that any liquid melting on the surface only penetrated about 5cm into the ice. Samples were retrieved from below this depth in all cases.
"There's really no chance for modern contamination," he explained.
The team suggests that because DNA in the old ice samples had degraded so much in response to exposure to cosmic radiation, life on Earth is unlikely to have hitched a ride on a comet or on debris from outside the Solar System - as some scientists have suggested.
"Given the extremely high cosmic radiation flux in space, our results suggest it is highly unlikely that life on Earth could have been seeded by genetic material external to this Solar System," they wrote in their scientific paper.
Dr Marchant added: "The other thing that's interesting about this is the connection to Mars. There's near-surface ice on Mars where the surface landform looks identical to what you'll see in Beacon Valley."
On Saturday, Nasa launched its Phoenix spacecraft on a nine-month journey to the Red Planet. It will dig below the surface of Mars' northern plains to collect samples of soil and near-surface ice for analysis.