What the scientists found is that the bacteria appear to have kept up their metabolism.
These barely living bacteria did not seem to be reproducing, but they were still taking in nutrients and giving off carbon dioxide, like humans do when they breathe.
The bacteria were using some of these resources to keep their DNA in good shape, the study authors said.
But the researchers found that bacteria couldn't keep chugging along like this forever.
"You see a large diversity [of bacteria] in the modern samples, and as you get older and older, the diversity declines," Willerslev said.
The amount of carbon dioxide the bacteria gave off also dropped with age.
The limit for life in the permafrost is somewhere around 600,000 years old, the researchers say.
In older permafrost, the team couldn't detect any carbon dioxide emissions or any large pieces of DNA indicative of living bacteria.
By about 750,000 years old, the bacteria trapped in the permafrost seemed to be completely dead.
Soil vs. Ice
Some scientists have claimed to be able to revive far older bacteria preserved in amber or salts, but Willerslev has doubts about these results.
"I've been extremely skeptical about these previous results," Willerslev said.
But in the much colder environments of Mars or Europa, life might be able to survive while frozen for much longer, Willerslev said.
At those lower temperatures, DNA damage would accumulate more slowly.
So the new results "could suggest that if you had similar life on Mars, it could exist for much longer," he said.
Brent Christner of Louisiana State University welcomes the new results, which he finds convincing.
Christner and others have been studying ancient ice from deep in the Antarctic ice sheet and have found live bacteria there that have been frozen in place for perhaps one to two million years.
These ancient bacteria seemed to be repairing themselves, but the team didn't have direct evidence showing how the microbes were surviving so long.
"This study confirms and corroborates everything we've been finding with ancient glacial ice," Christner said.
Still, Willerslev is cautious about making this connection.
Glacial ice, he said, "is a completely different environment from permafrost, which is basically frozen soil" and contains lots of nutrients.
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