Controversial claims have again emerged that Martian microbes could have established themselves here on Earth after hitching rides on meteorites.
D. radiodurans is one of the hardiest organisms known
A handful of bacteria on Earth today have the ability to survive exposure to extremely high levels of radiation that would kill other organisms.
Now, a team of scientists argues that the bugs could only have evolved this unusual ability on a planet like Mars.
The claims by a Russian-American team appear in the journal Astrobiology.
Recent discoveries of water in the permafrost on Mars and signs of subsurface water in mid-latitude regions have raised hopes that the Red Planet might host the right conditions for life.
According to one theory, impacts on the surface of the Red Planet could have thrown Martian rocks into space, which wandered the vaccum before tumbling through Earth's atmosphere and crashing down as meteorites.
If these Martian meteorites contained any life forms able to survive the journey, they might have been able to gain a foothold on our own planet, scientists speculate.
'Conan the bacterium'
The best-studied radiation-resistant microbe is Deinococcus radiodurans.
It can withstand several thousand times the lethal dose of radiation for humans, and has been nicknamed "Conan the bacterium" by microbiologists.
But other scientists say that radiation tolerance is a side effect of the defence mechanism bacteria such as D. radiodurans have developed to protect against dehydration.
D. radiodurans and other radiation-resistant bacteria survive because they are very efficient at repairing their DNA. But this is also useful for surviving extreme desiccation in arid environments.
The team led by Alexander Pavlov at the University of Arizona, US, rejects this alternative explanation.
"Our hypothesis of a Martian origin for radio-resistant bacteria provides an explanation for their ability to withstand ionizing radiation, a trait that appears to be of no value on Earth at any time in its history," the scientists write in Astrobiology.
The clement background levels of radiation on Earth are not thought to have changed significantly for the last four billion years.
As such, there has been no evolutionary pressure for bacteria to develop resistance to such high levels of radiation, the researchers argue.
But the Martian permafrost, where radiation levels are 100 times higher than on Earth, could provide a plausible environment where bacteria could pick up radiation-resistant genes.
In their paper, Professor Pavlov and his colleagues outline several pieces of evidence which cast doubt on the dehydration theory.
For example, they argue there is no evidence that the degree of resistance to radiation is related to the degree of resistance to dehydration in bacteria. Instead, Pavlov and his colleagues argue that these are independent attributes.
Scientists have also carried out experiments in which they blast "ordinary" bacteria with gamma rays, allow the survivors to recover, and then repeat the process again and again. After many cycles, the bacteria develop resistance to high levels of radiation.
But bugs exposed to successive cycles of dehydration and hydration in the lab develop resistance only to desiccation, not to radiation.
Scientists who are sceptical of the group's claims have pointed out that the genome of D. radiodurans is very similar to that of "ordinary" terrestrial bacteria, arguing against an extra-terrestrial origin.
Pavlov and his colleagues say that frequent exchanges of bacteria carried on meteorites between Mars and Earth could explain this similarity.
Nasa says around 34 of the 24,000 meteorites so far found on Earth have been identified as coming from Mars.