Again from NASA's Astrobiology website … and answered by: David Morrison
Astrobiology Senior Scientist, date: August 25, 2011:
Quote:
Question:
I understand that you have found a life form in the Mono desert that can stand high levels of arsenic, and began to wonder if an arsenic, if not phosphorus, was used by it in its DNA. Could not this be a representative of the earliest life on Earth, and that in some point phosphorus came into the picture and the organism survived, and then another story began.
Phosphorus is one of a handful of essential elements for life as we know it on Earth. This element is part of the molecular backbone of DNA and plays a key role in the storage and transfer of chemical energy within cells. Arsenic has a similar atomic structure to phosphorus, but it is not important in biochemistry, and in large quantities it is a poison.
The experiments you refer to were carried out by a team led by Felisa Wolfe-Simon (NASA and U.S. Geological Survey). As summarized by Dennis Overbye in the New York Times:
"Seeking evidence that life could follow a different biochemical path than what is normally assumed, Dr. Wolfe-Simon grew [microbes from Mono Lake] in an arsenic-rich and phosphorus-free environment, reporting in the paper and a NASA news conference on Dec. 2 [2010] that the bacterium, strain GFAJ-1 of the Halomonadaceae family of Gammaproteobacteria, had substituted arsenic for phosphorus in many important molecules in its body, including DNA."
But the same article notes that critics have said that "the experimenters had failed to provide any solid evidence that arsenic had actually been incorporated into the bacterium's DNA."
This debate is still continuing in the astrobiology community. There is no question that arsenic was chemically incorporated into the bacterium. What is less clear is whether the arsenic actually substituted for phosphorus in functional DNA molecules. This is an important debate that will likely not be resolved for many months.
As Overbye wrote at the end of his article, "Only more data ... will tell whether the GFAJ-1 bug is weird life that has found a new way to live, or just tough and able to survive in arsenic." We need to understand the answers to these questions before we can profitably speculate about possible arsenic-based lifeforms. You can read more about this research on the NASA Astrobiology website here and here.
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So, it seems that the 'evidence' for fundamental substitutions of base elements like phosphorus for arsenic in DNA sequences, is not necessarily agreed nor straightforward. So, the possibility of this permutation arising on Mars, doesn't look to be a straightforward 'given', either.
Cheers