When Nasa called a press conference “to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life” there was “snowballing speculation” about what they might have found, and calls to Nasa from the White House and Congress. The Science journal’s embargo didn’t help – paper abstract here.
What they’ve found is a bacterium in California’s Mono Lake that, apparently, can use arsenic instead of phosphorus in its DNA. As the Nasa Astrobiology website notes
Up until now, it was believed that all life required phosphorus as a fundamental piece of the ‘backbone’ that holds DNA together. The discovery of an organism that thrives on otherwise poisonous arsenic broadens our thinking about the possibility of life on other planets, and begs a rewrite of biology textbooks by changing our understanding of how life is formed from its most basic elemental building blocks. Astrobiology Magazine has the story.
The Guardian quotes co-author of the study, cosmologist Paul Davies
Christened GFAJ-1, the microbe lends weight to the notion held by some astrobiologists that there might be “weird” forms of life on Earth, as yet undiscovered, that use elements other than the basic six in their metabolism. Among those who have speculated is Prof Paul Davies, a cosmologist at Arizona State University and an author on the latest research.
“This organism has dual capability – it can grow with either phosphorus or arsenic,” said Davies. “That makes it very peculiar, though it falls short of being some form of truly ‘alien’ life belonging to a different tree of life with a separate origin. However, GFAJ-1 may be a pointer to even weirder organisms. The holy grail would be a microbe that contained no phosphorus at all.”
As the BBC’s Jason Palmer notes
Until now, the idea has been that life on Earth must be composed of at least the six elements carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus – no example had ever been found that violates this golden rule of biochemistry.
The bacteria were found as part of a hunt for life forms radically different from those we know.
“At the moment we have no idea if life is just a freak, bizarre accident which is confined to Earth or whether it is a natural part of a fundamentally biofriendly universe in which life pops up wherever there are Earth-like conditions,” explained Paul Davies, the Arizona State University and Nasa Astrobiology Institute researcher who co-authored the research.
“Although it is fashionable to support the latter view, we have zero evidence in favour of it,” he told BBC News.
“If that is the case then life should’ve started many times on Earth – so perhaps there’s a ‘shadow biosphere’ all around us and we’ve overlooked it because it doesn’t look terribly remarkable.”
Proof of that idea could come in the form of organisms on Earth that break the “golden rules” of biochemistry – in effect, finding life that evolved separately from our own lineage.
And a cautionary note from a detailed, and informative, report in the Washington Post
Chemist Steven Benner of the Foundation for Applied Molecular Evolution in Florida has been involved in “shadow biosphere” research for several years, and will speak at the NASA unveiling of Wolfe-Simon’s work. He says that the Mono Lake results are intriguing – “I do not see any simple explanation for the reported results that is broadly consistent with other information well known to chemistry” – but he says they are not yet proven. And a primary reason why is that arsenic compounds break down quickly in water while phosphorus compounds do not.
His conclusion: “It remains to be established that this bacterium uses arsenate as a replacement for phosphate in its DNA or in any other biomolecule.”
Nonetheless, the paper and its results have created an excitement reminiscent of the 1995 announcement at NASA headquarters of the discovery of apparent signs of ancient life in a meteorite from Mars found in Antarctica. That finding was central to establishing the field of astrobiology, but was also broadly challenged and a scientific consensus evolved that the case for signs of life in the meteorite had not been proven.
The Mono Lake discovery highlights one of the central challenges of astrobiology – knowing what to look for in terms of extraterrestrial life. While it remains uncertain whether the lake’s microbes represent another line of life, they show that organisms can have a chemical architecture different from what is currently understood to be possible.
“One of the guiding principles in the search for life on other planets, and of our astrobiology program, is that we should ‘follow the elements,’ ” said Ariel Anbar, an ASU professor and biogeochemist. “Felisa’s study teaches us that we ought to think harder about which elements to follow.”
And that report has a great quote from lead researcher Dr Felisa Wolf-Simon
“Sometimes I’m asked why something like this has never been found before, and the answer is that nobody has run the experiment before,” Wolfe-Simon said. “There was nothing really complicated about it – I asked a simple question that was testable and got an answer.”
Adds Nasa Science podcast (with script)