All life on Earth, from microbes to elephants and us, requires the element phosphorus as one of its six components.
But now researchers have discovered a bacterium that appears to have replaced the life-enabling phosphorus with its toxic cousin arsenic, raising new and provocative questions about the origins and nature of life.
News of the discovery caused a scientific commotion last week, including calls to Nasa from the White House asking whether a second line of earthly life has been found.
A Nasa press conference on Thursday and an accompanying article in the journal Science said the answer is no. But the discovery opens the door to that possibility and to the related existence of a theorised "shadow biosphere" on Earth - life that evolved from a different common ancestor from all we've known so far.
Felisa Wolfe-Simon, the biochemist who led the effort, said: Our findings are a reminder that life as we know it could be much more flexible than we generally assume or can imagine."
Prompted by debate about the possible existence of a shadow biosphere, Dr Wolfe-Simon, 33, set out specifically to see whether microbes that lived in California's briny, arsenic-filled Mono Lake naturally used arsenic instead of phosphorus for basic cellular functions, or were able to replace the phosphorus with arsenic.
She took mud from the lake into the lab and began growing bacteria in Petri dishes. She fed them sugars and vitamins but replaced phosphate salt with arsenic until the surviving bacteria could grow without needing the phosphates at all.
Her research found that some of the bacteria had arsenic embedded into their DNA, RNA and other basic underpinnings.
"If something here on Earth can do something so unexpected - that breaks the unity of biochemistry - what else can life do that we haven't seen yet?" said Dr Wolfe-Simon, a Nasa Astrobiology Research Fellow and member of the National Astrobiology Institute team at Arizona State University.
"This is different from anything we've seen before," said Mary Voytek, a senior scientist for Nasa's programme in astrobiology, the arm of the agency involved specifically in the search for life beyond Earth and for how life began here.
"These bugs haven't just replaced one useful element with another; they have the arsenic in the basic building blocks of their makeup," she said. "We don't know if the arsenic replaced phosphorus or if it was there from the very beginning - in which case it would strongly suggest the existence of a shadow biosphere."
The theoretical physicist and cosmologist Paul Davies, the director of the Beyond Centre at Arizona State, is a co-author on the new Science paper. He had been thinking about the idea of a shadow biosphere for a decade and had written a paper on it in 2005. The same year, the philosopher and astrobiologist Carol Cleland and the biochemist Shelley Copley, both of the University of Colorado at Boulder, teamed up to publish on the subject. Both asked why nobody was looking for life with different origins on Earth, and Dr Cleland coined the phrase "shadow biosphere".
At a Beyond Centre conference four years ago, Dr Wolfe-Simon, then in her late 20s, proposed a way to search for a possible shadow biosphere, and it involved Mono Lake and its arsenic.
"We were kicking vague ideas around, but she had a very specific proposal and then went out and executed it," Dr Davies said. "It defies logic to think she found the only example of this kind of unusual life. Quite clearly, this is the tip of a huge iceberg."
All life as we know it contains six essential elements - carbon, oxygen, hydrogen, nitrogen, sulphur and phosphorus - that have qualities that make them seemingly ideal for their tasks. A form of phosphorus, for instance, is near perfect for building the framework for the DNA molecule, and another form is crucial to the transfer of energy within cells.
These forms of phosphorus are well suited for their job because they are especially stable in the presence of water. Arsenic is not, and that fact is one that raises concerns for some researchers familiar with the Mono Lake organisms.
The paper and its results have created an excitement reminiscent of the announcement in 1995 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 proved.
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 has until now been understood to be possible.
Replacing phosphorus with arsenic may seem suicidal, but the two are very similar in their makeup. Arsenic is considered toxic because most living things take it in and treat it like phosphorus, only to be destroyed by the small differences.
While small amounts of the phosphorus remained in the arsenic-based bugs, it was definitely not enough to supply the presumed phosphorus needs of the cell. That was being done with the arsenic.
Dr Wolfe-Simon said she hopes to test her findings in northern Argentina, where there is an arsenic-heavy ecosystem that also seems to support microbial life.
* The Washington Post