WASHINGTON — All life on Earth — from microbes to elephants to us — is based on a single genetic model that requires the element phosphorus as one of its six essential components.
But now researchers have uncovered a bacterium that has five of those essential elements but has, in effect, replaced phosphorus with its look-alike but toxic cousin, arsenic.
News of the discovery caused a scientific commotion, including calls to NASA from the White House and Congress asking if a second line of Earthly life has been found.
A NASA news conference Thursday and an accompanying article in the journal Science, gave the answer: No, the discovery does not prove the existence of a so-called "second genesis" on Earth. But the discovery very much opens the door to that possibility, and to the related existence of a theorized "shadow biosphere" on Earth — life evolved from a different common ancestor than all that we've known so far.
"Our findings are a reminder that life-as-we-know-it could be much more flexible than we generally assume or can imagine," said Felisa Wolfe-Simon, the young biochemist who led the effort after being selected as a NASA Astrobiology Research Fellow and as a member of the National Astrobiology Institute team at Arizona State University.
"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?" she said.
The research, funded through NASA and conducted with samples from California's Mono Lake, found that some of the bacteria not only used arsenic to live, but had arsenic embedded into their DNA, RNA and other basic underpinnings.
"This is different from anything we've seen before," said Mary Voytek, senior scientist for NASA's program 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 make-up," 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."
Theoretical physicist and cosmologist Paul Davies, director of the Beyond Center at Arizona State University and a writer, had been thinking about the idea for a decade, and had written a paper in 2005. So had University of Colorado at Boulder philosopher and astrobiologist Carol Cleland. Both asked why nobody was looking for life with different origins on Earth, and Cleland coined the phrase "shadow biosphere."
At a Beyond Center conference four years ago, 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," 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, sulfur 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 raises concerns for some researchers familiar with the Mono Lake bugs.
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 Arizona State University professor and biogeochemist. "Felisa's study teaches us that we ought to think harder about which elements to follow."
Mono Lake receives runoff from the Sierra Nevada mountains, which have high concentrations of arsenic. When the water arrives, it has nowhere to go because there are no rivers carrying water further downstream. Arsenic is present in Mono Lake at a concentration 700 times greater than what the EPA considers safe.