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Arsenic-based bacterium may be 'second line' of life

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 usually 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 last week and an accompanying online 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 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."

Theoretical physicist and cosmologist Paul Davies, director of the Beyond Center at Arizona State University and a prolific writer, is a co-author on the paper. He had been thinking about the idea for a decade, and had written a paper in 2005.

So had University of Colorado, 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."

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