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 that 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 this week, including calls to NASA from the White House asking whether a second line of earthly life has been found.
A NASA press conference 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 theorized "shadow biosphere" on Earth - life evolved from a different common ancestor from all 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, 33, the biochemist who led the effort.
Prompted by debate about the possible existence of a shadow biosphere, Wolfe-Simon 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.