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The outer limits of life just got stranger. Two chemicals that swiftly kill most living things may be harmless or even helpful to some unusual forms of life, suggesting that there are more ways of sustaining life than we realised.

"Life as we know it could be much more flexible than we generally assume or can imagine," says Felisa Wolfe-Simon of NASA's Astrobiology Institute and the US Geological Survey in Menlo Park, California. Her team grew bacteria that are apparently able to substitute the deadly poison arsenic for phosphorus, one of the six chemical elements thought to be essential for life, even replacing the phosphate backbone of DNA with one based on arsenic (Science, DOI: 10.1126/Science.1197258). The bugs could represent part of a "shadow biosphere" - a parallel form of life on Earth with a different biochemistry to all others.

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