It is life, but not as we know it. The first life form on Earth to use deadly arsenic as one of its chemical building blocks has been discovered in a desert lake in California.

The find has astonished biologists who had believed that it was impossible for arsenic, normally considered a poison, to be used as building material for biological molecules such as DNA and proteins.

Scientists said last night that the discovery widens the scope for finding "weird" forms of extraterrestrial life on other planets that would have been considered – until now – to be too inhospitable for living organisms to evolve. For decades it has been accepted dogma in biology that all life on Earth, from the tiniest microbe to the biggest blue whale, relies on the same six chemical elements as the fundamental building blocks of biological molecules.

It was thought that the same principle must hold true for any extraterrestrial life forms living on other planets, which narrowed the search for habitable places where life could evolve. [ INDEPENDENT UK ]