BBC:

While bacteria have been found in inhospitable environments and can consume what other life finds poisonous, this bacterial strain has actually taken arsenic on board in its cellular machinery.

Until now, the idea has been that life on Earth must be composed of at least the six elements carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus - no example had ever been found that violates this golden rule of biochemistry.

The bacteria were found as part of a hunt for life forms radically different from those we know.

This reminds me of the point that Stephen Jay Gould makes in the essay "Eight Little Piggies." For many years, there was the idea that the pentadactyl limb (five fingers) was the original or primal form of limbs. Everywhere we looked, we found tetrapods (four-legged animals) with pentadactyl limbs. And this meant that something like the human hand was the canonical form, and animals with fewer fingers or fused fingers, like a horse's hoof, was the advanced form. The question was this: Why five? What made five important and original? This question was asked until paleontologists started finding fossils of tetrapods with six and seven fingers. Five was not the magic number. It was one out of several possibilities. Meaning, with life—its meandering manner, many materials, and multiplicity of modes—one should always be suspicious of "golden rules."