"There could be different building blocks used, different types of elements, or different types of energy sources," he says.
And now, along comes a terrestrial form of life that can replace phosphorus with arsenic and thrive.
Other bacteria in effect "breathe" arsenic or use it as a food source, converting it into something less harmful. But the newly discovered bacteria incorporate it into the very fabric of their most basic biological structures.
It's as though "the recipes to make the fundamental molecules of life are open for negotiation," adds James Elser, a biologist at Arizona State University who was not part of the group making the discovery.
Dr. Wolfe-Simon says the hunt for organisms that might be based on different groups of chemical elements began when she noticed that some of the elements organisms use in very small amounts – such as iron, zinc, and molybdenum – had ready substitutes. These substitutes are similar enough to the elements they would replace that many organisms can use them as stand-ins if the originals are unavailable.