Alien life might be hard to find for the simple reason that it is fundamentally unlike Earth life. It might not use DNA, or contain protein. But whatever and wherever it is, its tendency to chemically alter its environment might just give it away.
Life has had a radical impact on Earth's chemistry - perhaps most notably leading to soaring atmospheric oxygen concentrations around 2.2 billion years ago. If life has had a comparable impact elsewhere in the solar system, the relative abundances of chemicals key for its survival - whatever they may be - could betray its presence.
On Earth, those key chemicals, such as amino acids, are modestly sized, exist in relatively small numbers and act as building blocks to form all complex life. Identifying the alien equivalents of those chemical building blocks "is our only way to detect life not as we know it", says astrobiologist Chris McKay of NASA's Ames Research Center in Moffett Field, California. He calls this the Lego principle.
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