life involves molecules figuring out how to catalyze geochemical reactions to exploit the energy being released by them. These are some of the earliest known metabolisms (see methanogenesis). In a case such as this, scientists might search for a disequilibrium in the geochemical cycle, which would point to a reaction happening more or less often than it should. A disequilibrium such as this could be interpreted as an indication of life. Survivability A biosignature must be able to last for long enough so that a probe, telescope, or human can be able to detect it. A consequence of a biological organism's use of metabolic reactions for energy is the production of metabolic waste. In addition, the structure of an organism can be preserved as a fossil and we know that some fossils on Earth are as old as 3.5 billion years. These byproducts can make excellent biosignatures since they provide direct evidence for life. However, in order to be a viable biosignature, a byproduct must subsequently remain intact so that scientists may discover it. Detectability For a biosignature to be relevant in the context of scientific investigation, it must be detectable with the technology currently available. This seems to be an obvious statement, however there are many scenarios in which life may be present on a planet, yet remain undetectable because of human-caused limitations. False positives Every possible biosignature is associated with its own set of unique false positive mechanisms, or non-biological processes that can mimic the detectable feature of a biosignature. An important example of this is using oxygen as a biosignature. On Earth, the majority of life is centered around oxygen. It is a byproduct of photosynthesis and it is subsequently used by other forms of life to breathe. Oxygen is also readily detectable in spectra, with multiple bands across a relatively wide wavelength range, therefore it makes a very good biosignature. However, finding oxygen alone in a planet's atmosphere is not enough to confirm a biosignature because of the false-positive mechanisms associated with it. One possibility is that oxygen can build up abiotically via photolysis if there is a low inventory of non-condensible gasses or if it loses a lot of water. Finding and distinguishing a biosignature from its potential false-positive mechanisms is one of the most complicated parts of testing for viability because it relies on human ingenuity to break an abiotic-biological degeneracy, if nature allows.