Is Methane a good marker for life?
Space.com has an article discussing this very subject today. Recently the discovery and confirmation of persistent methane in Mars' atmosphere has fueled speculation that there could be active life on Mars today. Currently accepted theory is that methane would be broken down by solar radiation and must be constantly replaced to be in the atmosphere, with volcanism and life as the two most likely sources and no sign of active volcanism on Mars today.
Another theory that is less accepted, but which evidence is being found to support, states that methane, and other hydrocarbons such as oil and coal, are formed through chemical processes in a planets mantle rather than through the decay of organisms. Furthermore the theory states that the "signs of biological life" in the oil and coal deposits is not the unprocessed decayed matter that formed the hydrocarbons, but is in fact life the fed on those deposits deep under the crust. With the finding of radical extremophiles in every location on Earth, including deep underground, this idea may be gaining strength, and could have implications for finding active life on Mars deep under its crust feeding on the hydrocarbons being created in Mars' mantle.
This also has implications in the search for exoplanets. Some have stated that finding certain chemical signatures in the atmospheres of distant world's would be indicative of life, and methane in combination with oxygen, nitrogen, and CO2 would be the markers to look for, but if methane is a natural chemical byproduct of subsurface chemical reations, then maybe it wouldn't mean that we've found another Earth.