There is no amount of life that could generate those, forgive me, titanic amounts of methane ;-)
If there is some life there, it must be either very efficient in generating methane or very good at hiding itself ;-)
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Well, the Earth itself has been transformed by life! Most of the oxygen in the atmosphere comes from billions of years of photosynthesis:
- http://en.wikipedia.org/wiki/History_of_Earth#Photosynthesis...
- http://en.wikipedia.org/wiki/Earth%27s_atmosphere#Evolution_...
Most probably, a big chunk of the oxygen in the atmosphere comes from last month's photosynthesis - we use it to make CO2 and green stuff converts it back, so, it's always brand-new oxygen we see. But free oxygen indicates a far more active biosphere than lots of methane would and I get your point - methane makers are much lower profile than oxygen makers.
As for Titan being covered with life... We really don't know if the lakes we see from space aren't giant amoebas waiting patiently to eat any humans that land on their beaches and just cleverly pretending to be lakes. ;-)