Evidence of Life is Obvious (For some definition of obvious)
Written sometime in late 2004.
Lovelock wrote in one of his books about Gaia that since a biosphere is self-stabilizing, and can maintain a dynamically unstable state (e.g. an atmosphere with lots of volatile stuff like oxygen) then based on spectroscopic observations of the atmosphere it should be concluded that there is no extant life on Mars since it isn't obvious and widespread.
In retrospect, this is somewhat naive.
In the late 1800s, life "obviously" would have radios and collosal engineering, hence people listened for radio transmissions and saw imaginary canals on Mars.
In the mid 1900s, life "obviously" had a biosphere encompassing the atmosphere and surface.
Since then we've learned that the extreme subsurface and other locations are places life can hang out - thermal vents, deep ice lakes, deep cores, etc. We can also conceive of radically different timescales and substrates for metabolic processes.
I admit, my instincts say that if Mars had life for any significant period of time I'd have expected more obvious indicators. But then again we've had 3.5+ billion years of life on Earth and hence a ton more variability and sources for adaptation, and many deserts (hot and cold) still look pretty Mars-like on the coarse scale we've examined Mars at.