[sampo]

In a 1993 article, Carl Sagan considered the atmospheric oxygen content "at least suggestive of biology" (Conclusions, 3rd paragraph).

http://www-pw.physics.uiowa.edu/~dag/publications/1993_ASear...

[maaku]

Am I supposed to disagree with that? High atmospheric oxygen content is suggestive of biology. So is seasonal blooms of methane on a planet like Mars. Is that suggestive that there is life on Mars? Yes! Can we definitively infer that there is presently life on Mars? Not yet.