An oxygen-rich environment is so thermodynamically unstable (it would lead to oxidation and rusting of virtually every other prevalent element) that it would be exceedingly short-lived without the presence of oxygen-liberating biological metabolism. To that extent, a high-oxygen atmosphere is one of the very clear and detectable indicators of probable life which we are capable of detecting even on extra-solar planets (via spectroscopic analysis of reflected or filtered light).

Far more an Eden, then.