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ASTROBIOLOGY

than today. Instead, there was still no trace

of biogenic oxygen, whose production

would have started about a half-billion

years later ( 3 billion years ago or perhaps

even earlier), with the advent of photosyn-

thesis, the currently dominant metabolism

on our planet. Until the middle of the Pro-

terozoic ( 2. 0 - 0. 7 billion years ago), the

abundance of atmospheric oxygen re-

mained very scarce, reaching probably only

0. 1 % of the present level, and its presence

would have been inferred from a possible

faraway civilization only through the spec-

tral recognition of ozone (O 3 ), a photo-

chemical by-product of oxygen that pro-

duces a strong signal in the UV even when

oxygen itself is too scarce to leave a trace.

In the last half-billion years, however, the

disequilibrium in the atmospheric compo-

sition generated by life has become in-

creasingly evident. Being able to analyze

the terrestrial atmosphere through a spec-

troscope from a distant planet, you would

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