James Lovelock's search for life on Mars led him to conclude that the gases in a planet's atmosphere indicated whether it harboured life or was dead. Through a series of complicated evolutionary dance steps life contrives to keep a planet habitable for life. Whether that life is intelligent or not is another matter.
Any extra-terrestrial observing Earth would conclude something momentous happened to our own atmosphere in the last 60 years. A continent sized hole appeared in the ozone layer - the layer that protects life from harmful radiation. The delicate balance of greenhouse gases started rising rapidly. The most important, carbon dioxide, rose rapidly from 280 parts million in the 18th century to 400 parts per million this year - a 43% shift. Unsurprisingly the planet's temperature went up a notch (1°C or 1.8°F).
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