As well as looking for alien life in space, should we also be looking in time?

How do we know we are the first (indigenous or not) intelligent beings to inhabit this solar system? If there had been a prior civilisation on Earth we would probably have found some evidence of it, but if there had been one on Mars of Venus how would we know?

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"I worry that, especially as the Millennium edges nearer, pseudo-science and superstition will seem year by year more tempting, the siren song of unreason more sonorous and attractive." - Carl Sagan, 1995