On Intelligent Life

There's constant search for extra-terrestrial intelligence but could intelligent life have developed on earth in the past?

First let's define intelligent life - dolphins are intelligent, as are ravens, as are octopuses, as are wolves.  There are two things that make homo sapiens seemingly unique amongst intelligent species of today - tool usage and communication.  Whilst other species have tool-using abilities (ravens, octopuses, the great apes) and rich communication (dolphins and other cetaceans), no other current species combines these two characteristics, and it's allowed humans to dominate the planet.  It's unlikely that these two characteristics haven't occurred together in the same species at some point in the earth's evolutionary past.

So, could the kind of intelligence that we're looking for in the SETI programme have existed in the past history of earth?

We need to look at the artifacts left by our civilisation - if humans disappeared tomorrow, what artifacts of our culture would remain for future species to discover?   The world without us covers this in significant depth.  The plastics we've created should remain as a thin but detectable layer of strata though it's probable that a microbe will evolve to eat most of the synthetic polymers we produce.  The other significant relic of human civilisation is that from the nuclear industry - nothing in nature concentrates isotopes the way we do and these will leave a significant signature that should be relatively easy to detect - indeed the discovery of the natural nuclear reactor at Oklo was initially thought perhaps to be the product of an ancient culture.  The satellites placed in high earth orbit will probably be there until the sun expands to a red giant, but are extremely hard to detect.

Our culture also produces objects of high durability yet are likely difficult to detect over geologic time - glass and ceramics will eventually be ground to a powder or eroded; stainless steel cutlery and bronze sculpture lasts an incredible amount of time yet will eventually become an unusual deposit in rock strata; Gigantic engineering achievements such as the Suez canal or the Hoover dam would silt up and be gone within a few hundred years.

It's interesting that if we were looking for a civilisation equal to our own in the distant geologic past, we would not be looking for evidence of cities (eroded to unrecognisability in a few millennia), agriculture (reclaimed by native species within decades) or even the spike in atmospheric carbon dioxide (could be due to extremely large volcanic eruptions instead).   Instead, we'd be looking for buried bronze sculpture, rubbish dumps, and radioactive waste.

On balance, it's unlikely that a society as advanced as our own has existed on earth before - though the discovery of inhuman bronzes, or unpleasant waters would certainly change that.