The very recent re-evaluation of data gleaned from the Drake equation (the probabilistic estimate of the number of active, intelligent extra terrestrials in the Milky Way) has spawned some interesting scenarios. With the help of vastly superior technology and observational capacity than that available in 1961, a group of Caltech physicists have estimated that a significant number of intelligent civilisations may have come and gone already. We may, after all, be rather late to the game.

Given the vast age of our universe, there has been plenty of time for life to have evolved elsewhere, gradually developing intelligence and, through science, the capacity to dominate their planet. In our own galaxy, this may have occurred many times, only to hit a roadblock, or filter, at some stage. If the filter is great enough (nuclear war, runaway climate change etc) then the civilisation dies out.

This is highly speculative, of course, as there is no hard evidence. We have never observed any signs of intelligent life on an exoplanet, dead or alive. But if we ever do, then let's hope it hasn't gone into extinction. Dead civilisations are a warning sign that a giant filter is ahead of us and woe betide our chances of getting beyond it.