A brief thought experiment on the detection of previous intelligent life on Earth. What evidence is there that homo-sapien is the first intelligent species to have evolved on this planet? Right now we assume that finding no indication of previous intelligent species indicates that they did not exist. I have been conducting a small though experiment to determine what factors would influence the likelihood of finding leftovers from mankind in 50 million years time. Let us first assume that intelligent (technological) society will result in communal living for the exchange of ideas and the bringing to bear of sheer weight of numbers to the solution of problems. Communal living appears to have been a key stepping stone to technological society for mankind in the early Neolithic. What would be chance of any of our cities, towns or villages being excavated in 50m years and still being recognised for what they were? I believe this would be about zero. Many of our largest cities lie on coasts, likely to be subjected to coastal erosion and for those like LA subjected to subduction. For those in ideal conditions would be left? Quite a lot really for those in perfect fossilisation conditions. However the problem here becomes digging in the right spot. I have no idea how much of the Earth’s surface is currently built on but I would guess much less than 0.1%. This leaves a much reduced likelihood that we would accidentally stumble upon remains. Assuming that previous a civilisation has been technologically advanced enough o launch spacecraft then this must give us a much better chance to detect them. Like us the most obvious place to put spacecraft is in orbit. Over 50 million years I would expect all satellites to crash back to Earth due to small orbital deviations and atmospheric slowing. Even up at geo-stationary orbit the solar wind and just a few hits by dust particles will I would expect to de-orbit a satellite over a few million years. This leaves spacecraft orbiting the sun, I believe these would suffer the same fate as those orbiting the Earth and would eventually be swept up by planets or spiral into the sun. Spacecraft sent out of the solar system would fare better, but by now are so far away and so out of power that we will never find them. Spacecraft on the moon or other lifeless bodies may have fared much better, but are likely to be buried by regolith and so we are back to the problem of digging in just the right place. Perhaps Star Trek type orbital sensors will one day help us identify such things on the moon, but this is some time away. Why come up with this seemingly daft idea? Intelligent life has only evolved in one place that we know about. Only one planet known to us right now has the conditions that permit intelligence life to evolve. If it evolves once why not twice, why not several times, every time resulting in mass extinction and self termination once the means are available. We are already in the middle of a mass extinction event and I personally would not trust our current clutch of world politicians and religious fanatics as far as I could kick JDAM bomb. Any thoughts?