It is interesting to note that if we quietly pass away and 50 million years later intelligent lungfish build up a civilization, they would presumably have good evidence that we were here, and would have good reason to assume that civilizations arise about once every 50 million years on average. Our effect on the Earth has probably been great enough that they will not have significant evidence from previous periods to contradict this assumption. In the case of large scale planetary civilizations, only the first one is likely to be in a position to reliably notice a delay in the appearance of previous civilizations longer than the pause between themselves and the immediately previous civilization. Therefore it may be reasonable to believe that, if 10 civilizations arise on the average planet, 90% of them will believe that they are probably midway through a long succession of civilizations.