"The Ultimate Truth (Is There Intelligent Life on Earth ?)"
Study how bacteria grow in a petri dish.
They begin in the center where you put them.
As they multiply they expand out from the center as they encounter fresh food.
As new bacteria continue to expand toward the edge of the petri-dish
the original bacteria in the center stop multiplying,
then start dying due to lack of food / nutrients / fresh raw material.
Eventually the bacteria multiply to fill the entire petri-dish
and die out when all the food is consumed.
From Charles Krauthammer's "Saving Civilization" p.11A, Sun News, MB, SC Saturday 31 Dec 2011
The Fermi Paradox: modern satellite data suggests that there should be a very high number of
planets where intelligent life could exist, but we can find no evidence of them; why?
The answer may "be found, tragically, in the high probability that advanced civilizations destroy themselves."