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time frame. While life on earth may have begun quickly after the planet formed, 4 billion years ago, it took 3 billion more years to build the first multi-celled creature, and another half billion to build something as complex as a fish. Presumably neither single celled animals nor primitive fish have access to radios.

6) Detectable intelligent life. A real key is "when does intelligent life become technologically capable of being detected?" Once modern humans appeared, it took 200 million years to learn how to write. For 2,800 years most of each culture's writing was burned by invading cultures. Only in the last 200 years have we been able to throw off the chains of organized theocracy and develop a culture where it is safe to freely express an original, scientific thought. Only since Marconi have we been able to communicate at any distance. Will this phase last? How long does detectable intelligent life last? What are the threats? Asteroids, thermo-nuclear war, plague, famine, climate change brought about by intelligent life are all possible limitations on the continuation of detectable intelligent life.

In conclusion, an amended version of the question might be, "How much intelligent life could exist at any point in time in our galaxy?" Make your assumptions, assign the probabilities to each, and you've got your answer! All we can say for sure is that it will be close to a second or third generation star, and won't be any special distance from the center of the universe, because the universe has no center.

Learn more about this author, Rob Drew.
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