Cosmic accidents: 10 lucky breaks for humanity. Rare Earth hypothesis. Hypothesis that complex extraterrestrial life is a very improbable phenomenon and likely to be extremely rare The Rare Earth hypothesis argues that planets with complex life, like Earth, are exceptionally rare In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligence) required an improbable combination of astrophysical and geological events and circumstances.
A contrary view was argued in the 1970s and 1980s by Carl Sagan and Frank Drake, among others. It holds that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Given the principle of mediocrity (in the same vein as the Copernican principle), it is probable that we are typical, and the universe teems with complex life. Journal of Cosmology. 1.
Hawking's Alien's Squawkings, John Menninger, Ph.D., Department of Biology, University of Iowa. Aliens, as perceived by Hawking Could soon visit Earth for some gawking. They might do good, but Oy! , They might wish to destroy! We'll more likely be bored by their talking. (-- with apologies to E.