On Earth, it took more than 3 billion years for life to evolve from single-celled bacteria to Homo Sapiens. This is a long time compared to the emergence of “simple” life-forms, an argument often used to conclude that \(f_i \ll 1\). Scientists P. Ward and D. Brownlee in their book “Rare Earth” claim that “Intelligent life on Earth relied on so many unlikely accidents that we are probably alone in the Universe”.
Is there anybody out there? – silence –