In Search of Alien Life In a review of current debates about whether extraterrestrials have visited Earth, Elizabeth Kolbert discusses the Fermi paradox, which asks, in reference to aliens, “Where are they?” (Books, January 25th). Some useful context is the Rare Earth hypothesis, which argues that advanced life is an extremely unlikely outcome of Darwinian evolution; after all, life required nearly four billion years—almost one-third of the age of our universe—to develop on Earth. There is growing consensus that intelligent life here may well have depended on improbable contingencies such as the Chicxulub asteroid impact, sixty-six million years ago, which obliterated the dinosaurs. Fermi’s question, seen in the light of the Rare Earth hypothesis, could yield the answer “They are not there.”