Analysis of Potential Great Filter Events in Earth's Evolutionary History Suggests We May Be Alone in the Observable Universe

Abstract The “Great Filter” hypothesis proposes that the apparent absence of extraterrestrial civilizations, despite the vast number of potentially habitable planets, is the result of one or more extremely improbable steps, or “filters,” in the evolution of intelligent life. These filters may lie either in humanity’s future, predicting our eventual self- destruction, or in our evolutionary past, implying that intelligent civilizations are rare. Based on empirical evidence and evolutionary analysis, this study argues that no probable future filter exists with sufficient destructive capability to account for the absence of evidence of extraterrestrial intelligent life. Instead, several improbable and sequentially dependent events in Earth’s geological and biological history have been hypothesized as effective filters, including the Theia collision initiating plate tectonics, tectonic activity providing sufficient sustained chemical energy to enable abiogenesis, and the evolutionary dominance of mammals. By estimating the probabilities of these unlikely events, the probability of a civilization arising was calculated to be less than one per ~1040 star systems—while there are estimated to be only ~1030 stars in the observable universe. These findings suggest that Earth may host the only technological civilization within its cosmic horizon.