The probability that life is abundant in the universe should be high, yet we have located no undeniable signs of life beyond Earth.
This is what is known as Fermi's Paradox. One hypothetical solution is the Great Filter, which proposes that the conditions required for the evolution of intelligent life are rare. According to the hypothesis, at some point during the evolution of life, a wall or filter prevents it from progressing any farther. There may, this hypothesis proposes, be life out there, but it stopped at the bacterial stage (for example).
Aditya Chopra and Charley Lineweaver, astrobiologists from the Australian National University propose a modification on this scenario. Most life, they say, would become extinct before it gets very far along the evolutionary process at all.
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