Many astronomers think that advanced life is widespread in our galaxy.
The conditions that fostered evolution on Earth must exist in many other places, the reasoning goes, because the Milky Way contains hundreds of billions of stars around which hospitable planets might orbit. Chemistry and biology should produce rich stews of organisms on such worlds. And given enough time, those organisms could grow more complex and, ultimately, self-aware. With this view of a cosmic society, its just a matter of hunting in the right ways before we find our galactic brethren. The special circumstances that allowed animals and humans to thrive on Earth may arise almost nowhere else in the Milky Way, and indeed in the universe.