Carl Sagan opens this episode wondering whether life exists on other worlds, then turns to Earth's own life as the only working example available. He compresses the fifteen-billion-year history of the universe into a single calendar year, so the Big Bang lands on January 1 and every human being who has ever lived fits into the final seconds of December 31, a device that reframes how briefly our species has existed. From there Sagan traces the chemistry of DNA and the long chain of mutation and natural selection that built the tree of life on Earth, using it as a baseline for imagining what evolution might produce elsewhere. Animated sequences speculate on creatures that could survive in the crushing, freezing atmosphere of a Jupiter-like planet, floating organisms adapted to conditions nothing on Earth has to face. The episode keeps returning to its central argument: life's history is a matter of chance and chemistry playing out over immense time, which means it could plausibly have happened, differently, somewhere else.