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On Carl Sagan’s “Cosmic Calendar,” which reduces the entire span of the universe since the Big Bang to one year (1.58 million years per hour), September 21st is the date on which single-celled organisms called prokaryotes first appear. The oldest known fossilized prokaryotes, which are thought to be the first life forms on Earth, were laid down some 3.5 billion years ago. The Cosmic Calendar was first described by Sagan in 1980 in his book The Dragons of Eden and on the television series Cosmos. (In the 2014 revival of the series, the age of the universe was revised to 13.8 billion years from Sagan’s 15 billion.) On Sagan’s Cosmic Calendar, modern humans appear for the first time at eight seconds to midnight on December 31st. “It is far better to grasp the universe as it really is than to persist in delusion, however satisfying and reassuring.” —Carl Sagan