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Carl Sagan’s Calendar Analogy of Time Since the Big Bang

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Sagan (1977) created a powerful analogy, the cosmic calendar, to explain how much time has passed since the Big Bang, the primordial explosion that created the universe nearly 14 billion years ago. In his analogy, Sagan maps the time since the Big Bang onto a one-year calendar where January 1 is when the Big Bang happened and January 1 one year later is today. Let’s examine some of those dates:

•January 1 The Big Bang
•September 9 The Solar System forms
•September 14 The Earth forms
•September 25 Life on Earth begins (no fossil evidence remains for these early one-celled organisms)
•October 9 Oldest fossils known (bacteria and blue-green algae)
•December 16 First worms (fossil evidence of multicellular organisms)
•December 19 First fish (first vertebrates)
•December 20 Plants begin to colonize surface of Earth
•December 21 First insects
•December 22 First amphibians
•December 23 First reptiles
•December 24 First dinosaurs
•December 26 First mammals
•December 27 First birds
•December 28 Dinosaurs extinct
•December 29 First primates (our phylogenetic lineage)
•December 30 First hominins (human-like organisms)
•December 31 10:30 p.m. First humans
◦11:00 p.m. Extensive use of stone tools
◦11:46 p.m. Fire domesticated
◦11:56 p.m. Last ice age begins
◦11:58 p.m. Australia settled by sea-traveling peoples
◦11:59 p.m. European cave paintings created
◦11:59:20 p.m. Agriculture invented
◦11:59:35 p.m. First cities built and settled
◦11:59:53 p.m. Bronze Age and Trojan War
◦11:59:54 p.m. Iron Age and biblical Kingdom of Israel
◦11:59:56 p.m. High point of Roman Empire and birth of Jesus
◦11:59:57 p.m. Invention of number zero and Moslem conquests
◦11:59:58 p.m. Crusades and high point of Mayan civilization
◦11:59:59 p.m. Renaissance, discovery of “New World”, rise of science
◦12:00:01 a.m. Today
I’m always amazed whenever I re-read Sagan’s cosmic calendar because it so clearly illustrates how little time humans have been living on Earth. Although this chapter will cover some 4,000,000 years of hominin existence, that enormous span of years is small in comparison to the lifespan of the universe and of the planet.



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