What took so long for humans to appear on Earth? The Earth is 4.6 billion years old, and life began about 4 billion years ago, yet humans—the only intelligent, technological species we know of in the universe—have existed only for the last 200,000 years. Why didn’t we come sooner? What factor(s) delayed our appearance? And what can life’s timeline here say about the possibility of other technologically advanced lifeforms in the universe?
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