自洪荒時期，大約有一千億人在地球上留下了足跡。這是一個十分有趣的數字，因為無獨有偶的，在我們所處的銀河系中， 大約有一千億顆恆星。也就是說，在地球上出現的每一個人背後，都有一顆星星在太空中閃耀著。
"Since the dawn of time, roughly a hundred billion human beings have walked the planet Earth. Now this is an interesting number, for by a curious coincidence there are approximately a hundred billion stars in our local universe, the Milky Way. So for every man who has ever lived, in this Universe there shines a star."
Arthur C. Clark, 2001: A Space Odyssey
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