“To get a sense of the scale of Earth history, imagine walking back in time, a hundred years per step—every pace equal to more than three human generations. A mile takes you 175,000 years into the past. The twenty miles of Chesapeake cliffs, a hard day’s walk to be sure, correspond to more than 3 million years. But to make even a small dent in Earth history, you would have to keep walking at that rate for many weeks. Twenty days of effort at twenty miles a day and a hundred years per step would take you back 70 million years, to just before the mass death of the dinosaurs. Five months of twenty-mile walks would correspond to more than 530 million years, the time of the Cambrian “explosion”—the near-simultaneous emergence of myriad hard-shelled animals. But at a hundred years per footstep, you’d have to walk for almost three years to reach the dawn of life, and almost four years to arrive at Earth’s beginnings.”
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Robert M. Hazen,
The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet