““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.””
![“Perhaps no order of mammals presents us with so extraordinary a series of gradations as this [step by step, from humans to apes to monkeys to lemurs] - leading us insensibly from the crown and summit of the animal creation down to creatures, from which there is but a step, as it seems, to the…” quote by Thomas Huxley](https://sms4smile.com/quotes/img-quotes/f0b/thomas-huxley-1183_animal_perhaps-no-order-of-mammals-presents-us_3051.jpg)
