Time & Deep Time

The Geologic Timeline

To get a sense of the scale of Earth history, imagine walking back in time, 100 years per step—every pace equal to more than three human generations. A mile takes you 175,000 years into the past. [Twenty miles], 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 100 years per step would take you back 70 million years, to just before the mass death of 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 100 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.

* The Precambrian Supereon comprises the Hadean, Archean, and Proterozoic eons. The Precambrian begins with Earth’s formation 4.6 billion years ago and covers nearly 90 percent of our planet’s history. In the Precambrian, Earth was a place of intense chemical and geological processes in which life was virtually impossible. It ends with the Cambrian Period, 541.0 million years ago, when hard-shelled creatures appear in the fossil record. Because geological processes continuously recycle Earth’s crust, little evidence of the Precambrian exists.

** The Anthropocene is a proposed new epoch either after or within the Holocene. The word combines the root “anthropo”, meaning “human” with the root “-cene”, the standard suffix for “epoch” in geologic time. It is so named based on overwhelming global evidence that Homo sapiens has altered atmospheric, geologic, hydrologic, biospheric and other Earth processes. The start of the Anthropocene is being debated but is generally agreed to begin approximately 10,000 years ago, about BC 8000, with the end of the last glacial period.

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