Time & Deep Time

Want to study history? In American schools, most intro histories start with homo sapiens, us, and the European variety of us. But Earth’s history is far longer.

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.

—Robert M. Hazen, The Story of Earth, p. 27

Eon Era Period Epoch Began Years Ago
Hadean* 4.6 billion
Archean 4.0 billion
Proterozoic 2.5 billion
Phanerozoic Paleozoic Cambrian 541.0 million
Ordovician 485.4 million
Silurian 443.8 million
Devonian 419.2 million
Carboniferous 358.9 million
Permian 298.9 million
Mesozoic Triassic 252.17 million
Jurassic 201.3 million
Cretaceous 145.0 million
Cenozoic Paleogene 66.0 million
Neogene 23.03 million
Quaternary Pleistocene 2.58 million
Holocene 11.7 thousand
Anthropocene** 10.0 thousand

* 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.

Banded Iron Formation from the Great Lakes, among Earth’s oldest rocks, Elizabeth Fagan for Left Coast Creative, Lake Michigan’s Left Coast, Wisconsin, USA.
Photos and content © 2018 Elizabeth G Fagan – Left Coast Creative – Lake Michigan’s Left Coast


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