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John P. Rafferty, "Geochronology, Dating, and Precambrian Time: The Beginning of the World As We Know It"

Introduction

Planet Earth was formed roughly 4.6 billion years

ago. For human beings—used to measuring time in

terms of days, weeks, and months—such an enormous

span of time can be a difficult concept to grasp.

Geologists, scientists who study the Earth and the processes

that continue to shape it, have broken up this vast

expanse of “deep time” into major divisions based on

what they have learned from the study of ancient rocks

and fossils. The first of these divisions—from approximately

4.6 billion until 542 million years ago—is known

as the Precambrian, meaning everything that happened

before the Cambrian period. (Today some people prefer

to call this period the Cryptozoic, which means “hidden

life.”) Almost all of planet Earth’s history is Precambrian.

Until recently, however, it has remained the most unknown,

the strangest, and most perplexing period in all geologic

history—what some have referred to as the “Dark Ages”

of Earth’s existence.

For centuries, the Earth yielded no fossil record to

help humans envision Precambrian time. Discoveries of

rich caches of fossils from the Cambrian period enabled

scientists to assemble a vivid picture of the creatures that

inhabited the planet during that time, but the Precambrian

Earth remained largely unimaginable. What was known as

the “missing fossil record” of the Precambrian period

stood for more than a century as one of the great unsolved

mysteries of the natural sciences. This mystery perplexed

Charles Darwin and many other scientists who followed

in his footsteps. Humans had not yet developed the various

methods to accurately determine the age of rocks

formed during this interval of geologic time. They had not

yet identified and interpreted the remains of the microscopic

bacteria that formed in the earliest oceans. The

story scientists were beginning to piece together was full

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