What is the Pre-Cambrian era?
The Pre-Cambrian era represents the time of Earth’s beginning to just before the big explosion of life in the oceans—from about 4.54 billion to about 543 million years ago. During this time, Earth was cooling, developing its oceans, and building the continental crust; in addition, scientists believe that life began during the early part of the Pre-Cambrian. The following lists one interpretation of three Pre-Cambrian divisions, the approximate dates, and major evolutionary events during these times:
Hadean—4.5 to 3.8 billion years ago, the time when Earth was forming in the early solar system.
Archaean—3.8 billion years to 2.5 billion years ago, the oldest bacteria evolved.
Proterozoic Era—2.5 billion to 543 million years ago, a time in which multicelled eukaryotic (a cell with a definitive nucleus) evolved—in other words, animals.