Nearly 4 thousand million years passed after the Earth's inception before the first animals left their traces. This stretch of time is called the Precambrian. To speak of "the Precambrian" as a single unified time period is misleading, for it makes up roughly seven-eighths of the Earth's history. During the Precambrian, the most important events in biological history took place. Consider that the Earth formed, life arose, the first tectonic plates arose and began to move, eukaryotic cells evolved, the atmosphere became enriched in oxygen -- and just before the end of the Precambrian, complex multicellular organisms, including the first animals, evolved.
Find out more about the Precambrian paleontology and geology of North America at the Paleontology Portal.
Prekambrium is an exhibit on the Precambrian at the Paleontology Museum in Oslo (text is in Norwegian).