The Pre-Cambrian Era started when the Earth was very first formed about 4.6 billion years ago. This was the first geological period, and an important one (consists of 80% of the Earth’s history), as tectonic plates, oceans, continents, and eukaryotic cells were formed, oxygen came into the atmosphere, and near the end of the era, the first multicellular organisms evolved. Little is known about the animal life of the Pre-Cambrian. There are very little fossils of the fauna, but it is thought that some of these organisms included bacteria and algae, since spherical imprints have been found in some rocks of that era. Also, animals that resembled worms, sea pens, and jellyfish probably existed. The era ended about 590 million years ago.
Rocks from this era are often covered by rocks that have formed in more recent eras. The ones that have been found are greatly altered by metamorphism. Mainly, the rocks of the Pre-Cambrian Era were granite, schist, and gneiss. In the later time of this era, the continental crust formed from these rocks because the temperature of the Earth went down. After that, volcanic activity and mountain formation occurred. These rocks that still exist today from the Pre-Cambrian contain much of the world’s gold and nickel.