So, what is the Precambrian time? The Precambrian is the very beginning of the Earths time, this time is 4.5 billion years ago. Isn't that old? The Earth at this time was made of all sedimentary rocks. This tells us the past of Earth. The oldest animals that lived during this time were microscopic bacteria which we know because of the fossils found in the sedimentary rocks. The volcanic activity created the earth's crust and the clouds from the volcano's eruption created lots of rain which created the oceans. The atmosphere was very poor and kept the heat from the volcano inside of the Earth's atmosphere.