Precambrian time covers much of the Earth's history, starting with Earth's creation about 4.5 billion years ago and ending with the creation of complex, multicelled life-forms almost four billion years later. After the Earth was formed, oceans formed and covered the Earth. Shortly afterwards, oxygen became present in the Earth's atmosphere. Around the same time, the first sedimentary rocks formed and the first life forms (bacteria) appeared. The Earth's first ice age occured around 2.3 billion years ago. In the late Precambrian time, soft bodied, multicellular organisms developed. Lastely, the first mass extinction probably occured near the end of Precambrian time.