Precambrian

The Precambrian is the earliest part of Earth's history, before the current Phanerozoic Eon. It is named for coming before the Cambrian, the first period of the Phanerozoic Eon. This time stretches back to when Earth was formed, about 4.6 billion years ago, and continues until about 538.8 million years ago.

The Precambrian makes up most of Earth’s geologic time. It is divided into three main parts, called eons: the Hadean, Archean, and Proterozoic. During this time, Earth changed shape many times, from a very hot and molten place to a world where the first simple life forms began to appear.

Because it covers so much time, the Precambrian helps scientists understand how Earth went from being a newly formed planet to a place that could support life. Studying these ancient rocks gives clues about the early conditions that made our planet what it is today.

Overview

The Precambrian is the earliest part of Earth's history. It makes up most of the planet's time. Scientists have learned a lot about this time since the 1960s, but there is still much we do not know. Fossils from this period, like stromatolites, are harder to study because many rocks have changed over time or been buried deep under newer layers.

It is believed that the Earth formed about 4,543 million years ago and might have collided with another planet, creating the Moon. By 4,433 million years ago, the Earth had a stable surface, as shown by zircon crystals found in Western Australia. Geologists and paleontologists use the term "Precambrian" when they do not need a more specific time name. The Precambrian includes three smaller time periods called the Hadean, the Archean, and the Proterozoic. The oldest fossils help mark the start of a newer time period called the Cambrian.

Life forms

Further information: Origin of life, Avalon explosion, and Earliest known life forms

Scientists are not sure when life first began, but they have found clues in very old rocks. For example, carbon in rocks from 3.8 billion years ago near Greenland might have come from living things. Tiny fossils of bacteria that are even older than 3.46 billion years have been found in Western Australia. Some signs suggest life may have started even earlier, over 4 billion years ago. We have a good record of simple bacterial life during the later part of the Precambrian.

More complicated life forms might have begun as early as 2.1 billion years ago, but it is hard to tell from ancient fossils. Some possible early complex life includes a red alga-like fossil from the Kola Peninsula and other signs of life from China and Canada. The most widely accepted fossils of complex life come from a time called the Ediacaran Period, between 635 and 542 million years ago. These fossils show many soft-bodied creatures found all over the world. Hard-shelled animals appeared later, leading to a big increase in life forms during the Cambrian Period, known as the Cambrian explosion.

While land seemed empty of plants and animals, tiny microbes covered the land in thin layers. We also have found tracks in mud from 551 million years ago that might have been made by animals with leg-like parts.

Emergence of life

One idea is that a molecule called RNA came before DNA and proteins. During a very early time called the Hadean Eon, there were many natural places that could have helped RNA form and copy itself. These places might have allowed early life to develop.

Planetary environment and the oxygen catastrophe

Earth's early history, called the Precambrian, had many changes in its landscapes and air. Scientists think small pieces of land, called proto-continents, existed long ago. These later joined to form a big land named Rodinia. This big land broke apart much later. There were also times when Earth grew very cold, called glacial periods. One of these periods might have made the whole planet freeze over, known as "Snowball Earth".

A big change happened when simple life forms began making oxygen through a process called photosynthesis. Before this, Earth's air did not have much oxygen. When oxygen started to fill the air, it caused big changes for all living things. At first, oxygen mixed with rocks and metals on Earth’s surface. Once these surfaces were used up, oxygen stayed in the air, leading to the atmosphere we know today. We can see evidence of this in old rocks that contain layers of iron oxides, called banded iron formations.

Subdivisions

Main article: Timetable of the Precambrian

The Precambrian is the earliest part of Earth's history. It is split into three main parts, called eons: the Hadean, Archean, and Proterozoic.

The Hadean Eon was when Earth was just forming. The Archean Eon came next, when the first bits of crust began to form. The Proterozoic Eon was the time leading up to when life became more complex. Within the Proterozoic, there are three eras: the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. The Neoproterozoic includes the Ediacaran period, when early life forms appeared.

Precambrian supercontinents

The movement of Earth's plates has created and broken apart continents over time. Sometimes, almost all land joined together into one huge landmass called a supercontinent.

The earliest known supercontinent was Vaalbara. It formed about 3.636 billion years ago and later broke apart. Another early supercontinent, Kenorland, formed around 2.72 billion years ago. It broke up into smaller parts such as Laurentia and Baltica.

Later, the supercontinent Columbia, also called Nuna, formed between 2.1 and 1.8 billion years ago. It eventually broke apart. Rodinia is thought to have formed about 1.3 to 0.9 billion years ago. It later split into several continents around 750 to 600 million years ago.