Proterozoic

The Proterozoic /ˌproʊtərəˈzoʊɪk, ˌprɒt-, -əroʊ-, -trə-, -troʊ-/ is the third of the four geologic eons of Earth's history. It lasted from 2500 to 538.8 million years ago and is the longest eon on our planet's geologic time scale. It came after the Archean and before the Phanerozoic, making up the most recent part of the Precambrian.

During this time, Earth's atmosphere changed dramatically as free oxygen began to appear. This was an important step for life on our planet. Scientists believe this eon saw the beginning of complex cells called eukaryotes, which later led to many forms of life we see today.

The Proterozoic had three main parts, known as eras: the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. Some of the big events in this eon include long periods of cold weather, called glaciations, and the first clear signs of many different kinds of life in the rocks. These changes helped set the stage for the amazing variety of life that appeared later in Earth's history.

The Proterozoic record

The Proterozoic Eon has a more complete geologic record than the earlier Archean Eon. Instead of deep-water deposits, many rocks from this time were formed in shallow seas. These rocks are often less changed by heat and pressure than older rocks, making them easier to study.

During the Proterozoic, there were also the first clear patterns of supercontinents forming and breaking apart, as well as modern mountain-building activities. Evidence shows that the first known ice ages happened in this eon, with several glaciations, including possible "Snowball Earth" events towards the end of the Proterozoic.

Main article: Snowball Earth

The accumulation of oxygen

Main articles: Great Oxygenation Event and Neoproterozoic Oxygenation Event

During the Proterozoic, Earth's atmosphere changed dramatically as oxygen began to build up. This oxygen was produced by tiny plants through a process called photosynthesis. At first, there wasn't much oxygen because minerals in the ocean, like iron and sulfur, would remove it from the air. Over time, these minerals ran out, allowing oxygen levels to rise.

Two big increases in oxygen happened during this time. The first was called the Great Oxygenation Event, which changed the lives of many simple organisms on Earth. Later, another increase in oxygen helped complex, multicellular life to develop and thrive.

Subduction processes

The Proterozoic Eon was a time of great activity for Earth's structure. Oxygen in the air changed how rocks behaved, leading to many changes in the land. Volcanoes were also very active, causing more changes.

Scientists think that during this time, old pieces of Earth's crust were being pulled back down into the planet, a process called subduction. This helped create new rocks and made the first big landmasses stronger. Because of these processes, we can still find very old parts of Earth's surface from this time.

Supercontinent tectonic history

During the Proterozoic, Earth saw the rise and fall of several huge landmasses called supercontinents. One of the first was Columbia, which existed around 1,590 million years ago. Later, around 900 million years ago, Rodinia became the main supercontinent.

Another supercontinent, Pannotia, may have formed around 545 million years ago, though this is not fully certain. Finally, Gondwana appeared about 420 million years ago. These landmasses moved and changed due to the same tectonic forces that shape our planet today.

Life

See also: Symbiogenesis, Boring Billion, and Avalon Explosion

The Proterozoic eon saw important changes in life on Earth. During this time, advanced single-celled organisms called eukaryotes began to appear, likely because of more oxygen in the atmosphere. These eukaryotes formed important partnerships with tiny structures inside their cells, such as mitochondria and chloroplasts, which helped them survive.

Even though eukaryotes were growing more common, other simple life forms like cyanobacteria were still very successful. Fossil evidence shows that the earliest fungi appeared during the Proterozoic, and plants began to evolve during this time as well. While life was changing, the number of new species appearing stayed fairly low until later periods when life would burst forward in variety and complexity.