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 a lot. Free oxygen began to appear, which was important for life on our planet. Scientists think this eon saw the start of complex cells called eukaryotes. These cells later led to many kinds of life we see today.

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

The Proterozoic record

The Proterozoic Eon has a better geologic record than the earlier Archean Eon. Many rocks from this time formed in shallow seas, not deep water. These rocks were often less changed by heat and pressure, so they are easier to study.

During the Proterozoic, supercontinents formed and broke apart. Modern mountain-building also happened. Evidence shows the first known ice ages occurred in this eon, with several glaciations. There were possible "Snowball Earth" events near 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 air changed a lot as oxygen started to grow. This oxygen came from tiny plants through a process called photosynthesis. At first, there was not much oxygen because minerals in the ocean, like iron and sulfur, would take it away. Over time, these minerals ran out, so oxygen levels could rise.

Two big jumps in oxygen happened during this time. The first was called the Great Oxygenation Event, which changed life for many simple organisms on Earth. Later, another rise in oxygen helped more complex life to grow and flourish.

Subduction processes

The Proterozoic Eon was a busy time for Earth's structure. Oxygen in the air changed how rocks behaved, causing many changes in the land. Volcanoes were also very active, creating 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 huge landmasses called supercontinents rise and fall. 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 forces that shape our planet today.

Life

See also: Symbiogenesis, Boring Billion, and Avalon Explosion

The Proterozoic eon was a time when life on Earth changed in important ways. During this time, advanced single-celled organisms called eukaryotes began to appear. This happened because there was more oxygen in the air. These eukaryotes formed helpful partnerships with tiny structures inside their cells, like mitochondria and chloroplasts, which helped them live.

Even though eukaryotes were becoming 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 also began to evolve during this time. While life was changing, the number of new species stayed fairly low until later times when life would become much more varied and complex.