Proterozoic Eon: Oxygen, Supercontinents and Complex Cells

The Proterozoic Eon is the long span of Earth’s history from 2.5 billion to 538.8 million years ago, covering nearly half of the planet’s existence. During it, oxygen first built up in the atmosphere, complex cells with a nucleus evolved, supercontinents assembled and broke apart, and ice may have covered the entire globe. It ended with the first large animals, setting the stage for the explosion of animal life in the Cambrian.

Where the Proterozoic fits

Geologists divide Earth’s history into four eons: the Hadean, the Archean, the Proterozoic and the Phanerozoic. The first three are often grouped together as the Precambrian. The Proterozoic follows the Archean Eon, a world of microbes and an oxygen-poor atmosphere, and ends at the base of the Cambrian Period.

The eon has three eras. The Paleoproterozoic runs from 2.5 to 1.6 billion years ago, the Mesoproterozoic from 1.6 to 1.0 billion years ago, and the Neoproterozoic from 1.0 billion to 538.8 million years ago. The name means “earlier life” in Greek, contrasting with the Phanerozoic, or “visible life”.

The rise of oxygen

The Great Oxidation Event

The most important change of the early Proterozoic was the Great Oxidation Event, between about 2.4 and 2.1 billion years ago. Oxygen produced by cyanobacteria through photosynthesis began to accumulate in the atmosphere for the first time. Evidence includes the disappearance of certain sulphur-isotope signals that form only in oxygen-free air, and the appearance of red beds, soils and sediments coloured by oxidised iron.

Oxygen levels were still far below today’s, perhaps a few per cent of present levels or less for much of the eon. The change transformed weathering, ocean chemistry and the kinds of life that could thrive. It may also have helped trigger the Huronian glaciation of about 2.45 to 2.2 billion years ago by destroying methane, a powerful greenhouse gas.

Iron in the oceans

Before and during the oxidation, the oceans held large amounts of dissolved iron. As oxygen reacted with it, iron settled out to form banded iron formations, which became rare after about 1.85 billion years ago. Today these rocks supply most of the world’s iron ore.

Supercontinents

Plate tectonics shaped the Proterozoic world. The supercontinent Columbia, also called Nuna, assembled by about 1.8 billion years ago and broke up gradually between about 1.5 and 1.3 billion years ago. A second supercontinent, Rodinia, formed around 1.1 to 0.9 billion years ago through collisions such as the Grenville orogeny, and began to break apart about 750 million years ago.

Large asteroid impacts also left their mark. The Vredefort crater in South Africa, about 2.0 billion years old, and the Sudbury structure in Canada, about 1.85 billion years old, are among the oldest and largest known impact sites on Earth.

Complex cells and early multicellular life

Eukaryotes

Sometime in the Paleoproterozoic, eukaryotes appeared. These are cells with a nucleus and internal compartments, including mitochondria, which descend from bacteria that came to live inside another cell. Fossils that are probably eukaryotic are known from about 1.6 billion years ago and possibly earlier, and the coiled fossil Grypania, about 1.87 billion years old, may be an early alga.

Unusual large fossils from about 2.1 billion years ago in Gabon, the Francevillian biota, have been interpreted by some as early multicellular organisms, though this remains debated.

The “boring billion”

The middle Proterozoic, roughly 1.8 to 0.8 billion years ago, is sometimes called the “boring billion” because oxygen levels, climate and the carbon cycle appear to have been unusually stable. Yet life was evolving. The red alga Bangiomorpha, about 1.05 billion years old, is one of the oldest known multicellular organisms with distinct cell types and possibly sexual reproduction.

Snowball Earth

The Neoproterozoic brought the most extreme climate events in Earth’s history. During the Cryogenian Period, two great glaciations, the Sturtian (about 717 to 660 million years ago) and the Marinoan (ending about 635 million years ago), left glacial deposits even in regions that lay near the equator. The Snowball Earth hypothesis, developed by Joseph Kirschvink and Paul Hoffman among others, proposes that ice covered most or all of the planet.

Key evidence includes glacial deposits capped directly by thick layers of carbonate rock, known as cap carbonates, which normally form in warm water. Their sudden appearance suggests a rapid switch from ice to hothouse conditions. A return of banded iron formations at this time also points to oceans cut off from the atmosphere beneath the ice.

Each glaciation ended abruptly, probably when carbon dioxide from volcanoes, unable to be removed by weathering on frozen land, built up enough to melt the ice. Scientists still debate whether the oceans froze completely or whether open water survived near the equator, a version sometimes called Slushball Earth. Life clearly survived, perhaps in patches of open water, beneath thin ice or around volcanic hot springs. Oxygen levels rose again in the Neoproterozoic, perhaps helping larger and more active organisms to evolve.

The first animals

The final period of the Proterozoic, the Ediacaran, ran from 635 to 538.8 million years ago. From about 575 million years ago, soft-bodied organisms such as Dickinsonia, Charnia and Kimberella appeared in seas around the world. Together they are known as the Ediacaran biota. Some were early animals; others are hard to place in any living group.

The Proterozoic ended about 538.8 million years ago. The boundary is defined at Fortune Head in Newfoundland, Canada, by the first appearance of a distinctive branching burrow called Treptichnus pedum, a sign that animals had begun to move through the sediment. Its numerical age comes from dating volcanic ash layers at other sites, mainly in Namibia and Oman. The rapid diversification of animals that followed is known as the Cambrian Explosion.

Proterozoic rocks today

Proterozoic rocks are exposed on every continent. Famous examples include the layered rocks of the lower Grand Canyon in Arizona, the Belt Supergroup of Montana and Alberta, and the ancient sandstones of the Torridonian in north-west Scotland. They also host major mineral deposits, from the iron ores of the Lake Superior region and Western Australia to the nickel and copper of the Sudbury Basin in Ontario.

Because the eon is so long and its fossils so sparse, scientists rely heavily on chemistry to read its history. Ratios of carbon, sulphur and other isotopes in Proterozoic rocks reveal changes in oxygen, the burial of organic matter and the activity of microbes, even where no fossils survive.

By the numbers

Timeline

• 2.5 billion years ago: Start of the Proterozoic Eon.
• c. 2.45-2.2 billion years ago: Huronian glaciation.
• c. 2.4-2.1 billion years ago: Great Oxidation Event.
• c. 1.8 billion years ago: Supercontinent Columbia (Nuna) assembled.
• c. 1.6 billion years ago: Eukaryotic fossils become common.
• c. 1.1-0.9 billion years ago: Assembly of Rodinia.
• c. 717-635 million years ago: Sturtian and Marinoan glaciations of the Cryogenian.
• c. 575 million years ago: First Ediacaran organisms appear.
• 538.8 million years ago: Beginning of the Cambrian and end of the Proterozoic.

Why it matters

The Proterozoic turned a microbial world without oxygen into one that could support animals. Almost every feature of the modern Earth system, from breathable air to complex cells and moving continents, took shape during these two billion years.