The Proterozoic Eon (2500-541 Ma)

The  Proterozoic  Eon, meaning “earlier life,” comes after the Archean Eon and ranges from 2.5 billion to 541 million years old. During this time, most of the central parts of the continents had formed, and plate tectonic processes had started. Photosynthesis by microbial organisms, such as single-celled cyanobacteria, had been slowly adding oxygen to the oceans. As cyanobacteria evolved into multicellular organisms, they completely transformed the oceans and the atmosphere by adding massive amounts of free oxygen gas (O₂) and initiating the Great Oxygenation Event (GOE ).

This drastic environmental change decimated the anaerobic bacteria, which could not survive in the presence of free oxygen. On the other hand, aerobic organisms could thrive in ways they could not earlier. An oxygenated world also changed the chemistry of the planet in significant ways. For example, iron remained in solution in the non-oxygenated environment of the earlier  Archean   Eon. In chemistry, this is known as a reducing environment. Once the environment was oxygenated, iron combined with free oxygen to form solid precipitates of iron oxide, such as the mineral hematite or magnetite. These precipitates accumulated into large mineral deposits with red chert known as banded-iron formations dated at about 2 billion years.

The formation of iron oxide minerals and red chert in the oceans lasted a long time and prevented oxygen levels from increasing significantly since precipitation took the oxygen out of the water and deposited it into the rock strata. As oxygen continued to be produced and mineral precipitation leveled off, dissolved oxygen gas eventually saturated the oceans and bubbled out into the atmosphere. Oxygenation of the atmosphere is the biggest event that distinguishes the Archean and  Proterozoic environments.

In addition to changing mineral and ocean chemistry, the GOE is also tabbed as triggering Earth’s first glaciation event around 2.1 billion years ago, the Huron Glaciation. Free oxygen reacts with methane in the atmosphere to produce carbon dioxide. Carbon dioxide and methane are called greenhouse gases because they trap heat within the Earth’s atmosphere, like the insulated glass of a greenhouse. Methane is a more effective insulator than carbon dioxide, so as the proportion of carbon dioxide in the atmosphere increased, the greenhouse effect decreased, and the planet cooled.

Rodinia

By the Proterozoic Eon, lithospheric plates had formed and were moving according to plate tectonic forces that were similar to current times. As the moving plates collided, the ocean basins closed to form a  supercontinent called Rodinia. The supercontinent formed about 1 billion years ago and broke up about 750 to 600 million years ago, at the end of the Proterozoic. One of the resulting fragments was a continental mass called Laurentia that would later become North America. Geologists have reconstructed  Rodinia by matching and aligning ancient mountain chains, assembling the pieces like a jigsaw puzzle, and using paleomagnetic to orient to the magnetic north.

The disagreements over these complex reconstructions are exemplified by geologists proposing at least six different models for the breakup of  Rodinia to create Australia, Antarctica, parts of China, and the Tarim craton north of the Himalayas, Siberia, or the Kalahari craton of eastern Africa. This breakup created many shallow-water, biologically favorable environments that fostered the evolutionary breakthroughs marking the start of the next eon, the  Phanerozoic.

Life Evolves

Early life in the  Archean and earlier is poorly documented in the fossil record. Based on chemical evidence and evolutionary theory, scientists propose this life would have been single-celled photosynthetic organisms, such as the cyanobacteria that created stromatolites. Cyanobacteria produce free oxygen in the atmosphere through photosynthesis. Cyanobacteria, archaea, and bacteria are prokaryotes —primitive organisms made of single cells that lack cell nuclei and other organelles.

A large evolutionary step occurred during the  Proterozoic Eon with the appearance of eukaryotes around 2.1 to 1.6 billion years ago. Eukaryotic cells are more complex, having nuclei and organelles. Nuclear DNA is capable of more complex replication and regulation than that of prokaryotic cells. The organelles include mitochondria for producing energy and chloroplasts for photosynthesis. The eukaryote branch in the tree of life gave rise to fungi, plants, and animals.

Another important event in Earth’s biological history occurred about 1.2 billion years ago, when eukaryotes invented sexual reproduction. Sharing genetic material from two reproducing individuals, male and female, greatly increased genetic variability in their offspring. This genetic mixing accelerated evolutionary change, increasing complexity among individual organisms and ecosystems.

Proterozoic land surfaces were barren of plants and animals, and geologic processes actively shaped the environment differently because land surfaces were not protected by leafy and woody vegetation. For example, rain and rivers would have caused erosion at much higher rates on land surfaces devoid of plants. This resulted in thick accumulations of pure quartz sandstone from the  Proterozoic Eon, such as the extensive quartzite formations in the core of the Uinta Mountains in Utah.

Fauna during the Ediacaran Period, 635.5 to 541 million years ago, is known as the Ediacaran fauna and offers a first glimpse at the diversity of ecosystems that evolved near the end of the Proterozoic. These soft-bodied organisms were among the first multicellular life forms and probably were similar to jellyfish or worm-like. Ediacaran fauna did not have hard parts like shells and was not well preserved in the rock records. However, studies suggest they were widespread in the Earth’s oceans. Scientists still debate how many species were evolutionary dead-ends that became extinct and how many were ancestors of modern groupings. The transition of soft-bodied Ediacaran life to life forms with hard body parts occurred at the end of the  Proterozoic and the beginning of the Phanerozoic Eons. This evolutionary explosion of biological diversity dramatically changed scientists’ understanding of the history of life on Earth.