2.5: Proterzoic Eon

Figure \(\PageIndex{1}\): Diagram showing the main products and reactants in photosynthesis. The one product that is not shown is sugar, which is the chemical energy that goes into constructing the plant, and the energy that is stored in the plant which is used later by the plant or by animals that consume the plant.

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. Cyanobacteria were incorporated into larger collaborations through endosymbiosis, making algae and then plants, and the spread of photosynthesis allowed them to completely transformed the oceans and later the atmosphere by adding massive amounts of free oxygen gas (\(\ce{O2}\)) and initiated what is called 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, which are dated at about 2 billion years.

Figure \(\PageIndex{2}\): Alternating bands of iron-rich and silica-rich mud, formed as oxygen combined with dissolved iron.

The formation of iron oxide minerals and red chert (see figure) 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 started bubbling out into the atmosphere. Oxygenation of the atmosphere is the single 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 reacted 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

Figure \(\PageIndex{3}\): One possible reconstruction of Rodinia 1.1 billion years ago. (John Goodge, modified from Dalziel (1997).)

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 paleomagnetics to orient to magnetic north.

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

Life Evolves

Figure \(\PageIndex{4}\): Modern cyanobacteria (as stromatolites) in Shark Bay, Australia.

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 produced 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.

Figure \(\PageIndex{5}\): Fossil stromatolites in Saratoga Springs, New York.

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. The 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, contributing to more complexity among individual organisms and within ecosystems (see An Introduction to Geology: Chapter 7).

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.

Figure \(\PageIndex{6}\): Dickinsonia, a typical Ediacaran fossil.

Fauna during the Ediacaran Period, 635.5 to 541 million years ago, are known as the Ediacaran fauna, and offer 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 were 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 beginning of the Phanerozoic Eons. This evolutionary explosion of biological diversity made a dramatic difference in scientists’ ability to understand the history of life on Earth.