8.3: Proterozoic Eon

The Proterozoic Eon, meaning “earlier life,” is the eon of time after the Archean eon and ranges from 2.5 billion years old to 541 million years old. During this time, most of the central parts of the continents had formed and the plate tectonic process had started. Photosynthesis in organisms such as single-celled cyanobacteria (like stromatolites) had already been slowly adding oxygen to the oceans. As cyanobacteria evolved into multicellular organisms, they completely transformed the oceans and later the atmosphere by adding massive amounts of free oxygen gas (O₂) and initiated what is known as the Great Oxygenation Event. This drastic environmental change decimated the anaerobic (non-oxygen) bacteria, which could not survive in the presence of free oxygen [47]. In an oxygenated world, aerobic organisms could thrive in ways they could not earlier.

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.

Oxygen also changed the chemistry of the planet in significant ways. For example, iron can be carried in solution in a non-oxygenated environment. In chemistry, this is known as a reducing environment. However, once the environment was oxygenated, iron combined with free oxygen to form solid precipitates of iron oxide, such as the minerals hematite or magnetite. These precipitates accumulated into large deposits with red chert known as banded-iron formations, which are dated at around 2 billion years old [48].

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

The formation of the banded iron lasted a long time and prevented the oxygen levels from increasing significantly in the oceans since precipitation took the oxygen out of the water and formed alternating layers of iron-oxide minerals and red chert. Eventually, as oxygen continued to be made, absorption of oxygen in mineral precipitation leveled off, and 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 the Proterozoic environments [49]. In addition to changing mineral and ocean chemistry, the Great Oxygenation Event is also tabbed as the likely cause of Earth’s first glaciation, the Huron Glaciation, that occurred around 2.1 billion years ago [50]. Free oxygen reacted with methane in the atmosphere, turning it into 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, and as CO₂ increased in the atmosphere, the greenhouse effect actually decreased, thus cooling the planet.

Rodinia

By the Proterozoic Eon, lithospheric plates had formed and started moving according to plate tectonic motions similar to today. As the moving plates collided, the ocean basins closed to form a supercontinent called Rodinia. It formed about 1 billion years ago and broke up at the end of the Proterozoic, about 750-600 million years ago. One of the resulting fragments was a continental mass called Laurentia, that would later become North America. The reconstruction of Rodinia has been accomplished by matching and aligning ancient mountain chains to assemble the pieces like a jigsaw puzzle, and using paleomagnetic data to orient them with respect to magnetic north.

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

As examples of the complexity of the issue and disagreements among geologists over the reconstructions, there are at least six different models of what broke away from Laurentia in the Panthalassa Ocean (early Pacific), including Australia, Antarctica [53], parts of China, the Tarim craton north of the Himalaya [54], Siberia [55], or the Kalahari craton of eastern Africa [56]. Regardless of the exact details, it was this breakup that created lots of biologically-favorable shallow water environments that fostered the evolutionary breakthroughs that mark the start of the next eon, the Phanerozoic.

Life Evolves

Early life in the Archean and earlier is poorly documented in the fossil record, but based on chemical evidence and evolutionary theory, scientists propose life would have been single-celled photosynthetic organisms such as cyanobacteria in stromatolites. Fossil cyanobacteria in these stromatolites produced free oxygen in the atmosphere through photosynthesis. Cyanobacteria, archaea and bacteria are prokaryotes, i.e. single-celled organisms with simple cells that lack a cell nucleus and other organelles.

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

However, during the Proterozoic, a large evolutionary step occurred with the appearance of eukaryotes. Evolving around 2.1-1.6 billion years ago, eukaryotic cells are more complex, having a nucleus and cell 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. About 1.2 billion years ago, another important event in Earth’s biological history occurred when some eukaryotes invented sexual reproduction [59]. By sharing genetic material between reproducing individuals (male and female), genetic variability was greatly increased in their offspring. This genetic mixing accelerated evolutionary change, allowed more complexity among individual organisms, and eventually, 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 (635.5-541 million years ago, [60]), known as the Ediacaran fauna, offers the first glimpse at the diversity of ecosystems that evolved toward the end of the Proterozoic. These soft-bodied organisms were among the first multicellular life forms and may have been similar to soft jellyfish or worm-like organisms [61; 62]. Since the Ediacaran fauna did not have hard parts like shells, they are not well preserved in Proterozoic rocks. However, studies suggest that they were widespread around Earth's oceans [63]. Scientists still debate how many of these are extinct evolutionary dead-ends or the ancestors to modern biological groups [61]. The transition of life from the soft-bodied Ediacaran forms to the explosion of forms with hard parts at the end of the Proterozoic and beginning of the Phanerozoic made a dramatic difference in our ability to understand Earth history and the history of life.

Figure \(\PageIndex{5}\): Dickinsonia, a typical Ediacaran fossil.

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