15.5: Proterozoic Eon
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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 like stromatolites) had already been adding oxygen slowly to the atmosphere, but it was quickly absorbed in minerals. Evolutionary advancements in multicellular cyanobacteria completely transformed the atmosphere by adding free oxygen gas (O2) and causing the decimation of the anaerobic (non-oxygen) bacteria that existed at the time . This is known as the Great Oxygenation Event. In an oxygenated world, organisms could thrive in ways they could not earlier. Oxygen also changed the chemistry of the planet in significant ways. For example, iron can be carried in solution in a non-oxygenated environment. However, when iron combines with free oxygen, it creates a solid precipitate to make minerals like hematite (iron oxide). This is the reason large deposits of iron known as banded iron formations are common during this time, ending around 2 billion years ago .
The formation of the banded iron lasted a long time and prevented the oxygen level from increasing significantly in the oceans since the rocks literally 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 started filling the oceans and eventually bubbling out into the atmosphere. Oxygenation of the atmosphere is the single biggest event that distinguishes the Archean Earth and the Proterozoic Earth . In addition to changing mineral and ocean chemistry, this event is also tabbed as the likely cause of Earth’s first glaciation, the Huron Glaciation that occurred around 2.1 billion years ago . Free oxygen reacted with methane in the atmosphere, turning it into carbon dioxide. Methane is a more effective greenhouse gas than carbon dioxide, and as CO2 increased in the atmosphere, the greenhouse effect actually decreased, thus cooling the planet.
By the Proterozoic eon, lithospheric plates had formed and started moving according to plate tectonic motions similar to today. As these plates formed and started moving, eventually a supercontinent formed from collisions as the ocean basins closed. The exact number of supercontinents during the Proterozoic (or earlier) is unknown, but Rodinia is the best understood. It formed about 1 billion years ago and broke up at the end of the Proterozoic, about 750-600 million years ago. Laurentia, the name for the continental mass that became North America, most likely was in the center of Rodinia. The reconstruction of Rodinia has been accomplished matching and aligning ancient mountain chains to assemble the pieces like a jigsaw puzzle, and paleomagnetic to orient them with respect to magnetic north.
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 Panthallasa Ocean (early Pacific), including Australia, Antarctica , parts of China, the Tarim craton north of the Himalaya , Siberia , or the Kalahari craton of eastern Africa . 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.
Early life in the Archean and earlier is poorly documented in the fossil record, but chemical evidence and evolutionary theory state that this 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 are prokaryotes, i.e. single-celled organisms (archaea and bacteria) with simple cells that lack a cell nucleus and other organelles.
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 with cell organelles and a nucleus with more complex DNA replication and regulation, mitochondria for additional energy, and chloroplasts to perform photosynthesis and produce energy. Certain organelles even have their own DNA, like mitochondria. Eukaryotes are the branch of the tree of life that 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 sex . By sharing genetic material between reproducing individuals (male and female), evolutionary change was enhanced by increasing genetic variability. This allowed more complexity among individual organisms, and eventually, ecosystems.
It is important to realize that the Proterozoic land surfaces were barren, at least of plants like grasses, trees, and animals. Geologic processes were active just like today, but the application of the Uniformity Principle requires the realization of differences in the environments in which the processes operate. For example, rain and rivers were present but erosion on barren land surfaces would have operated at different rates than on modern land surfaces protected by plants.
The Ediacaran fauna (635.5-541 million years ago, ) offers the first glimpse at these evolving ecosystems toward the end of the Proterozoic. These 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 the earth . Scientists still debate how many of these are extinct evolutionary dead-ends or the ancestors to modern biological groups . 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.
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