History of the Earth Part 3: Proterozoic
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This time, I will be translating the English Wikipedia entry for History of the Earth. I am not a professional translator, so there may be mistranslations, but please be lenient.
I use tools like DeepL and Google Translate for my translations.
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History of the Earth
Proterozoic
The Proterozoic is the period from 2.5 billion years ago to 542 million years ago. During this period, cratons grew into continents of modern size. The change to an oxygen-rich atmosphere was a significant development. Life evolved from prokaryotes to eukaryotes and multicellular organisms. During the Proterozoic, there were several severe ice ages known as "Snowball Earth." After the last Snowball Earth about 600 million years ago, the evolution of life on Earth accelerated. Around 580 million years ago, the Ediacaran biota formed a precursor to the Cambrian explosion.
Oxygen Revolution
Early cells absorbed energy and food from their surrounding environment. In doing so, they utilized "fermentation," which breaks down more complex compounds into less complex ones with less energy, and used that energy to grow and reproduce. Fermentation only occurs in anaerobic (oxygen-free) environments. The evolution of photosynthesis allowed cells to obtain energy from the sun.
Most organisms covering the Earth's surface depend directly or indirectly on photosynthesis. Oxygenic photosynthesis, the most common form, converts carbon dioxide, water, and sunlight into food. In photosynthesis, the energy of sunlight is captured in energy-rich molecules such as ATP (translator's note: adenosine triphosphate), which are then used as an energy source to create sugars. In the process, to supply electrons, hydrogen is taken from water, leaving oxygen as a waste product. Some organisms, such as purple bacteria and green sulfur bacteria, perform anoxygenic photosynthesis, using hydrogen sulfide, sulfur, or iron as electron donors instead of hydrogen extracted from water. Organisms with such extreme affinities are limited to inhospitable environments such as hot springs and hydrothermal vents.
The simpler anoxygenic type emerged around 3.8 billion years ago, shortly after the appearance of life. There is debate regarding the timing of oxygenic photosynthesis; it had certainly appeared by around 2.4 billion years ago, but some researchers date it back to around 3.2 billion years ago. Oxygenic photosynthesis is thought to have "increased the Earth's productivity by at least two to three orders of magnitude." The oldest remains of oxygen-producing organisms are fossil stromatolites.
Initially, the released oxygen combined with minerals such as limestone and iron. Oxidized iron appeared as banded iron formations—a type of geological stratum that formed in abundance during the Siderian period (2.5 to 2.3 billion years ago)—as red layers. Once the exposed, easily reactive minerals were mostly oxidized, oxygen finally began to accumulate in the atmosphere. Although the amount of oxygen produced by each individual cell was minute, the cumulative metabolism of countless cells over vast amounts of time transformed Earth's atmosphere into its current state. This is Earth's third atmosphere.
Some of the oxygen was stimulated by solar ultraviolet radiation to form ozone, which gathered near the upper layers of the atmosphere. This ozone layer absorbed a significant amount of the ultraviolet radiation that had previously passed through the atmosphere, and it continues to do so today. Without the ozone layer, cellular mutations caused by ultraviolet radiation raining down on the oceans and land would have been impossible to sustain.
Photosynthesis also had a major impact. Oxygen is toxic, and as its concentration rose, it is believed that much of the life on Earth perished, an event known as the "Great Oxygenation Event." Resistant organisms survived and thrived, and some even developed the ability to use oxygen to enhance their metabolism, gaining more energy from the same food.
Snowball Earth
Due to the natural evolution of the Sun, its brightness increased by 6% every billion years from the Archean to the Proterozoic. As a result, Earth began to receive more heat from the Sun during the Proterozoic. However, this did not mean the Earth became warmer. On the contrary, geological records show that it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa date back 2.2 billion years, and paleomagnetic evidence suggests they were located near the equator. Therefore, this ice age, known as the Huronian glaciation, may have been global in scale. Some scientists propose the "Snowball Earth" hypothesis, suggesting that this ice age was so intense that the Earth froze from the poles to the equator.
The Huronian glaciation may have been caused by an increase in atmospheric oxygen concentration, which led to a decrease in atmospheric methane (CH₄). Methane is a potent greenhouse gas, but it reacts with oxygen to become CO₂, which is less effective as a greenhouse gas. Once free oxygen existed in the atmosphere, methane concentrations would have dropped dramatically, counteracting the effects of the increased heat flow from the Sun.
However, the term "Snowball Earth" is often used to describe the later, extreme ice ages of the Cryogenian period. Between 750 million and 580 million years ago, there were four periods, each lasting about 10 million years, during which it is believed that the Earth was covered in ice except for the highest mountains, with an average temperature of about -50°C (-58°F). The presence of the supercontinent Rodinia straddling the equator is also considered one of the causes of the snowball state. Carbon dioxide can be removed from the atmosphere by combining with rock weathered by rain to form carbonic acid, which is then washed into the ocean. When continents are near the poles, advancing ice covers the rocks, slowing the reduction of carbon dioxide; however, during the cold period of Rodinia, weathering continued unabated until the ice reached the tropics. This process may have eventually been reversed by the release of carbon dioxide from volcanoes or the destabilization of methane clathrates. Additionally, the "Slushball Earth" theory suggests that water existed on the equator even at the height of the ice age.
The emergence of eukaryotes
Modern taxonomy classifies life into three domains. It is unclear when these domains originated. The Bacteria domain likely separated from other life forms (sometimes called Neomura) first, though this hypothesis is debated. Shortly thereafter, by 2 billion years ago, the Neomura split into Archaea and Eukaryota. Eukaryotic cells (eukaryotes) are larger and more complex than prokaryotic cells (bacteria and archaea), but the origin of this complexity is only just beginning to be understood. The oldest fossils with fungal characteristics were discovered in the Paleoproterozoic, about 2.4 billion years ago; they were multicellular benthic organisms with anastomosing filamentous structures.
Around this time, the first primitive mitochondria were formed. A bacterial cell similar to modern Rickettsia, which had evolved to metabolize oxygen, entered a larger prokaryotic cell that lacked the ability to metabolize oxygen. The larger cell may have tried to digest the smaller one but failed (perhaps because the smaller one had evolved defensive mechanisms against predators). The smaller cell may have tried to parasitize the larger one. In any case, the smaller cell survived inside the larger one. It used oxygen to metabolize the larger cell's waste products, gaining more energy. Some of this surplus energy was returned to the host. The smaller cell multiplied inside the larger one. Eventually, a stable symbiotic relationship developed between the large cell and the small cell inside it. The large cell could not live without the energy produced by the small cell, and the small cell could not live without the raw materials provided by the large cell. Today, the entire cell is considered a single organism, and the small cell is classified as an organelle called a mitochondrion.
It is also the case that cyanobacteria that perform photosynthesis entered large heterotrophic cells and became chloroplasts. Perhaps as a result of these changes, more than a billion years ago, groups of photosynthetic cells separated from other eukaryotes. There were likely several such integration events. Regarding the cellular origins of mitochondria and chloroplasts, in addition to the well-established endosymbiotic theory, there are theories that cells originated from peroxisomes, cilia and flagella from spirochetes, and perhaps the cell nucleus from DNA viruses, but none of these are widely accepted.
Archaea, bacteria, and eukaryotes continued to diversify, becoming more complex and adapted to their environments. Each domain repeatedly split into multiple lineages, but little is known about the history of Archaea. Around 1.1 billion years ago, the supercontinent Rodinia was assembled. The plant, animal, and fungal lineages had split, but they still existed as isolated cells. Some of these may have been forming colonies, and gradually division of labor may have begun to occur, such as peripheral cells taking on different roles than internal cells. The distinction between colonies with specialized cells and multicellular organisms is not always clear, but the first multicellular plants, likely green algae, appeared around 1 billion years ago. True multicellularity in animals likely also progressed around 900 million years ago.
Initially, they would have possessed totipotent cells, which could reassemble a destroyed organism, much like modern sponges. Once the division of labor was completed in all lineages of multicellular organisms, cells became more specialized and dependent on each other, and isolated cells would die.
Proterozoic supercontinents
Reconstruction of crustal movements over the past 250 million years (Cenozoic and Mesozoic) can be done reliably by matching continental margins, seafloor magnetic anomalies, and paleomagnetic poles. Because oceanic crust from before that time does not exist, earlier reconstructions are more difficult. Paleomagnetic poles are complemented by geological evidence, such as orogenic belts marking the edges of ancient plates and the distribution of past flora and fauna. The older the data, the scarcer it becomes, the harder it is to interpret, and the more uncertain the reconstruction.
Throughout Earth's history, continents have collided to form supercontinents, which later split into new continents. Between approximately 1 billion and 830 million years ago, most continents were integrated into a supercontinent called Rodinia. Before Rodinia, there may have been early-to-mid Proterozoic continents called Nuna or Columbia.
After Rodinia split about 800 million years ago, the continents may have formed another short-lived supercontinent around 550 million years ago. This hypothetical supercontinent is sometimes called Pannotia or Vendia. Evidence for this includes a phase of continental collision called the Pan-African orogeny, which joined the continental masses of modern Africa, South America, Antarctica, and Australia. Whether Pannotia existed depends on the timing of crustal extension between Gondwana (which includes most of the current Southern Hemisphere landmasses, the Arabian Peninsula, and the Indian subcontinent) and Laurentia (roughly corresponding to modern North America). At the very least, it is certain that by the end of the Proterozoic, most of the continents were gathered in a position centered on the South Pole.
Late Proterozoic climate and life
At the end of the Proterozoic, there were at least two intense Snowball Earth events, where the ocean surface froze completely. These occurred during the cold periods 716.5 million and 635 million years ago. The intensity and mechanisms of these two ice ages are still under investigation and are more difficult to explain than the early Proterozoic Snowball Earth. Many paleoclimatologists believe these cold episodes are linked to the formation of the supercontinent Rodinia. Because Rodinia was located directly under the equator, chemical weathering accelerated, sequestering atmospheric carbon dioxide (CO₂). Similarly, during Snowball Earth, most of the continental surface was covered in permafrost, which reduced chemical weathering again and ended the ice age. Another hypothesis is that volcanic eruptions released enough carbon dioxide, which then raised Earth's temperature through the greenhouse effect. Also, volcanic activity became more active due to the breakup of Rodinia around the same time.
Following the cryogenic period, the Ediacaran period began, characterized by the rapid development of multicellular organisms. While it is not certain whether there is a connection between the end of the severe ice age and the increase in biological diversity, it does not seem to be a coincidence. New life forms known as the Ediacaran biota were larger and more diverse than ever before. The classification of most Ediacaran organisms is unknown, but some were ancestors of modern biological groups. What is important is the origin of muscle cells and nerve cells. Ediacaran fossils did not have hard body parts like skeletons. These only appear after the boundary between the Proterozoic and Phanerozoic eons, or after the Ediacaran period and the Cambrian period.
Plate tectonics, Paleogeography, & Ice Ages (dual hemispheres)
Christopher Scotese
* The video is from the following Phanerozoic eon. You can see the Gondwana continent near the South Pole during the Cambrian period, as well as the Rodentia continent, which corresponds to present-day North America. Eventually, the massive supercontinent Pangea will emerge.
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