Energy and the Human Journey: Where We Have Been; Where We Can Go - Wade Frazier

[Magda Hassan]

The Cryogenian Ice Age and the Rise of Complex Life

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Earth's Major Ice Ages

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Major Ice Age

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Duration

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[TD="class: Normal, width: 182"] Impact on Ecosphere
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[TD="class: Normal, width: 233"] Suspected Primary Cause(s)
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[TD="class: Normal, width: 201"] Huronian
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[TD="class: Normal, width: 153"] c. 2.4 to 2.1 bya
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[TD="class: Normal, width: 182"] Perhaps little only prokaryotes existed.
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[TD="class: Normal, width: 233"] Early stage of Great Oxygenation Event.
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[TD="class: Normal, width: 201"] Cryogenian
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[TD="class: Normal, width: 153"] c. 850 to 635 mya
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[TD="class: Normal, width: 182"] Perhaps great life may have been nearly extinguished, and rise of complex life followed Cryogenian.
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[TD="class: Normal, width: 233"] Supercontinent breakup and resultant runaway effects.
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[TD="class: Normal, width: 201"] Andean-Saharan
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[TD="class: Normal, width: 153"] c. 460 to 430 mya
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[TD="class: Normal, width: 182"] Caused the first great mass extinction.
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[TD="class: Normal, width: 233"] Gondwana drifted over the South Pole.
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[TD="class: Normal, width: 201"] Karoo
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[TD="class: Normal, width: 153"] c. 360 to 260 mya
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[TD="class: Normal, width: 182"] Destroyed Earth's first rainforests and resulted in a mass extinction that led to the rise of reptiles.
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[TD="class: Normal, width: 233"] Carbon sequestering by rainforests and Gondwana at South Pole.
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[TD="class: Normal, width: 201"] Pleistocene
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[TD="class: Normal, width: 153"] c. 2.5 mya to present
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[TD="class: Normal, width: 182"] Growing and retreating ice sheets led to cooling and drying, warming and moistening phases.
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[TD="class: Normal, width: 233"] The ultimate cause is declining carbon dioxide levels. The first proximate cause was probably Antarctica covering South Pole and becoming isolated. The second proximate cause was probably the formation of a land bridge between the Americas. The third proximate cause is variation in Earth's orientation to the Sun.
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Reconstruction of supercontinent Rodinia at 1.1 bya (Source: Wikimedia Commons)

Chapter summary:

• History of thought about ice ages and the Cryogenian Ice Age
  
  
• Beginnings of plate tectonics as a science
  
  
• Role of positive and negative feedbacks in geophysical and geochemical processes
  
  
• Oxygenation of the global ocean
  
  
• Shuram Excursion and Cryogenian Ice Age and oxygenation of the global ocean as examples of the nature of scientific controversies
  
  
• Use of "molecular clock" dating
  
  
• Formation of fossils
  
  
• Appearance of the first animals
  
  
• Appearance of the first large animals
  
  
• Appearance of the first Tethyan ocean
  
  
• First mass extinction of large complex life
  

This essay's section will provide a somewhat detailed review of the Cryogenian Ice Age and its aftermath, including some of the hypotheses regarding it, evidence for it, and its outcomes, as the eon of complex life arose after it. The Cryogenian Period ran from about 850 mya to 635 mya. This review will sketch the complex interactions of life and geophysical processes, and the increasingly multidisciplinary methods being used to investigate such events, which are yielding new and important insights.
The idea of an ice age is only a few hundred years old, and was first publicly proposed as a scientific hypothesis by Louis Agassiz in 1837, who got his first ideas from Karl Schimper and others.[108] There had also been proposals for ice ages in the preceding decades. By the 1860s, most geologists accepted the idea that there had been a cold period in Earth's recent past, attended by advancing and retreating ice sheets, but nobody really knew why.[109] Hypotheses began to proliferate, and in the 1870s, James Croll proposed that the idea that variations in Earth's orientation to the Sun caused the continental ice sheets. Because of problems with matching his hypothesis with dates adduced for ice age events, it fell out of favor and was considered dead by 1900.[110] Croll's work regained its relevance with the publication of a paper by Milutin Milanković (usually spelled Milankovitch in the West) in 1913, and by 1924, Milankovitch was widely known for explaining the timing of the advancing and retreating ice sheets of the current recent ice age.[111]
The book that made Milankovitch famous (Croll's work is still obscure, even though Milankovitch gave full credit to Croll in his work) was co-authored by Alfred Wegener, who a decade earlier first published his hypothesis that the continents had moved over the eons. As is often the case with radical new hypotheses, aspects of it previously existed in various stages of development, but Wegener was the first to propose a comprehensive hypothesis to explain an array of detailed evidence. Wegener was a meteorologist working outside of his specialty when he proposed his "continental drift" hypothesis. His hypothesis was harshly received and dismissed by the day's orthodoxy, and Wegener died in 1930 while setting up a research station on Greenland's ice sheet. His continental drift hypothesis quickly sank into obscurity. It was not until my lifetime, when paleomagnetic studies confirmed his views, that Wegener's work returned from exile and plate tectonics became a cornerstone of geological theory. Ice age data and theory does not pose an immediate threat to the global rackets or "national security," so the history of developing the data and theories has been publicly available.
Wegener concluded, based on his gathered evidence, that there was a global ice age in the Carboniferous and Permian periods. He was right.[112] Nearly 50 years later, in 1964, the same year that the first symposium of the plate tectonic era was held, Brian Harland proposed, based on paleomagnetic evidence, that there was a global ice age immediately preceding the Cambrian Period, where even the tropics were buried under ice. That was the first time that a truly global glaciation was proposed, and Harland's idea developed into what is today called the Snowball Earth hypothesis.
Ice ages are an important area of scientific investigation. Humanity's colossal burning of Earth's hydrocarbon deposits may well be delaying the return of the ice sheets, which have been advancing and retreating in rhythmic fashion for the previous million years.[113] Today, it is accepted that the tipping point for the current pattern has been Earth's orientation toward the Sun, particularly the eccentricity of Earth's orbit, which has a roughly 100,000-year cycle. Earth's orientation is universally considered to be the tipping point variable, but it is not the only influence. The ultimate cause has been steadily declining atmospheric carbon dioxide levels. Antarctica began developing its ice sheets about 35 mya due to its position near the South Pole and declining carbon dioxide levels. The current ice age began 2.5 mya, and it may have been initiated by the formation of Panama's isthmus three mya, which separated the Atlantic and Pacific oceans and radically altered oceanic currents. Also, the Arctic Ocean is virtually landlocked. Those factors all contributed to the current ice age.
In investigating how ice ages begin and end, positive and negative feedbacks are considered. A positive feedback will accentuate a dynamic, and a negative feedback will mute it. In the 1970s, James Lovelock and the author of today's endosymbiotic theory, Lynn Margulis, developed the Gaia hypothesis, which posits that Earth has provided feedbacks that maintain environmental homeostasis. Under that hypothesis, environmental variables such as atmospheric oxygen and carbon dioxide levels, ocean salinity levels, and Earth's surface temperature have been kept in relatively constant by a combination of geophysical, geochemical, and life processes, which have maintained Earth's inhabitability. The homeostatic dynamics were mainly negative feedbacks. If positive feedbacks dominate, then "runaway" conditions happen. In astrophysics, runaway conditions are responsible for a wide range of phenomena. A runaway greenhouse effect may be responsible for the high temperature of Venus's surface. Climate scientists today are concerned that burning the hydrocarbons that fuel the industrial age may result in runaway climatic effects. Mass extinctions are the result of Earth's becoming largely uninhabitable by the life forms existing during the extinction event. The ecosystems then collapse as portions of the food chains go extinct. Mass extinction specialist Peter Ward recently proposed his Medea hypothesis as a direct challenge to the Gaia hypothesis.
Gaian and Medean dynamics have both played roles in the development of Earth and its biosphere, and positive and negative feedbacks have had impacts. Life saved Earth's oceans with its negative feedback on hydrogen's loss to space, without which life as we know it on Earth probably would not exist. But there is also evidence that life contributed to mass extinction events.
Investigating the Cryogenian Ice Age led to finding evidence of runaway effects causing dramatic environmental changes, and the Cryogenian Ice Age's dynamics will be investigated and debated for many years. The position of Antarctica at the South Pole and the landlocked Arctic Ocean have been key variables in initiating the current ice age, and another continental configuration that could contribute to initiating an ice age is when a supercontinent is near the equator, which was the case during the Cryogenian Ice Age. A hypothesis is that Canfield Oceans can accompany supercontinents, so warm water is not pushed to the poles as vigorously.[114] A supercontinent near the equator would not normally have ice sheets, which means that silicate weathering would be enhanced and remove more carbon dioxide than usual. Those conditions could initiate an ice age, beginning at the poles. It would start out as sea ice, floating atop the oceans.
Around when Harland first proposed a global ice age, a climate model developed by Russian climatologist Mikhail Budyko concluded that if a Snowball Earth really happened, the runaway positive feedbacks would ensure that the planet would never thaw and become a permanent block of ice.[115] For the next generation, that climate model made a Snowball Earth scenario seem impossible. In 1992, a Cal Tech professor, Joseph Kirschvink, published a short paper that coined the term Snowball Earth. Kirschvink sketched a scenario where the supercontinent near the equator reflected sunlight, as compared to tropical oceans that absorb it. Once the global temperature decline due to reflected sunlight began to grow polar ice, the ice would reflect even more sunlight, and Earth's surface would become even cooler. This could produce a runaway effect where the ice sheets grew into the tropics and buried the supercontinent in ice. Kirschvink also proposed that the situation could become unstable. As the sea ice crept toward the equator, it would kill off all photosynthetic life, and a buried supercontinent would no longer engage in silicate weathering. Those are the two primary ways that carbon is removed from the atmosphere in the carbon cycle. Volcanism would have been the main way that carbon dioxide was introduced to the atmosphere (animal respiration also releases carbon dioxide, but this was before the eon of animals), and with the two primary dynamics for removing it suppressed by the ice, carbon dioxide would have increased in the atmosphere, and the resultant greenhouse effect would have eventually melted the ice and runaway effects would quickly turn Earth from an icehouse into a greenhouse. Kirschvink proposed the idea that Earth could vacillate between icehouse and greenhouse states.
Kirschvink noted that BIFs reappeared in the geological record during the possible Snowball Earth times, after vanishing about a billion years earlier. Kirschvink noted that iron cannot increase to levels where they would create BIFs if the global ocean was oxygenated. Kirschvink proposed that the sea ice not only killed the photosynthesizers, but it also separated the ocean from the atmosphere so that the global ocean became anoxic. Iron from volcanoes on the ocean floor would build up in solution during the icehouse phase, and during the greenhouse phase the oceans would become oxygenated and the iron would fall out in BIFs. Other geological evidence for the vacillating icehouse and greenhouse conditions was the formation of cap carbonates over the glacial till. It was a global phenomenon; wherever the Snowball Earth till was, cap carbonates were atop them. In geological circles, carbonate layers deposited during the past 100 million years are considered to be of tropical origin, so scientists think that the cap carbonates reflected a tropical environment. The fact of cap carbonates atop glacial till is one of the strongest pieces of evidence for the Snowball Earth hypothesis. Kirschvink finished his paper by noting that the eon of complex life came on the heels of the Snowball Earth, and scouring the oceans of life would have presented virgin oceans for the rapid spread of life in the greenhouse periods, and this could have initiated the evolutionary novelty that led to complex life.
Kirschvink is a polymath, was soon pursuing other interests, and left his Snowball Earth musings behind.[116] Canadian geologist Paul Hoffman had been an ardent Arctic researcher, but a dispute with a bureaucrat saw him exiled from the Arctic.[117] He landed at Harvard and soon picked Precambrian rocks in Namibia to study, as it was largely unexplored geological territory. The Namibian strata were 600-700 million years old, instead of the two billion years that Hoffman was familiar with. In the Namibian desert, he soon found evidence of glacial till among what were considered tropical strata when created.
Glacial till is composed of "foreign" stones that had been transported there by ice. When ice ages were first conceived, a key piece of evidence was "erratics," which were large stones found far from their place of origin. Erratics found in ocean sediments are called dropstones. Eventually, after plenty of controversy, scientists decided that erratics had usually been deposited by glaciers.[118] Oceanic dropstones were deposited by melting icebergs, the land-based erratics by retreating glaciers.
Hoffman's team tested the carbon-13/12 ratios of the cap carbonates and found them to be lifeless. That was key evidence presented in their 1998 paper that supported Kirschvink's Snowball Earth hypothesis.[119] As Kirschvink did, Hoffman and his colleagues argued that BIFs were evidence of Snowball Earth conditions, and they concluded their paper as Kirschvink did, by stating that the alternating icehouse and greenhouse periods would have produced extreme environmental stress on the ecosystems and may well have led to the explosion of complex life in their aftermath. A few months after publication of the Hoffman team's paper came another seminal paper, by Donald Canfield.[120] Those papers resulted in a flurry of scientific investigations and controversy, as Hoffman engaged in feuds, being Snowball Earth's front man. The Snowball Earth hypothesis has won out, so far. There is a "Slushball Earth" hypothesis that states that the Cryogenian Ice Age was not as severe as Hoffman and his colleagues suggest, and there are other disputes over the Snowball Earth hypothesis, but the idea of a global glaciation is probably here to stay, with a great deal of ongoing investigation. The record during the Cryogenian Ice Age shows immense swings in organic carbon burial, coinciding with forming the late-Proterozoic BIFs.[121] The Proterozoic Eon is the last one before complex life appeared on Earth.
Canfield's original hypothesis, which seems largely valid today, is that the deep oceans were not oxygenated until the Ediacaran Period, which followed the Cryogenian; the process did not begin until about 580 mya and first completed about 560 mya.[122] The wildest carbon-13/12 ratio swing in Earth's entire geological record begins about 575 mya and ends about 550 mya, and is called the Shuram excursion.[123] Explaining the Shuram excursion is one of the most controversial areas of geology today, with numerous proposed hypotheses. Ediacaran fauna, the first large, complex organisms to ever appear on Earth, also first appeared about 575 mya, when the Shuram excursion began.[124] I strongly doubt that Earth's first appearance of large complex life at the exact geological timescale moment of the biggest carbon-isotope anomaly in Earth's history will prove to be a coincidence. The numerous competing hypotheses regarding Shuram excursion include:

• The oxidation of a vast pool of organic carbon in the oceans, aided by the carbon-removal effect of animal feces and dead animals dropping to the ocean floor;[125]
  
  
• The excursion does not mark a genuine event relating to life processes, but is an artifact of geological processes (called diagenesis); this has a high hurdle to overcome, as the excursion has been measured globally and diagenesis is usually a local phenomenon, and no global mechanism has yet been proposed for it;[126]
  
  
• The excursion is the result of an asteroid impact that changed Earth's tilt;[127]
  
  
• The vaporization of methane hydrates on the ocean floor;[128]
  
  
• It was related to a global glaciation, like previous Snowball Earth glaciations;[129]
  
  
• The excursion was real, but there were others, and none of them significantly impacted Precambrian evolution.[130]
  

Deep-ocean currents, taking atmospheric gases deep into the oceans as they do today, do not seem to have existed during supercontinental times, and atmospheric oxygen was only a few percent at most when the Cryogenian Period began. Canfield's ocean-oxygenation evidence partly came from testing sulfur isotopes. As with carbon, nitrogen, and other elements, life prefers the lighter isotope of sulfur, and sulfur-32 and sulfur-34 are two stable isotopes that can be easily tested in sediments. Canfield proposed that in the pre-Cryogenian ocean's depths, sulfate-reducing bacteria, which are among Earth's earliest life forms and produce hydrogen sulfide as its waste product, abounded. Hydrogen sulfide gives rotten eggs their distinctive aroma, and is highly toxic to plants and animals, as it disables the enzymes used in mitochondrial respiration. Hydrogen sulfide would react with dissolved iron to form iron pyrite and settle out in the ocean floor, just as the iron oxide did that formed the BIFs. The sulfate-reducing bacteria will enrich the sulfur-32/34 ratio by 3% and did so before the Cryogenian, but the Ediacaran iron pyrite sediments showed a 5% enrichment, and a persuasive explanation is recycling sulfur in the oceanic ecosystem, which can only happen in the presence of oxygen.[131]
Part of the hypothesis for skyrocketing oxygen levels during the late Proterozoic was that high carbon dioxide levels, combined with a continent that had been ground down by glaciers, and the resumption of the hydrological cycle, which would have vanished during the Snowball Earth events, would have created conditions of dramatically increased erosion, which would have buried carbon (the cap carbonates are part of that evidence) and thus helped oxygenate the atmosphere. Evidence for that increased erosion also came in the form of strontium isotope analysis. Two of strontium's stable isotopes are strontium-86 and 87. Earth's mantle is enriched in strontium-86, while the crust is enriched in strontium-87, so basalts exposed to the ocean in the oceanic volcanic ridges are enriched in strontium-86, while continental rocks are enriched in strontium-87. If erosion is higher than normal, then the ocean sediments will be enriched in strontium-87, which analysis of Ediacaran ocean sediments confirmed. That evidence, combined with carbon isotope ratios, provided strong evidence of high erosion and high carbon burial, which would have increased atmospheric oxygen levels.[132] There is other evidence of increasing atmospheric oxygen content during the late Proterozoic, such as an increase in rare earth elements in Ediacaran sediments, and the consensus today is that the Cryogenian is when atmospheric oxygen levels began dramatically rising to modern levels, where they have largely stayed, although as this essay will later discuss, oxygen levels have varied widely since the late Proterozoic (from perhaps 13% to 35%).
An increase in atmospheric oxygen usually meant a decline in carbon dioxide, which would have cooled the planet. Recent data and models suggest that during the Cryogenian Period, global surface temperatures declined from around 40[SUP]o [/SUP]C to around 20[SUP]o [/SUP]C, and it has been below 30[SUP]o [/SUP]C ever since, generally fluctuating between 25[SUP]o [/SUP]C and 10[SUP]o [/SUP]C. Today's global surface temperature of around 15[SUP]o [/SUP]C is several degrees warmer than during the glacial periods of the current ice age, but is still among the lowest that Earth has ever experienced, and is generally attributed to atmospheric carbon dioxide's consistent decline during the past 100-150 million years.
Paleontologists were lonely fossil hunters for more than a century, but in my lifetime they found allies in geologists, and with DNA sequencing and genomics, molecular biologists have provided invaluable assistance. In 1996, a paper was published that created a huge splash in paleontological circles.[133] It was the work of molecular biologists that used the concept of the "molecular clock" of genetic divergence among various species. Their work concluded that the stage was set for animal emergence hundreds of millions of years before they appeared in the fossil record, particularly during the Cambrian Explosion. That paper initiated its own explosion of genetic research, and the current range of estimates has the genetic origins of animals somewhere between 1.2 bya and 700 mya, but this field is in its infancy, and more results are surely coming.[134] From an early optimism that molecular clocks could finely calibrate the timing of events, scientists have come to admit that "molecular clocks" do not reliably keep time. Today, the molecular evidence is used more to tell what happened than when. The geological and archeological record is considered more accurate for dating, and that evidence is sued for calibrating the molecular evidence. Even though "molecular clocks" keep far from perfect time, they are being used to do some timekeeping, when they can be bounded by other timing evidence, with a kind of interpolation of the data points.
In particular, the synergies of molecular biology and paleontology have identified the importance of Hox genes in early animals. In bilaterally symmetric animals, Hox genes dictate body development and are effectively identical in a fly and a chicken, which diverged from their common ancestor nearly 700 mya. Hox genes became an anchor in animal development, with the basics still unchanged after more than 600 million years.
In summary, today's orthodox late-Proterozoic hypothesis is that the complex dynamics of a supercontinent breakup somehow triggered the runaway effects that led to a global glaciation. The global glaciation was reversed by runaway effects primarily related to an immense increase in atmospheric carbon dioxide. During the Greenhouse Earth events, oceanic life would have been delivered vast amounts of continental nutrients scoured from the rocks by glaciers, and the hot conditions would have combined to create a global explosion of photosynthetic life. A billion years of relative equilibrium between the prokaryotes and eukaryotes was ultimately shattered, and oxygen levels dramatically rose during the Cryogenian and Ediacaran periods toward modern levels. Largely sterilized oceans, which began to be oxygenated at depth for the first time, are now thought to have prepared the way for what came next: the rise of complex life.
Fossils are created by undisturbed life form remains that become saturated with various chemicals, which gradually replace the organic material with rock by several different processes of mineralization.[135] Few life forms ever become fossils, but are instead consumed by other life; rare dynamics lead to fossil formation, usually by anoxic conditions leading to undisturbed sediments that protect the evidence and fossilize it. Scientists estimate that only about 1%-2% of all species that ever existed have left behind fossils that have been recovered. Geological processes are continually creating new land, both on the continents and under the ocean. The seafloor strata do not provide much insight into life's ancient past, particularly fossils, because the process recycles the oceanic crust in "mere" hundreds of millions of years. The basic process is that, in the Atlantic and Pacific sea floors in particular, oceanic volcanic ridges spew out basalt, and the plates flow toward the surrounding continents. When oceanic plates reach continental plates, the heavier mafic (basaltic) oceanic plates are subducted below the lighter felsic (granitic) continental plates. Parts of an oceanic plate were entirely subducted into the mantle more than 100 mya, and left behind plate fragments. On the continents, however, as they have floated on the heavier rocks, tectonic and erosional processes have not obliterated all ancient rocks and fossils. The oldest "indigenous" rocks yet found on Earth are more than four billion years old. Stromatolites have been dated to 3.5 bya, and fossils of individual cyanobacteria have been dated to 1.5 bya.[136] There are recent claims of finding fossils of individual organisms dated to 3.4 bya. The oldest eukaryote fossils found so far are of algae dated to 1.2 bya. The first amoeba-like vase-shaped fossils date from about 750 mya, and there are recent claims of finding the first animal fossils in Namibia, of sponge-like creatures which are up to 760 million years old.[137] Fossils of animals from 665 mya in Australia might be the first animal fossils, and some scientists think that animals may have first appeared about one bya. The first animals, or metazoans, were probably descended from choanoflagellates. The flagellum is a tail-like appendage that protists primarily used to move, and it could also be used to create a current to capture food. Flagella were used to draw food into the first animals, which would have been sponge-like. When the first colonies developed in which unicellular organisms began to specialize and act in concert, animals were born, and it is currently thought that the evolution of animals only happened once.[138] In interpreting the fossil record, there are four general levels of confidence: inevitable conclusions (such as ichthyosaurs were marine reptiles), likely interpretations (ichthyosaurs appeared to give live birth instead of laying eggs), speculations (were ichthyosaurs warm-blooded?), and guesses (what color was an ichthyosaur?).[139]
During the eon of complex life, the geologic time scale is divided by the distinctive fossils found in the sedimentary layers attributed to that time. Before the eon of complex life (that ancient time before complex life first appeared, which represents about 90% of Earth's existence so far, is called the Precambrian supereon today), fossils were microscopic and rare. Over time, geophysical forces eradicate sedimentary layers, and for the earliest animals, their fossils are found in only a few places on Earth. The first animal fossils of significance formed about 600 mya, and are strange creatures to modern eyes. They were first noticed in 1868 in Newfoundland, but the fledgling paleontological profession dismissed them, not recognizing them as fossils.[140] In Namibia in 1933, those Precambrian fossils were again noted but given a Cambrian chronology because the day's prevailing theory placed the beginning of animal life during the Cambrian Explosion. In 1946, in the Ediacara Hills in Australia, more such strange fossils were found in what were thought to be Precambrian rocks, but it was not until 1957, when those fossils were found in England, in rocks positively identified as Precambrian, that the first period of animal life, the Ediacaran, was on its way to recognition (it was not officially named the Ediacaran until 2004, for the first new period recognized since the 19[SUP]th[/SUP] century). In China, the Doushantuo Formation has provided fossils from about 635 mya to 550 mya, which covers the Ediacaran Period (c. 635 to 541 mya), and Ediacaran fossils have been found in a few other places. Microscopic algae spores and animal embryos abound in the Doushantuo cherts, and the spores look like little suns and other fanciful shapes. Almost all of them went extinct within a few million years of appearing in the fossil record, for an "invisible" mass extinction.[141] That mass extinction directly preceded the appearance of the first large organisms that Earth ever saw: Ediacaran fauna (also called "Ediacaran biota," in certain scientific circles, as there is debate whether those Ediacaran fossils were animal remains[142]).
Early Ediacaran fossil finds were often dismissed as pseudofossils because they did not fit the prevailing idea of an animal or plant, and Dickinsonia left the most famous Ediacaran fossils. Today, the most likely interpretation seems to be that Dickensonians flopped themselves down on bacterial mats and fed on them. When one finished eating a mat, it flopped its way to another. It was a bilateral-like creature, and is today classified into an extinct phylum with other Ediacaran fauna. Charnia looked like a plant but almost certainly was not, and is classified into another extinct phylum. Phyla are body plans, and the Ediacaran fauna are indeed strange looking. There is debate whether the Ediacaran fauna were plants, animals, or neither, and that debate will not end soon. Spriggina resembled a trilobite, and may have been its ancestor. Paths in the sediments, called feeding traces, have been found, but there was no deep burrowing in the Ediacaran Period. In the last few million years of the Ediacaran, the first skeletons appeared, particularly of the Cloudinids.[143] That characteristic Ediacaran fauna suddenly appeared in the fossil record about 575 mya, and all abruptly disappeared about 542 mya. Below are images of those Ediacaran forms, which can appear so bizarre to people today. (Source for all images: Wikimedia Commons)

There has been controversy regarding why Ediacaran fauna quickly disappeared, and even if their disappearance qualifies as a mass extinction.[144] One prominent idea is that their disappearance was due to predation by what became Cambrian fauna, and another is that they ate their food sources to extinction, but it appears more likely that it may have been an extinction brought on by anoxic oceans, and Cambrian fauna filled the vacant niches and then some, when the oceans became oxygenated again. Although Ediacaran fauna did not move much, their existence was probably owed to some oxygenation of the oceans, and although their metabolisms would have been slow compared to the animals that followed them, they may not have been able to survive in anoxic oceans. Ediacaran anoxic events are also when the first Middle East oil deposits were formed. The Proto-Tethys Ocean appeared in the Ediacaran, followed by the Paleo-Tethys and the Tethys, and those oceanic basins eventually all disappeared and their seafloors were subducted by colliding continents. Those subducted basins became the primary source of Middle East oil, which are extracted from Earth's most gigantic hydrocarbon deposits.
As with all "big idea" hypotheses such as those that gird the foregoing narrative of a global glaciation and the rise of complex life, there are challenges aplenty coming from various corners, and some are:

• There was not really a Snowball Earth, but several regional plateau glaciations have been misinterpreted as a global glaciation, and the reappearing BIFs were only local in nature;[145]
  
  
• There was not really a Snowball Earth, and a naturally wandering axis of rotation has created the illusion of tropical glaciation;[146] another version is that the magnetic poles wandered more than currently believed and made the paleomagnetic evidence invalid, which has created an illusion of tropical glaciation;
  
  
• The trigger for the Snowball Earth episodes was the drawdown in atmospheric carbon dioxide caused by life processes; one hypothesis is that land plants did it, as they colonized the continents hundreds of millions of years before popularly supposed, and another is that early animal life did it;[147]
  
  
• Reconstructions of the oxygen record are subject to a wide range of error, so the levels used to make life-related arguments may be invalid;[148]
  
  
• Animal activities may have been responsible for ventilating the oceans, especially near shore, so animals were a cause, not a consequence, of oxygenating the oceans;[149]
  
  
• Even if rising oxygen levels in the atmosphere and oceans coincided with the rise of complex life, it was not necessarily a causal relationship; some animals can respire anaerobically (at up to four times the usual rate for anaerobic respiration and fermentation), and perhaps the rise of complex life happened in an anaerobic environment, and animals only switched to aerobic respiration when oxygen became available;[150]
  
  
• Canfield's sulfur evidence may not be evidence of an oxygen increase, but of an increase in burrowing animals in the ocean sediments;[151]
  
  
• Oceanic salinity may have prevented complex life forming in the ocean, and maybe complex life first evolved on land and only entered the ocean when it was safe to do so, but the fossil record is too sparse to currently prove it; maybe even life itself first evolved in fresh water, not in oceanic volcanic vents;[152]
  
  
• Atmospheric oxygen levels really did not change around the ventilation episode; oxygen may have been no more important to the appearance of complex life than water or photosynthesis were;[153]
  
  
• The coming eon of complex life had no single underlying cause, but was the result of fortuitous circumstances and dynamics that happened when they did.[154]
  

Some hypotheses are stronger, others weaker, and some have already come and gone (and might be resurrected one day, like Birkeland's hypothesis was?). The coming generation of research may resolve most of these issues, but new ones will undoubtedly arise, and there is obviously a long way to go before significant consensus will be reached on those ancient events.
Again, the purpose of this chapter's presentation is to cover, in some depth, the scientific process and the kinds of controversies and numerous competing hypotheses that can appear, and to show how intersecting lines of evidence, brought from diverse disciplines and using increasingly sophisticated tools, are providing new and important insights, not only into the distant past, but which can also have modern-day relevance.
Readers for the collective task that I have in mind need to become familiar with the scientific process, partly so they can develop a critical eye for the kinds of arguments and evidence that attend the pursuit of FE and other fringe science/technology efforts. For the remainder of this essay, I will attempt to refrain from referring to too many scientific papers and getting into too many details of the controversies. Following my references will help readers who want to go deeply into the issues, and many of them are as deep and controversial as the Snowball Earth hypothesis and aftermath has proven to be, or attempts to explain the Shuram excursion. These are relatively new areas of scientific investigation, partly due to an improved scientific toolset and ingenious ways to use them. It is very possible that the controversies in those areas will diminish within the next generation, as new hypotheses account for increasingly sophisticated data, and paradigmatic changes in the near future are nearly certain. But science is always subject to becoming dogmatic and hypotheses can prevail for reasons of wealth, power, rhetorical skill, and the like, not because they are valid. The history of science is plagued with that phenomenon, and probably will be as long as humanity lives in the era of scarcity.
As will become a familiar theme in this essay, whether it was suffering from predation, a food shortage, or a lack of oxygen, in each instance it was primarily an energy issue. Ediacaran fauna either became an energy source for early Cambrian predators, they ran out of food energy, or they ran out of the oxygen necessary to power their metabolisms or lacked some other energy-delivered nutrient. For this essay's purposes, the most important understanding is that the Sun provides all of earthly life's energy, either directly or indirectly (all except nuclear-powered electric lights driving photosynthesis in greenhouses, and that energy came from dead stars). Today's hydrocarbon energy that powers our industrial world comes from captured sunlight. Exciting electrons with photon energy, then stripping off electrons and protons and using their electric potential to power biochemical reactions, is what makes Earth's ecosystems possible. Too little energy, and reactions will not happen (such as ice ages, enzyme poisoning, the darkness of night, food shortages, and lack of key nutrients that support biological reactions), and too much (such as ultraviolet light, ionizing radiation, temperatures too high for enzyme survival), and life is damaged or destroyed. The journey of life on Earth is all about adapting to varying energy conditions and finding levels where life can survive. For the many hypotheses about those ancient events and what really happened, the answers are always primarily couched in energy terms, such as how it was obtained, how it was preserved, and how it was used. For life scientists, that is always the framework, and they devote themselves to discovering how the energy game was played.

Speciation, Extinction, and Mass Extinctions

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[align=center]Earth's Largest Mass Extinction Events

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Major Extinction Event

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Minor Extinction Event

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Date

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Percent of Species or Genera that Went Extinct

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Suspected Primary Cause(s)

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Aftermath Dynamics

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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Microscopic organisms
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[TD="class: Normal, width: 86"] May have happened numerous times before eon of complex life.
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[TD="class: Normal, width: 128"] [/TD]
[TD="class: Normal, width: 176"] Changing sea temperatures and chemistry.
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[TD="class: Normal, width: 166"] The last microscopic mass extinction directly preceded the rise of the first animals that could be seen with the naked eye.
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Ediacaran
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[TD="class: Normal, width: 86"] c. 542 mya
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[TD="class: Normal, width: 128"] Unknown, but almost all Ediacaran forms disappeared.
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[TD="class: Normal, width: 176"] Anoxia
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[TD="class: Normal, width: 166"] Cambrian Explosion
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Mid-Cambrian
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[TD="class: Normal, width: 86"] c. 517 mya
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[TD="class: Normal, width: 128"] Unknown, but small shelly fauna largely disappear.
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[TD="class: Normal, width: 176"] Anoxia and changing sea levels.
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[TD="class: Normal, width: 166"] Trilobite radiation
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Dresbachian
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[TD="class: Normal, width: 86"] c. 502 mya
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[TD="class: Normal, width: 128"] 40% of marine genera
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[TD="class: Normal, width: 176"] Anoxia
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[TD="class: Normal, width: 166"] End of Golden Age of Trilobites, and brachiopods diminished.
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] End-Cambrian
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[TD="class: Normal, width: 86"] c. 485 mya
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[TD="class: Normal, width: 128"] Unknown, but half of trilobite species go extinct. Might be regional, but could be a major mass extinction.
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[TD="class: Normal, width: 176"] Rising sea levels and anoxia.
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[TD="class: Normal, width: 166"] Ordovician radiation
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[TD="class: Normal, width: 186"] OrdovicianSilurian
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[TD="class: Normal, width: 142"] [/TD]
[TD="class: Normal, width: 86"] c. 443 mya
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[TD="class: Normal, width: 128"] c. 85% of all species
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[TD="class: Normal, width: 176"] Temperature and sea level changes and anoxia.
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[TD="class: Normal, width: 166"] Ecosystem functioning not fundamentally altered.
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Ireviken
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[TD="class: Normal, width: 86"] c. 433
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[TD="class: Normal, width: 128"] 50% of trilobite and 80% of conodont species in seafloor event.
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[TD="class: Normal, width: 176"] Climate and sea level changes; it was a late ice age event. Chemistry and/or currents changes or anoxia.
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[TD="class: Normal, width: 166"] Disaster taxa appear afterward, followed by recovery.
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Mulde
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[TD="class: Normal, width: 86"] c. 427 mya
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[TD="class: Normal, width: 128"] Seafloor communities devastated
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[TD="class: Normal, width: 176"] Climate change, sea level changes, and anoxia.
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Lau
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[TD="class: Normal, width: 86"] c. 424 mya
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[TD="class: Normal, width: 128"] Seafloor communities devastated
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[TD="class: Normal, width: 176"] Climate change, sea level changes, and anoxia.
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[TD="class: Normal, width: 186"] Late Devonian
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[TD="class: Normal, width: 142"] [/TD]
[TD="class: Normal, width: 86"] c. 375 to 360 mya
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[TD="class: Normal, width: 128"] c. 70% of all species
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[TD="class: Normal, width: 176"] Series of extinctions. Sea level changes and anoxia. Mountain-building and volcanism could have triggered ice age that caused it.
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[TD="class: Normal, width: 166"] Arthropod and vertebrate colonization of land halted for 14 million years.
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[TD="class: Normal, width: 186"] [/TD]
[TD="class: Normal, width: 142"] Mid-Carboniferous
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[TD="class: Normal, width: 86"] ...