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In the early 19th century, a dispute was personified by , a British lawyer and geologist, and , a French paleontologist. Their respective positions came to be known as and . Just as , so did uniformitarianism prevail in scientific circles. Under the comforting uniformitarian worldview, there was no such thing as a global catastrophe. Changes had only been gradual, and only the present geophysical, geochemical, and biological process had ever existed. The British Charles Darwin explicitly made Lyell’s uniformitarianism part of his evolutionary theory and he proposed that extinction was only a gradual process. Cuvier was , which contradicted the still-dominant Biblical teachings, even in the . Although Cuvier did not subscribe to the , his catastrophic extinction hypothesis was informed by his fossil studies. But Lyell and Darwin prevailed. Suggesting that there might have been catastrophic mass extinctions in Earth’s past was an invitation to be branded a pseudoscientific crackpot. That state of affairs largely prevailed in orthodoxy until the 1980s, after the was posited for the dinosaurs’ demise. An effort led by a scientist publishing outside of his field of expertise (a ) removed from its primacy. Only since the 1980s have English-speaking scientists studied mass extinctions without facing ridicule from their peers, which has never been an auspicious career situation. Since then, many and mass extinction events have been studied, but the investigations are still in their early stages, partly due to a dogma that prevailed for more than a century and a half, and Lyell’s uniformitarianism is influential. The ranking of major mass extinctions is even in dispute, , and a was recently .

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, scientists have discovered planets in other star systems by using a variety of methods that reflect the improving toolset that scientists can use, especially space-based telescopes. Before those discoveries, there was controversy whether planets were rare phenomena, but scientists now admit that planets are typical members of star systems. , so planets hosting intelligent life may not be all that rare.

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The genetic testing that has been performed on humanity in the past generation has shown that the founder group’s pattern of migration was to continually spread out, and once the original settlement covered the continents, people did not move much at all, at least until Europe began conquering the world (and there were some ). There is little sign of warfare in those early days of migration, and the leading hypothesis is that people moved to the next valley rather than be close enough to fight each other. Any conflict would have been easily resolved by moving farther out, where more easily killed animals lived. Also, in those virgin continents, people need not have roamed far to obtain food. Today, an !Kung woman will carry her child more than 7,000 kilometers before the child can walk for himself/herself. If an !Kung woman bears twins, it is her duty to pick which child to murder, because she cannot afford to carry two. That demonstrates the limitations of today’s hunter-gatherer lifestyle, but in those halcyonic days of invading virgin continents (which had to be the Golden Age of the Hunter-Gatherer), those kinds of practices probably waned and bands grew fast. When they they split, and the new group moved to new lands where the animals, again, never saw people before. Unlike the case with humans, there would not have been a grapevine so that animals told their neighbors about the new super-predator. The first time that those megafauna saw humans was probably their last time. It is very likely, just as with all predators for all time, and as can be seen with historical hunting events such or , that those bands soon took to killing animals, harvesting the best parts, and moving on. To them it would not have been a “blitzkrieg,” but more like kids in candy stores. After a few thousand years of grabbing meat whenever the fancy took them, or perhaps less, those halcyonic days were over as the far coasts of Australia were reached and the easy meat was gone. When that land bridge formed to Tasmania about 43 kya, people crossed and were able to , until all the megafauna was gone on Tasmania. They also may have worked their way through the food chain, in which the first kills were the true mother lode. Nobody even deigned to raise a spear at anything less than a until they were gone. Then they started killing smaller prey, which eventually did wise up and were harder to kill, so humans had to work at it again and the brief golden age was over. The as they shaped the new continent to their liking, maybe recreating the savanna conditions that they left in Africa, may have also been used to flush out animals if they began to avoid humans.

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, which would have cooled the planet. Recent data and models suggest that during the Cryogenian Period, global surface temperatures declined from around 40o C to around 20o C, and it has been below 30o C ever since, generally fluctuating between 25o C and 10o C. Today’s global surface temperature of around 15o C is several degrees warmer than during the glacial periods of the current ice age but is still , and is generally attributed to .

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In summary, today’s orthodox late-Proterozoic hypothesis is that the complex dynamics of a supercontinent breakup somehow triggered . The global glaciation was reversed by runaway effects primarily related to an immense increase in atmospheric carbon dioxide. During the 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 prokaryotes and eukaryotes was ultimately shattered, and oxygen levels began rising 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.

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are created by undisturbed organism remains that become saturated with various chemicals, which gradually replace the organic material with rock by . 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. Seafloor strata do not provide much insight into life’s ancient past, particularly fossils, because the process in “mere” . 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 (basaltic) oceanic plates are subducted below the lighter (granitic) continental plates. Parts of an oceanic plate were 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 . have been dated to 3.5 bya, and fossils of individual cyanobacteria have been dated to 1.5 bya. There are recent claims of finding . The oldest eukaryote fossils found so far are of . 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 . Fossils from might be the first animal fossils, and some scientists think that animals may have first appeared about one bya. The first animals, or , probably descended from . The 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 probably only happened . 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?).