The process of transforming requires millions of years. When organic sediments are buried, most of the oxygen, nitrogen, hydrogen, and sulfur of dead organisms is released, leaving behind carbon and some hydrogen in a substance called , in a process that is . Plate tectonics can subduct sediments, particularly where oceanic plates meet continental plates. There is an “oil window” roughly between 2,000 and 5,000 meters deep; if kerogen-rich sediments are buried at those depths for long enough (millions of years), (which produce high temperature and pressure) break down complex organic molecules and the result is the hydrocarbons that comprise petroleum. If organic sediments never get that deep, they remain kerogen. If they are subducted deeper than that for long enough, bonds are broken and the result is , which is also called . Today, the geological processes that make oil can be reproduced in industrial settings that can in a matter of hours. Many hydrocarbon sources touted today as replacements for conventional oil were never in the oil window, so were not “refined” into oil and remain kerogen. The so-called and are made of kerogen ( is soluble kerogen). It takes a great deal of energy to refine kerogen into oil, which is why kerogen is an inferior energy resource. Nearly a century ago in it took less than one barrel of oil energy to produce one hundred barrels, for an energy return on investment ("EROI" or "") of more than 100, in the Golden Age of Oil. Global EROI is declining fast and will fall to about 10 by 2020. The EROIs of those oil shales and oil sands are less than five and as low as two.
In recent years, Neogene temperatures have been the focus of intensive research. What appears to be the proximate cause of elevated temperatures was a dramatic change in global ocean currents. The final closing of the , the isolation of Antarctica, the creation of , and the opening and closing of land bridges, such as in the Bering Sea and ultimately the land bridge between North and South America, created dramatic changes in ocean currents and global climate. One result was fluctuating . Its production declined beginning about 24 mya, and its weakness lasted until about 14 mya. Consequently, Earth’s oceans were not stratified as they are today, and warm water extended far lower into the oceans than it does today. Also, it reduced the temperature gradient between the equator and poles, which drives global currents: the greater the differential, the more vigorous the currents. It was still an Icehouse Earth, but the “mid-Miocene climatic optimum” was relatively warm. The past three million years are the coldest that Earth has seen since the that ended 260 mya, but this . While the steadily declining carbon dioxide levels of the past 150-100 million years is the ultimate cause of this Icehouse Earth phase, relatively short-term and regional fluctuations have had their proximate causes rooted in other geophysical, geochemical, and celestial dynamics.
Most plants produce seeds, which would have largely survived the catastrophe and began growing when conditions improved. Ferns came back first, in what is called a , as ferns are a . Crocodiles, modern birds (which included ), mammals, and amphibians also survived, and all could have found refuge in burrows, swamps, and shoreline havens, lived in tree holes and other crevices that they were small enough to hide in, and all could have eaten the catastrophe’s detritus. In general, freshwater species fared fairly well, especially those that could eat detritus. Also, the low-energy requirements of ectothermic crocodiles would have seen them survive when the mesothermic/ dinosaurs starved. The primary determinants seem to have been what could survive on detritus or energy reserves and what could not, and what could find refuge from the initial conflagration. While there may have been some evidence of dinosaur decline before the end-Cretaceous extinction (it was gradually growing colder), and the may have caused at least some local devastation, the complete extinction of non-avian dinosaurs, ammonites, marine reptiles, and others that would have been particularly vulnerable to the bolide event’s aftermath has convinced most dinosaur specialists that the bolide impact alone was sufficient to explain the extinction and no other hypothesis explains the pattern of extinction and survival that the bolide hypothesis does. In general, the key to surviving the end-Cretaceous extinction was being a marginal species, and all of those on center-stage paid the ultimate price. The end-Cretaceous extinction's toll was nearly 20% of all families, half of all genera, and about 75% of all species, and marked the end of an era; the Mesozoic ended and made way for the Age of Mammals, also called the , which used to have the .
When sea levels rise as dramatically as they did in the Cretaceous, coral reefs will be buried under rising waters and the ideal position, for both photosynthesis and oxygenation, is lost, and reefs can die, like burying a tree’s roots. About 125 mya, reefs made by , which thrived on , began to displace reefs made by stony corals. They may have prevailed because they could tolerate hot and saline waters better than stony corals could. About 116 mya, an , probably caused by volcanism, which temporarily halted rudist domination. But rudists flourished until the late Cretaceous, when they went extinct, perhaps due to changing climate, although there is also evidence that the rudists . Carbon dioxide levels steadily fell from the early Cretaceous until today, temperatures fell during the Cretaceous, and hot-climate organisms gradually became extinct during the Cretaceous. Around 93 mya, , perhaps caused by underwater volcanism, which again seems to have largely been confined to marine biomes. It was much more devastating than the previous one, and rudists were hit hard, although it was a more regional event. That event seems to have , and a family of . On land, , some of which seem to have , also went extinct. There had been a decline in sauropod and ornithischian diversity before that 93 mya extinction, but it subsequently rebounded. In the oceans, biomes beyond 60 degrees latitude were barely impacted, while those closer to the equator were devastated, which suggests that oceanic cooling was related. shows rising oxygen and declining carbon dioxide in the late Cretaceous, which reflected a general cooling trend that began in the mid-Cretaceous. Among the numerous hypotheses posited, late Cretaceous climate changes have been invoked for slowly driving dinosaurs to extinction, in the “they went out with a whimper, not a bang” scenario. However, it seems that dinosaurs did go out with a bang. A big one. Ammonoids seem to have been brought to the brink with nearly marine mass extinctions during their tenure on Earth, and it was no different with that late-Cretaceous extinction. Ammonoids recovered once again, and their lived in the late Cretaceous, but the end-Cretaceous extinction marked their final appearance as they went the way of and other iconic animals.
While oxygen level changes of the model show early fluctuations that the model does not, both models agree on a huge rise in oxygen levels in the late Devonian and Carboniferous, in tandem with collapsing carbon dioxide levels. There is also virtually universal agreement that that situation is due to rainforest development. Rainforests dominated the Carboniferous Period. If the Devonian could be considered terrestrial life’s , then the Carboniferous was its . In the Devonian, plants developed vascular systems, photosynthetic foliage, seeds, roots, and bark, and true forests first appeared. Those basics remain unchanged to this day, but in the Carboniferous there was great diversification within those body plans, and Carboniferous plants formed the foundation for the first complex land-based ecosystems. Ever since the episodes, there has , and the that have prominently shaped Earth’s eon of complex life probably always began with ice sheets at the South Pole, and the current ice age arguably is the only partial exception, but today’s cold period really began about 35 mya, .
Polar forests reappeared in the Eocene after the , and the Eocene’s was the Cenozoic’s warmest time and . Not only did alligators live near the North Pole, but the continents and oceans hosted an abundance and diversity of life that Earth may have not seen before or since. That ten million year period ended as Earth began cooling off and headed toward the current ice age, and it has been called the original Paradise Lost. One way that methane has been implicated in those hot times is that leaves have , which regulate the air they take in to obtain carbon dioxide and oxygen, needed for photosynthesis and respiration. Plants also lose water vapor through their stomata, so balancing gas input needs against water losses are key stomata functions, and it is thought that in periods of high carbon dioxide concentration, . Scientists can count stomata density in fossil leaves, which led some scientists to conclude that carbon dioxide levels were not high enough to produce the PETM, so that produced the PETM and , and the controversy and research continues.
For this essay’s purposes, the most important ecological understanding is that the Sun provides all of earthly life’s energy, either (all except nuclear-powered electric lights driving photosynthesis in greenhouses, as 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 , ionizing radiation, temperatures too high for enzyme activity), and life is damaged or destroyed. The journey of life on Earth has primarily been 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 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.
Trees first appeared during a plant diversity crisis, and the arrival of seed plants and ferns ended the dominance of the first trees, so the plant crises may have been more about evolutionary experiments than environmental conditions, although a carbon dioxide crash and ice age conditions would have impacted photosynthesizers. The that gave rise to trees and seed plants largely went extinct at the Devonian’s end. But what might have been the most dramatic extinction, as far as humans are concerned, was the impact on land vertebrates. During the about 20% of all families, 50% of all genera, and 70% of all species disappeared forever.
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.
Whatever the case may be, it appears clear that the population in Africa and Neanderthal population in Europe and the Middle East were isolated for tens of thousands of years, perhaps far more than 100,000 years, and humans used a toolkit like the Neanderthals’ until something happened between 70 and 50 kya. Just happened is a matter of great controversy, and in recent years, several disciplines have converged on the issue and are drawing a clearer picture today. Some key findings that shed light came from global DNA studies, linguistics partnering with evolutionary theory, and brain studies. In the past generation, as has been applied to many areas, a startling picture of the human journey has emerged. , probably for flexible power generation. For animals that reproduce sexually, the mother’s mitochondria are passed to her offspring, while virtually none comes from the father, if any. Geneticists can measure mutations in and approximate when two different animals shared a common ancestor, whether they belong to the same species or not. Similarly, regarding nuclear DNA, the produces a male mammal, and mutations in the Y chromosome can also be analyzed to estimate when two men shared the same ancestor. , but scientists have been aligning DNA results with fossil dates, which are considered more reliable, and have been resolving some limitations. But if the timing is suspect for such genetic analyses, far more confidence exists for descent relationships. Human DNA testing is a burgeoning business, used for everything from freeing to to examining the genetic heritage of the .