As the war continued, the Athenian hinterland was turned into a desert. Plato described the deforestation of , which remained barren until my lifetime, when the Greek government began to reforest it; many trees could only be planted by blasting holes in the limestone bedrock. When Attica's residents returned home after the Spartan occupation, they built their homes with a southern orientation to take advantage of sunlight, as wood was scarce. After five years of peace with Sparta subsequent to , Athens took to the offensive again and pretended to intervene in a war in Sicily to protect Ionian colonists, but they really did it to conquer Sicily and plunder its forests and other resources, and thereby build another naval fleet to conquer Sparta. The was a catastrophe for Athens, and it lost most of its navy. There were other setbacks and victories, but a starving and besieged Athens finally surrendered to the Spartans in 404 BCE. The environment around Athens could feed nothing but “bees,” and where wolves once abounded, not a rabbit could be found. As Athens slowly became the center of a wasteland, the changing perceptions could be seen in contemporary writing. When forests were plentiful in 700 BCE, Greek authors wrote of trees in pragmatic fashion or as impediments to progress. As the forests disappeared along with the ecosystems they supported, an ecological consciousness began to appear. Plato and Aristotle placed forests at the root of a civilization’s health, and . Conservation only became an idea when the environment had already been ruined by “progress." Numerous commentators of the day wrote about the connections between forests and a healthy water supply, and many clearly saw the relationship between deforestation, erosion, and desertification, including Plato. and his professional heir wrote about ecological ideas. Theophrastus could be considered the first ecological writer, and he had the beginnings of an ecosystems approach. He noted that when the region surrounding was deforested, it became dryer and warmer.
Because the Western Hemisphere’s inhabitants were virtually all in their Stone Age, they as greatly as Old World civilizations did, and many societies were environmentally sustainable and provided seeming answers to questions that scientists have asked about Old World civilizations’ development. The natives of coastal California were familiar with agriculture, as it was practiced by nearby inland tribes, but they never adopted it. California was so bountiful, and its climate was so human-friendly, that its natives retained their hunter-gatherer lifestyle. Similarly, northward on the Pacific Northwest's coast, natives created an economy in which half of its calories derived from salmon runs, and those peoples were relatively sedentary without agriculture. Natives turned the Great Plains into a big pasture for bison, and the biome was partly maintained by annual burning of the grasslands. In Mesoamerica, farming has been sustainable for thousands of years. In the Amazon, the natives transformed the rainforest, and a higher proportion of plants and trees provided human-digestible foods than in any other “wild” place on Earth, those natives also terraformed thin tropical soils with ceramics (maybe unintentional) and charcoals (intentional) and made super-soils called and . In summary, native practices in the Western Hemisphere were often sustainable if not quite abundant. But when civilizations arose, they had problems that were like their Old World counterparts'. Their problems were also environmental and not just the injustices of hierarchal societies, often steeply hierarchical.
Until my lifetime, scientists thought of dinosaurs as slow and stupid, but that view has changed. In the 1970s, scientists realized that prior depictions of bipedal dinosaurs such as . Their actual posture had the tail, spine, and head all on a line largely parallel with the ground. Not until the release of did the public begin to see more realistic portrayals of bipedal dinosaur posture. That posture may have been critical for the success of dinosaurs, as becoming bipedal, with their legs in an upright position under their bodies, allowed them to overcome . Also, the notion of overcoming Carrier’s Constraint transformed the view of dinosaurs from lumbering, slow creatures to nimble runners. The dinosaur line is considered , and the first dinosaurs were bipeds. All quadrupedal dinosaurs re-evolved their four-legged stances from the original bipedal posture, which is obvious in that nearly all quadrupedal dinosaurs had rear legs longer than their front ones.
The ecosystems may not have recovered from Olson’s Extinction of 270 mya, and at 260 mya came another mass extinction that is called the mid-Permian or extinction, or the , although a recent study found only one extinction event, in the mid-Capitanian. In the 1990s, the extinction was thought to result from falling sea levels. But the first of the two huge volcanic events coincided with the event, in . There can be several deadly outcomes of major volcanic events. As with an , massive volcanic events can block sunlight with the ash and create wintry conditions in the middle of summer. That alone can cause catastrophic conditions for life, but that is only one potential outcome of volcanism. What probably had far greater impact were the gases belched into the air. As oxygen levels crashed in the late Permian, there was also a huge carbon dioxide spike, as shown by , and the late-Permian volcanism is the near-unanimous choice as the primary reason. That would have helped create super-greenhouse conditions that perhaps came right on the heels of the volcanic winter. Not only would carbon dioxide vent from the mantle, as with all volcanism, but the late-Permian volcanism occurred beneath Ediacaran and Cambrian hydrocarbon deposits, which burned them and spewed even more carbon dioxide into the atmosphere. Not only that, great salt deposits from the Cambrian Period were also burned via the volcanism, which created hydrochloric acid clouds. Volcanoes also spew sulfur, which reacts with oxygen and water to form . The oceans around the volcanoes would have become acidic, and that fire-and-brimstone brew would have also showered the land. Not only that, but the warming initiated by the initial carbon dioxide spike could have then warmed up the oceans enough so that methane hydrates were liberated and create even more global warming. Such global warming apparently warmed the poles, which not only melted away the last ice caps and ended an ice age that had , but deciduous forests are in evidence at high latitudes. A 100-million-year Icehouse Earth period ended and a 200-million-year Greenhouse Earth period began, but the transition appears to have been chaotic, with wild swings in greenhouse gas levels and global temperatures. Warming the poles would have lessened the heat differential between the equator and poles and further diminished the lazy Panthalassic currents. The landlocked Paleo-Tethys and Tethys oceans, and perhaps even the Panthalassic Ocean, may have all become superheated and anoxic as the currents died. Huge also happened, which may have and led to ultraviolet light damage to land plants and animals. That was all on top of the oxygen crash. With the current state of research, all of the above events may have happened, in the greatest confluence of life-hostile conditions during the eon of complex life. A recent study suggests that the extinction event that ended the Permian may have lasted only 60,000 years or so. In 2001, a bolide event was proposed for the Permian extinction with great fanfare, but it does not appear to be related to the Permian extinction; the other dynamics would have been quite sufficient. The Permian extinction was the greatest catastrophe that Earth’s life experienced since the previous supercontinent existed in the .
The Triassic began hot and ended hot, and the Jurassic and Cretaceous were also hot, so staying warm was not a significant issue for dinosaurs. stayed cool by becoming aquatic, and for land-based dinosaurs, features such as plates apparently replaced the sails of for both heating and cooling, and like the synapsid sail, those plates may have also been used for display. Also, like the cliché, many large herbivorous dinosaurs lived near cooling swamps, although the issue has been controversial. Cooling swamps and protective water holes that we see in the tropics today were a major aspect of Mesozoic landscapes. But the thermoregulatory aspect that most work is directed toward today is how dinosaurs kept warm. There is compelling evidence that dinosaurs regulated their body temperature in myriad ways, including internal chemistry. All bipedal animals today are endotherms and they all have four-chambered hearts, as dinosaurs did. , dinosaurs living near the poles (, ), and of dinosaur bones all support the idea that , but one of the more intriguing areas is that of . Like tree rings, bones have seasonal growth rings and they have been read for many dinosaur fossils. They have been used to determine dinosaurian life expectancies. could live to be about 30, giant could live to be 50, and smaller dinosaurs, as with smaller mammals, lived shorter lives. The tiny ones only lived three-to-four years and the mid-sized ones lived seven-to-fifteen years. Growth rates also provide thermoregulation evidence. Tyrannosaurs had juvenile growth spurts and largely stopped growing as adults, and sauropods had growth rates equivalent to today’s whales, which are Earth’s fastest growing animals. But there is also evidence of ectothermic dynamics. The great size of dinosaurs would have led to relatively easy ways to stay warm, as large animals have a greater mass-to-surface area ratio, like the way in which . Also, in the generally hot Mesozoic times, staying warm would have been fairly easy, particularly for huge dinosaurs.
This chapter 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 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.
During that “,” , , and the rise of grazing and predation had eonic significance. While many critical events in life’s history were unique, one that is not is multicellularity, , and some prokaryotes have multicellular structures, some even with specialized organisms forming colonies. There are , but the primary advantage was size, which would become important in the coming eon of complex life. The rise of complex life might have happened faster than the billion years or so after the basic foundation was set (the complex cell, oxygenic photosynthesis), but geophysical and geochemical processes had their impacts. Perhaps most importantly, the oceans probably did not get oxygenated until just before complex life appeared, as they were sulfidic from 1.8 bya to 700 mya. Atmospheric oxygen is currently thought to have remained at only a few percent at most until about 850 mya, although there are recent arguments that it remained low until only about 420 mya, when large animals began to appear and animals began to colonize land. Just as the atmospheric oxygen content began to rise, then came the biggest ice age in Earth’s history, which probably played a major role in the rise of complex life.
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 . As is often the case with radical new hypotheses, , 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 , and . His continental drift hypothesis quickly sank into obscurity. It was not until my lifetime, when , that Wegener’s work returned from exile and became a cornerstone of geological theory. Ice age data and theory does not pose an immediate threat to the or "," so the history of developing the data and theories has been publicly available.
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. The wildest swing in Earth’s entire geological record begins about 575 mya and ends about 550 mya, and is called the Shuram excursion. Explaining the Shuram excursion is one of the most controversial areas of geology today, with numerous proposed hypotheses. When the controversies are finally resolved, if they resolved, the Shuram and excursions, even though they go in opposite directions, I suspect will likely be both related to the dynamics of ice ages and the rise of oxygen levels. Ediacaran fauna, the first large, complex organisms to ever appear on Earth, also first appeared about 575 mya, when the Shuram excursion began. I strongly doubt that Earth’s first appearance of large complex life at the exact geological timescale moment of the wildest carbon-isotope swing in Earth’s history will prove to be a coincidence. The numerous competing hypotheses regarding the Shuram excursion include:
Just as were “invented,” somewhere between 1.6 bya and 600 mya a eukaryote ate a cyanobacterium and both survived, and that cyanobacterium became the ancestor of all chloroplasts, which is the photosynthetic organelle in all plants. As with similar previous events, it appears that it , and all plants are descended from that unique event. The invention of the chloroplast , which were the first plants. The first algae fossils are from about 1.2 bya. Most algae species are not called plants, as they are not descended from that instance when a eukaryote ate a cyanobacterium. The non-plant algae, such as , also have chloroplasts, from various “envelopment” events when algae chloroplasts were eaten and the grazers and chloroplasts survived. Below is the general outline of the tree of life today, in which bacteria and archaea combined to make eukaryotic cells, and in which the bacterium enveloped into a protist to make plants, and all complex life developed from protists. (Source: Wikimedia Commons)