This essay has presented Earth’s many changing faces during its journey. Earth had molten beginnings, was , and may have . Later, and and . Earth experienced swings from to conditions as atmospheric gases dramatically changed, continents moved, and vast and of complex life played out on land and sea. But the changes happened over timescales of millions and billions of years, not hundreds. No climate scientist will deny that carbon dioxide traps infrared radiation and warms Earth’s atmosphere. The vented enough carbon dioxide into the atmosphere to create 200 million years of Greenhouse Earth conditions, when reptiles ruled Earth. Volcanism waned and around 150-to-100 mya. By 35 mya, and the Antarctic ice sheet began forming. Every paleoclimate study I have seen places greenhouse gas (and primarily carbon dioxide) concentrations as the primary determinant of global surface temperatures, after the Sun's radiation, but the Sun's output is considered to have been exceptionally stable and has risen slowly over the eons. , usually by accentuating the carbon dioxide with a positive feedback effect that may have reached runaway conditions at times.
The other critical innovation was the modern steam engine, which was intimately related to coal. Burgeoning coal mines quickly exhausted deposits above the water table and began digging deeply into the earth, and water in the mines became a great problem. Not only were floods killing miners, but standing water made mines inoperable. Romans pumped water from their mines (). So did British mining operations, and around 1710, combined the ideas of a and an to make the , to pump water from coal mines. In a parallel case of using coal for smelting, the coal-fired Newcomen engine was . It was the first of its kind, primitive compared to later engines, and its spread was gradual. . He eventually invented an improved version with a that was . The steam engine that powered the Industrial Revolution was thus born, although, as with coal, its spread was gradual, and wind and water power were competitive with coal for nearly a century. The hydrocarbon-fueled steam engine was the key to the Industrial Revolution, in which the energy of ancient sunlight was exploited to generate previously unimaginable power. A steam locomotive of 1850 roaring through the English countryside would have been inconceivable to an English peasant of 1500. From a to to to less than five hundred years, the duration of each Epochal Event continued to shrink as levels of energy use increased dramatically and with each event.
What was most relevant to humans, however, was the almost-complete extinction during the Kellwasser event of the tetrapods that had come ashore. Tetrapods did not reappear in the fossil record until several million years after the Kellwasser event, and has even been referred to as the Fammenian Gap (the is the Devonian’s last age). The Kellwasser event also appeared to be a period of low atmospheric oxygen content, and some evidence is the lack of charcoal in fossil deposits. Recent research has demonstrated that getting wood to burn at oxygen levels of less than 13-15% may be impossible. Because all periods of complex land life show evidence of forest fires, it is today thought that oxygen levels have not dropped below 13-15% since the Devonian, but during the “charcoal gap” of the late Devonian, when the first landlubbing tetrapods went extinct, oxygen levels reached their lowest levels since the , which must have impacted the first animals trying to breathe air instead of water. During the , there is no charcoal evidence at all, which leads to the notion that oxygen levels may have even dropped below 13%. This drop may be related to severe climatic stresses on the new forests, which are probably related to the ice age that the forests helped bring about due to their carbon sequestering. That is an attractively explanatory scenario, but the continues. The first seed plants probably appeared before the Kellwasser event, but it was not until after the Fammenian Gap that seed plants began to proliferate.
But it was not only flying insects that became huge: giant , , and other arthropods also lived in the Carboniferous, such as mayflies with half-meter wingspans. The (more than two meters long) has been featured in popular culture as a nightmare creature, although it was vegetarian. The lived in the Carboniferous and reached seven meters long. The high-oxygen hypothesis is challenged for and giant animals in general, and the controversy will probably continue for many more years.
In the oceans, the Carboniferous is called the Golden Age of Sharks, and ray-finned fish arose to a ubiquity that they have yet to fully relinquish. Ray-finned fish probably prevailed because of their high energy efficiency. Their skeletons and scales were lighter than those of armored and lobe-finned fish, and their increasingly sophisticated and lightweight fins, their efficient tailfin method of propulsion, changes in their skulls, jaws, and new ways to use their lightweight and versatile equipment accompanied and probably led to the rise and subsequent success of ray-finned fish in the Carboniferous and afterward. , which are amoebic protists, rose to prominence for the first time in the Carboniferous. Reefs began to recover, although they did not recover to pre-Devonian conditions; those vast Devonian reefs have not been seen again. did not appear until the . Trilobites steadily declined and nautiloids familiar today, and straight shells became rare. The first , which were ancestral to squids and octopi, first appeared in the early Carboniferous, but some Devonian specimens might qualify. Ammonoids flourished once again, after barely surviving the Devonian Extinction. This essay is only focusing on certain prominent clades, and there are many and . The early Carboniferous, for example, is called the Golden Age of , which are a kind of , which is a phylum that includes starfish. The crinoids had their golden age when the fish that fed on them disappeared in the end-Devonian extinction. Earth’s ecosystems are vastly richer entities than this essay, or essay, can depict.
The Cambrian Explosion’s iconic animal was the . As a child, I read every paleontology text in my elementary school’s library, and I have fond memories of imagining trilobite lives. Was there love among the trilobites? Among the protists? The bacteria? To a scientist, those questions might be unanswerable and even meaningless, but a mystic might pursue them. I will not wax too mystically in this essay (I do it ), but that may well be the big question of life on Earth and . The nature of consciousness and love in the Cambrian, or the lack thereof, as much as it may always be a mystery, does not invalidate life’s arc through the evolutionary process; it only challenges materialism.
consist of body plans, which scientists have used to classify all life forms, and all significant animal phyla had appeared by the Cambrian Period’s end. The Cambrian Explosion has been difficult to explain and there is still great controversy and many unanswered questions, and it has also been difficult to explain why significant change stopped the explosion. Once the basic body plans appeared and biomes were filled, new plans never appeared again. Why did all fundamental change stop? The emerging view is the same for why complex life with and never changed since then. Not only could innovation confer great benefits, but , further travel along the developmental path made it continually less feasible to backtrack, start over, and take another path, or choose a fundamentally different path. The history of life’s choices was reflected in organisms in several ways, and the source of that inertia began to be understood when biology and chemistry at the cellular and subcellular levels were investigated, particularly after DNA was sequenced and studied. The fact that have not significantly changed in several hundred million years points to the issue. Hox genes have not changed because they control key developmental steps in embryonic development. Not only do Hox genes work, there are no practical ways to significantly change them, as they lay the animal’s structural foundation. Hox genes are called regulatory genes, and the nature of seems to be why animals have not fundamentally changed since the Cambrian Explosion.
But the branch of the that readers might find most interesting led to humans. Humans are in the phylum, and the last common ancestor that founded the Chordata phylum is still a mystery and understandably a source of controversy. Was our ancestor a ? A ? Peter Ward made the case, as have others for a long time, that it was the sea squirt, also called a tunicate, which in its larval stage resembles a fish. The nerve cord in most bilaterally symmetric animals runs below the belly, not above it, and a sea squirt that never grew up may have been our direct ancestor. Adult tunicates are also highly adapted to extracting oxygen from water, even too much so, with only about 10% of today’s available oxygen extracted in tunicate respiration. It may mean that tunicates adapted to low oxygen conditions early on. Ward’s respiration hypothesis, which makes the case that adapting to low oxygen conditions was an evolutionary spur for animals, will repeatedly reappear in this essay, as will . Ward’s hypothesis may be proven wrong or will not have the key influence that he attributes to it, but it also has plenty going for it. The idea that fluctuating oxygen levels impacted animal evolution has been gaining support in recent years, particularly in light of recent reconstructions of oxygen levels in the eon of complex life, called and , which have yielded broadly similar results, but their variances mean that much more work needs to be performed before on the can be done, if it ever can be. Ward’s basic hypotheses is that when oxygen levels are high, ecosystems are diverse and life is an easy proposition; when oxygen levels are low, animals adapted to high oxygen levels go extinct and the survivors are adapted to low oxygen with body plan changes, and their adaptations helped them dominate after the extinctions. The has a pretty wide range of potential error, particularly in the early years, and it also tracked atmospheric carbon dioxide levels. The challenges to the validity of a model based on data with such a wide range of error are understandable. But some broad trends are unmistakable, as it is with other models, some of which are generally declining carbon dioxide levels, some huge oxygen spikes, and the generally relationship between oxygen and carbon dioxide levels, which a geochemist would expect. The high carbon dioxide level during the Cambrian, of at least 4,000 PPM (the "RCO2" in the below graphic is a ratio of the calculated CO2 levels to today's levels), is what scientists think made the times so hot. (Permission: Peter Ward, June 2014)
Entropy is another important concept for this essay. Entropy is, in its essence, the tendency of hot things to cool off. The concept is now introduced to students as . Even though science , it can measure its effect. At the molecular level, entropy is the tendency of mass to become disordered over time, as the random motion of molecules spreads in collisions with other molecules, until the interacting molecules have the same . Life had to overcome entropy in order to exist, as it brought order out of disorder and maintained it while alive, and it takes energy to do that. The prevailing theory is that net entropy can only increase, and life has to create more entropy in its surroundings so that it can reduce entropy internally and produce and maintain the order that sustains itself. Life is called a negentropic phenomenon, in which it uses energy to reverse entropy to make the order of its organism’s structures, and it is continually using energy to reverse the natural entropy that is called decay.