Short essay on Human Circulatory System

Those oceanic changes profoundly impacted Earth’s ecosystems. Not only did most warm-climate species go extinct, at least locally, but new species appeared that were adapted to the new environment. about 35 mya and were replaced by whales adapted to the new oceanic ecosystems that are still with us today: , which include dolphins, orcas, and porpoises; and , which adapted to the rich plankton blooms caused by of the new circulation, particularly in the . Sharks adapted to the new whales, which culminated with in the Oligocene. With the land bridges and small seas between the northern continents unavailable in colder times, the easy travel between those continents that characterized the Eocene’s warm times ended and the continents began developing endemic ecosystems. Europe became isolated from all other continents by the mid-Eocene and developed its own peculiar fauna. At the Oligocene’s beginning, the was no longer a barrier between Europe and Asia. More , although from competition, an extinction event, or other causes is still debated, and competition is favored. About half of European mammalian genera went extinct, replaced by immigrants from Asia, and some from North America via Asia.

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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)

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appeared in the Ediacaran, and Cambrian Period skeletons became a key aspect of the coming arms race between predator and prey. appeared in which about was transferred to the animal that ate it. Unlike the internal skeletons that characterized fish, amphibians, reptiles, birds, and mammals, the first skeletons were external. Hard shells protected from predation, and the bigger the animal, the more likely it would survive (but a bigger animal also meant a bigger energy windfall if it could be eaten). But size presented immense challenges. Similar to how complex cells needed to , increasing size presented numerous problems to early complex life. How could a large organism supply energy and other nutrients to its cells? Remove waste? Move? Life solved the problems by making structures and organs from specialized cells. By the Cambrian Period’s end, animals had developed skeletons, gills, muscles, brains, circulatory systems, digestive and eliminative systems, nervous systems, respiratory systems, and internal organs which included eyes, livers, kidneys, etc.

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Here is a brief summary of this essay. Ever since more than three billion years ago and about a billion years after the Sun and Earth formed, organisms have continually invented more effective methods to acquire, preserve, and use energy. after three billion years of evolution and, pound-for-pound, it used energy . The story of life on Earth has been one of , and in turn influencing them. During the eon of complex life that began more than 500 million years ago, there have been many brief for some fortunate species, soon followed by increased energy competition, a relatively stable struggle for energy, and then cleared biomes and set the stage for another golden age by organisms adapted to the new environments. Those newly dominant organisms were often marginal or unremarkable members of their ecosystems before the mass extinction. That pattern has characterized the journey of complex life over the past several hundred million years. among some animals, which provided them with a competitive advantage.

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Energy and the Human Journey: Where We Have Been; …

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