Chapter One:
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So life began about 3.5 billion years ago. Eventually more sophisticated organisms developed in an Earth without oxygen in its atmosphere, an atmosphere rich in carbon dioxide and unbreathable, indeed poisonous, to humans. The oxygen in the atmosphere has been mostly biologically generated: For example, countless cyanobacteria breathed in CO2 and exhaled O2 for hundreds of millions of years. Plants, when they settled on the continents, did the same. As a result, the CO2 in the atmosphere has decreased while the O2 has steadily gone up.
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Chapter Two:
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When an extinction event takes place, it is a great opportunity for life's survivors.
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This was the case for the Permian-Triassic extinction. Reptiles "seemed to seize" the "moment" to become the ruling class of Earth. In fact, the Mesozoic (245 - 65 million years ago) is known as the era of the "ruling reptiles." Not only did the dinosaurs dominate Earth's land, but also pterosaurs filled the skies, and swimming reptiles occupied the seas. These giant (and not-so-giant) reptiles prevailed for more than 150 million years, only to be extinguished in a cataclysmic event, [possibly] an asteroid that struck near the Yucatán Peninsula. Not only did dinosaurs die but many other life forms also did so. At this point, birds (which had evolved from coelurosaur-like dinosaurs about 150 million years ago via Archaeopteryx) and, figuratively speaking, mammals "seized" the opportunity to "take control" of Earth.
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During the Cenozoic Era (65 million years ago to the present), the mammals, which started out rather small, grew larger, diversified and developed in complexity. Among the mammals were primates, and the among the primates eventually would appear apes, monkeys, australopithecines and
man.
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So here we are today. Perhaps now is the time to lift the wineglass and make a toast to our existence.
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