Astrobiologists commonly use the term "Earthlike Planet" in their search of the heavens for another potentially habitable planet nearby in the Milky Way Galaxy. But just what does "Earth-like" mean? This question is far more complex than it seems. If we define Earth-like as having oceans, continents, and atmosphere in modern quantities, it turns out that Earth-like qualities are quite new. Over time the continents have repeatedly changed position and size, and the oceans have risen and fallen in their basins based on the effects of gravity and tectonic changes within the ocean basins themselves. But these latter two aspects of the Earth show far less change than does the atmosphere. Our current atmosphere is but a slice of a forever changing entity, and greatly different from the atmosphere at most times in Earth's history. Since the composition of the atmosphere greatly affects planetary temperatures, it is probably safe to say that Earth life is well adapted to the current atmosphere. Yet should atmospheric conditions found at the end of the Permian and end of the Triassic Period suddenly exist in our current world, the result would be calamitous. Those two not-so-ancient versions of an Earth-like atmosphere very nearly wiped out our furry ancestors some 250 million years ago, and then again some 200 million years ago. Yet while it is well-known that the mass extinctions, of which the Permian and Triassic events are but two of many (albeit two of the five most catastrophic), there is far less discussion about the effects that "optimal" as well as dangerous levels of various atmospheric components may have had on Earth life.
In this chapter we will briefly summarize how one of the most important of atmospheric gases (and for all those organisms with aerobic respiration, the most important), oxygen, may have stimulated formation of the most efficient respiratory system ever evolved, and in so doing paved the way for the existence of birds.
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