4 - A Thin Sliver of Existence

Summary

Here, I describe the Earth’s biosphere, which is not a sphere at all but rather a spherical shell. I discuss how far it extends in both directions. The question of how deep the biosphere goes takes us underground and to the bottoms of deep ocean trenches. The question of how far up it extends takes us to the stratosphere and the ozone layer that it contains. Next, I deal with plate tectonics. The recycling of the mobile plates that form the base of the biosphere has many consequences, including the obliteration of impact craters, in contrast to their near-permanence on the Moon. I then consider the extent to which the biosphere can be divided up into areas in which the predominant life-forms are different from each other. The marine component covers about 70% of Earth’s surface and is sometimes referred to as the global ocean, to emphasize its lack of real boundaries. Although the land component is smaller (about 30% of Earth’s surface), this component can be divided into biogeographic realms in which evolution has operated quasi-independently. Finally, I look back at the biosphere’s history, including such phenomena as glaciations, supercontinents, and the Great Oxygenation Event that occurred some 2.5 billion years ago.