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Excerpt of Coal by Barbara Freese
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Even more fascinating than what went into coal, though, is what has come out of it: enough energy to change the world profoundly. For billions of years, almost every life form on earth depended for its existence on energy fresh from the sun, on the "solar income" arriving daily from outer space or temporarily stored in living things. Like living solar collectors handily dispersed all over the planet, plants capture sunshine as it arrives and convert it into chemical energy that animals can eat. And plants don't just convert energy, they store it over time--holding that energy within their cells until they decay, burn, or get eaten (or, in rare but important cases, are buried deep within the planet as a fossil fuel).

Animals eating plants take that stored energy into their bodies, where they not only store it in concentrated form but disperse it through space. A flock of geese, a pod of whales, a herd of caribou--they are all, on some level, mobile battery-packs. They gather solar energy that falls upon one patch of the planet and deliver it to another as they migrate; in this way, they make life possible for their predators even when, for example, the snow is thick and there is not a green leaf in sight. Life on earth is, in short, a vast and sophisticated system for capturing, converting, storing, and moving solar energy, the evolutionary success of each species depending in significant part on how well it taps into that system.

In the animal kingdom, one of the species that can most efficiently turn the calories of its food into useful mechanical energy is our own; humans need about half the calories that, say, a horse needs to exert the same physical energy. Our metabolisms are astonishingly energy-efficient, and that undoubtedly gave us an evolutionary advantage over other species. Perhaps this advantage helped give us the big brains we needed to figure out yet another way to tap into the stream of solar income captured by plants: fire.

By burning plants--especially plants we couldn't eat, like trees--humanity leapt beyond the physical limits imposed by its own gastric and metabolic systems and released far more solar energy than ever before. It was, of course, a momentous step. Fire is one of the distinguishing features of our species. Only people use fire, if by "people" we include the primates that would eventually evolve into people, because we began controlling fire perhaps some half-million years ago, long before Homo sapiens emerged. This new means of controlling energy reduced our vulnerability to the forces of nature, particularly during the long ice ages that repeatedly gripped the earth, and helped make us fully human.

Eventually, people stopped wandering across the land hunting and gathering food and began to grow it instead, a milestone archaeologists generally consider the beginning of civilization. Fire--and the unusually stable climate that has prevailed over the last 10,000 years--made this settled agricultural life possible. Fire let people clear land for crops (using much the same slash-and-burn methods threatening our rainforests today) and made digestible the cereals they planted. In these more permanent settlements, people eventually learned basic manufacturing skills, like firing pottery, baking bricks, and smelting metals--ways to make products that would last for societies that would last, at least as long as they had fuel.

Many of these early artisans turned to a fuel that would be an important bridge between wood and coal, and is akin to both of them: charcoal. Charcoal is wood that has already been partially burned. For thousands of years, charcoal was made by heaping wood into large piles, or partially burying it, and then burning it in a slow, oxygen-poor smolder that left behind almost pure carbon. The resulting charcoal burned hotter and cleaner than wood, but the process of making it wasted much of the wood's original fuel content, putting an even greater strain on the forests.

Excerpted from Coal: A Human History by Barbara Freese. Copyright 2002 by Barbara Freese. All rights reserved. Excerpt reproduced by permission of the publisher, Perseus Publishing.