Energy has changed how people live. Muscle power, fueled by food and translated into mechanical energy, was the primary energy resource used by early humans for millions of years

Muscle power, fueled by food and translated into mechanical energy, was the primary energy resource used by early humans for millions of years. But over time, people have taken advantage of other kinds of energy transformations, using machines and fuels to help do their work. As an economy becomes more developed, a larger variety of energy sources are tapped, and the energy consumed per person increases.

It's not a coincidence that civilization-science, education, and the arts-has advanced as energy usage has increased. Harnessing energy and putting it to work

frees people from the time-consuming labor of making food, clothing, and shelter. Consider the energy that goes into preparing a bath. In five minutes, you can fill a tub with hot water. 150 years ago, you had to chop wood, light a fire, fetch water, heat the water, and pour it into a washtub. Before modern heating, plumbing, and food processing, most people had very little time for education or leisure activities.

Wood was long the principal-in some areas the only-fuel. It was used not only for heating and cooking, but also as source of charcoal for reducing ores to metals. For many centuries, smoke and ash blanketed towns in colder climates, due to wood's high carbon content and relatively low burning point.

The first fossil fuels to be used were surface deposits of asphalt, peat, and coal, along with oil that seeped to the surface and gas vented from underground reservoirs. Coal was difficult to mine with primitive tools. Although it was used to some extent in England during Roman times, it did not become widely used until the 1700s, when it was understood that coke from coal could be substituted for charcoal in the reduction of ores.

Petroleum has been used throughout recorded history, to line canals, seal joints in boats, build roads, and fuel lamps. Initially, people relied on oil that seeped to the surface. But as early as 600 BC, Confucius described 100-foot-deep wells yielding water and natural gas along the Tibetan border. To transport oil and natural gas from well sites, the Chinese built extensive pipelines of bamboo.

While oil became an important commercial commodity in Europe in the 1700s, the modern petroleum industry was born in the 1800s in the United States, capitalizing on advances in drilling. The invention of the internal-combustion engine stimulated petroleum production starting in the early 1900s. Further advances in the refining of petroleum led to petrochemicals. These became the building blocks of a host of new products in the 20th century, including plastics, pharmaceuticals, and fabrics.

Oil has fueled most of global energy consumption since World War II. In 1950, oil accounted for approximately one quarter of the world's energy use; today it accounts for 40 percent.

U.S. energy consumption has grown dramatically since World War II. As developing countries industrialize, world energy consumption is increasing at a faster rate than in the U.S.

From the first steam engines to today's best gas turbines, efficiency in energy use has increased 50 fold, while the amount of carbon in the world's energy mix has declined at an average annual rate of 0.3 percent.

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