Since the dawn of civilization, humanity's level of development has been inextricably linked to the use of energy. All animals consume food to provide the energy needed for metabolism, movement and work, but ours is the only species which uses external energy, other than food, to any significant extent. The process began with the discovery of fire - a process in which the energy contained within organic molecules (primarily in wood) could be converted into heat. This provided the ability to cook food, and thus the ability to use a much wider range of food items. Fire also provided heating which enabled humans to survive in colder climates. The next breakthrough in harnessing energy came with the use of animals such as the donkey as a means of transport and a beast of burden, and oxen to till the soil. Fire was then used in the production of improved tools made of bronze and iron, which transformed the way of life for much of humanity. Machines, such as the wheel, were developed to permit more efficient use of external energy. The pyramids are a monument to the devices which must have been developed to provide mechanical advantage, although the energy used was still primarily manpower. Further harnessing of energy was achieved by the use of streams to turn a wheel, and, in due course, windmills. Wood or charcoal was burnt to produce lime, which permitted the construction of more elaborate buildings. Right up to the time of large-scale use of coal, which led to the Industrial Revolution, humans had only been using renewable energy, supplied by the sun.
Combustion of coal, followed by oil and natural gas, permitted a quantum leap in transportation (and hence trade) and manufacture of a vast range of products, all of which enabled human societies to become more complex, and, for most people at least, to enjoy an enhanced quality of life. Our consumption of energy continues to grow, despite major efforts at increasing efficiency.
Solar energy has thus permitted the development and civilization of the human race but we must not forget that it also facilitates the growth and survival of all forms of life and it drives the global circulation systems. Thus ocean currents, climate, rainfall and all freshwater resources, are dependent on the energy of the sun. Without the sun there would be no fresh water!
Thus for most of history, all activity, human and otherwise, was dependent on energy in the form of solar radiation. This can be regarded as solar capital. In relatively recent times humanity has learned to exploit the natural resources of the earth to produce energy, but this is using a legacy which is not being replaced. Coal, oil and natural gas are fossil fuels. For a few centuries they are providing an enormously useful bonus, but the reserves are being depleted and we can only rely on them for a few more generations. Fossil fuels represent a major component of what can be regarded as earth capital. This includes natural resources such as minerals and human resources. These two forms of capital support and sustain all life and all economies on earth.
Thus energy is essential for all human activities, and by harnessing external energy on a massive scale, humans have managed to dominate the earth. Figure 1 indicates the range of human activities, driven by energy.