What did primitive men use as a source of energy?

Primitive men made use of fire as an external source of energy for heating, lighting and cooking. He then learnt to harness natural energy to fulfill his needs. The energy of the wind was the first to be harnessed. During the Middle Ages windmills were used extensively for grinding grain. The energy of falling water was harnessed by the use of waterwheels and turbines. Now he generates energy from natural sources such as Sun, water and fuels through efficient and advanced techniques. He has devised combustion engines, thermoelectric, hydroelectric and nuclear power plants as powerful and efficient means of generating energy.