Answer-The oldest type of energy known to man is –

The oldest energy known to man is Geothermal energy. This geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. It has been used for space heating and bathing since ancient Roman times, but is now better known for generating electricity. Geothermal energy is thermal energy generated and stored in the Earth.