Economic and technological development is linked with shifts in sources of energy. The tendency has been the adoption sources that have a higher energy content, as the shift from coal (solid) to oil (liquid) and natural gas (gas) indicates. This shift can be simplified into five major phases, one about the future being speculative: From the beginning of history up to the industrial revolution (18th century), mankind's use of energy relied only on muscular and biomass sources. Most work was provided by manual labor and animals, while the biomass (mainly firewood) provided for heating and cooking energy needs. Other sources of energy, such as windmills and watermills were present but their overall contribution was marginal. By the mid 19th century, the industrial revolution brought a major shift in energy sources with the usage of coal, mainly for steam engines, but increasingly for power plants. As the 20th century began, the major reliance was on coal, but a gradual shift towards higher energy content sources like oil began. This second major shift inaugurated the era of the internal combustion engine and of oil-powered ships.