During the time that the human economy depended solely on human energy, life was crude. The harnessing of animal energy (beasts of burden) and wind energy(sailing vessels) greatly improved economic capabilities. Animal energy for the economy was obtained, primarily, from horses, donkeys, mules, and from oxen, buffalo, and camels. Our measuring unit for mechanical power is horse power.
Fossil fuel reserves are finite, and fossil fuel energy may be altering the environment of our planet. Nuclear fuel residues remain toxic for thousands of years. Animals were the energy bridge to our modern economy. What next?