For most of human history we have been dependent on muscle power to perform intensive labour. During our early history (which isn't particularly early since it constitutes between 90 and 99 percent of Man's time on planet Earth) this muscle power came from ourselves. As we went through the gigantic cultural and technological shift that was the neolithical revolution and became farmers, the muscles more and more came to belong to other animals.
The animal body, be it that of man or beast, was indispensable to us, as it was our only means of converting energy in any meaningful and reliable way. From an intake of food, which is basically solar energy, we got movement energy. In comparison most machines and tools we could produce could only retain the original energy form that was applied upon them and none of them could convert another energy form into movement energy. So if the millstone or saw moved, it was because the water wheel moved, and the water wheel moved because the water moved.
Even if many technological breakthroughs made this harnessing of energy more efficient, the lack of energy conversien severely hampered our possibilities to take the next great technological and cultural leap (not necessarily a leap for the better, but a leap nonetheless). The means to take this leap didn't arrive until the 18th century, when the British invention of the steam engine made energy conversion from heat to movement energy possible. With this, and to the tumultous sound of pistons and gears, the industrial revolution made its entrance.