Descartes may have started it all. Technically our split with nature could even go back to ancient Greek philosopher Plato, but it was mainly the "doubting" Frenchman who started thinking of animals as mere mechanical machines. René Descartes claims that animals, unlike humans, do not have souls, and that a major difference between the two is our innate ability for language and hence reason. His scientific philosophy was an attempt of combining philosophy with Newtonian physical laws, which was the growing trend back in those days.
His fellow French philosopher Julien Offray de La Mettrie took it one step further in the 18th century and included humans into his mechanistic vision of life. Humans were not that different from animals; remarkably, such “blasphemous” ideas appeared in the pre-Darwinian age. Darwin would later give humanity its second blow (the first one having come courtesy of Kepler and Galileo) which would take away the privileged position of humanity through his theory of evolution. It was the birth of materialism; a world where science reigned and religion was viewed with mistrust, if not outright hostility.
Nonetheless, it is interesting to note that La Mettrie's materialism was not the same as our modern notion. He actually used the terms “transformism” and “vitalism” and followed an age-old tradition, going back to the ancient Egyptians, that matter was in fact "alive" and "vital." Chop off the head of a chicken and it still runs away; take out the heart, and it keeps beating. Discoveries of these kinds made him assume that matter indeed contained living organism and, as such, contained (tissues of) life.
While proposing such groundbreaking assumptions, his goal was actually quite different from our experience of present-day materialism. We tend to think in a Cartesian manner and see nature, at best, as our plaything and bend it according to our wishes. Materialism, in its more modern version, has also been injected with a dose of financial greed at the expense of the destruction of our living space.
Yet La Mettrie thought that his philosophy would actually do the opposite. By seeing ourselves not as the lords of creation dominating over animals, by disposing the accepted religious view of human superiority, he wanted to raise the status of both animals and nature to create a kind of equilibrium. In such a way, we would ideally both respect and appreciate all the marvelous things nature has to offer. By becoming a firm and established part of nature, we should be able to identify and consequently value its vital importance for our own lives.
Unfortunately, we took a completely different path. The state of our environment, the egotistical quest for money and fame at the expense of destruction of nature and human lives has been his unwanted legacy. Some might claim that with Descartes we lost our soul, with La Mettrie our humanity. Lest we forget, their intentions were not evil; we just chose what suited our best interests. After all, we are the lords of nature, of planet Earth, the rest of the universe ... especially ever since Nietzsche killed God.