As most of you already know, in late 14th through late 15th centuries the kings of Spain and Portugal signed a treaty dividing the ownership of the "new (undiscovered) world" between them. This included the people, the land and all other living and non-living things.

"The Treaty of Tordesillas (signed at Tordesillas, June 7, 1494) divided the world outside of Europe in a exclusive duopoly between the Spanish and the Portuguese along a north-south meridian 370 leagues (1770 km) west of the Cape Verde Islands (off the coast of Senegal in West Africa), roughly 46° 37' W. The lands to the east would belong to Portugal and the lands to the west to Spain. The treaty was ratified by Spain, July 2, and by Portugal, September 5, 1494.
After new negotiations, the Treaty of Saragossa of April 22, 1529 decided that the line should lay 297.5 leagues west of the Moluccas. Spain got a monetary compensation in return." - http://www.nationmaster.com/encyclopedia/Treaty-of-Saragossa


I think that during the Enlightenment era Europeans began to further develop the idea that the world was created for man and was to be conquered by man (the European). Thus, the idea that man would manifest it's own destiny by seeking out worlds to own conquer supported this idea of white (European) supremacy. This is the foundation for Western thought, capitalism and evolution of the human in the so-called "Great Chain", from hunter-gatherers to farmers, to the industrial and technological revolutions.

Also, I think that there was a disconnect with nature and that was what separated Europeans from so-called "primitive" peoples. The latter were seen as uncivilized because they represented all that Europeans and other Westerners had left behind. This break with nature, evolution from agriculturalist to explorer/scientist/laborer may eventually be the downfall of the planet. This is arguable, of course, because we still live on this planet and we continue to dig deeper to tap the Earth's resources and fly further in space to discover any new worlds. Western man's biggest flaw is greed and thus Western imperialism is solely responsible for the destruction of nature.

No other group of people sought to step past the boundaries of their world to take control and conquer others. Japanese and other civilizations warred but stayed within certain boundaries. Upon the arrival of the Europeans, the entire world was at the mercy of the West. The Earth is round but the explorers flatten the globe, broke it up amongst themselves, to possess and control. I do not think other cultures would have done the same.