Hunting and gathering remained the main mode of subsistence of humanity for hundreds of thousands of years, beginning some 1.8 million years ago, and until the Neolithic Revolution (some 10,000 years ago), when agriculture gradually spread through human societies (Marlowe 2005). Hunter-gatherer societies obtained their food directly from "natural" ecosystems, by hunting wild animals and collecting wild plants (Richerson et al. 1996). Early agrarian societies started planting desired crops on suitable lands, competing with wildlife for space and resources. As agrarian societies evolved, techniques for planting and harvesting became technologically more advanced and more efficient (Richerson et al. 1996). Innovations thus allowed the human population to grow and to colonize nearly every terrestrial ecosystem type on Earth.