"[The Sixth Extinction] did not happen yesterday because we suddenly became aware of the dangers of global warming. It began 50,000 years ago when a relatively hairless primate stumbled out of equatorial Africa and began wiping out the megafauna of the time. Wherever this creature (our ancestor) went, their arrival was followed by large die-outs of megafauna. Primitive hominids were well-organized, efficient, slaughter crews. As they advanced, the mammoth, sabre-toothed cats, cave bears, giant sloths, camels, horses, and wholly rhinos fell to their stone weapons and deliberately set fires. The extinction of all of these great mega-species is directly attributable to "primitive" human hunters. The hunting down of the mega-fauna was followed by the advent of agriculture and the domestication of selected animals. Domesticated cows, goats, sheep, and pigs grew in numbers and denuded large areas of grasslands. Irrigation systems began to toxify land. Then agriculture was followed by industrial activities, and finally, by the burning off of vast amounts of fossil fuels."