"Thus we discover that this blind and impersonal process produced humans not in a lightning flash, not in a sudden instant of creation, but as the result of accumulation. The origin of humans is not something that can be pinpointed at five million years, or one million years, or 100,000 years in the past, but, rather, occurs continuously over time. Our origin is the whole pattern of evolution, although there are key events that we must discover and identify. The things that make us human are acquired as a complex mosaic—we became upright four million years ago; we began to make tools two million years ago; we began to live all over the earth less than one million years ago; and possibly we only acquired language in the last 100,000 years or so. Each of these factors is an essential part of the process of becoming human. What makes human evolution such an endlessly fascinating story is trying to visualize the stages, imagining what sort of a creature could walk upright but not talk, make tools but not use fire, survive the rigors of the Ice Age but know nothing of agriculture and a settled way of life."