Humans, like other species, are integral to the ecology of the earth. We display a series of adaptations that allow us to eat, grow, find mates, and raise offspring. Do humans have a fundamental ecological niche? How have humans adapted to climate change over time and space, for example, as we spread out of Africa, into Eurasia and then into the Americas? We consider how climate, environmental, and habitat reconstructions are made by scientists; and how diets, food acquisition strategies, geographic distribution, and social structure are known to paleoanthropologists. We consider changes in the human gut, tooth size, body size, and social behaviors. We also explore how long humans have made a significant impact on the environment, via hunting and overhunting, planting food, population growth, and greenhouse gas emissions. When did the "Anthropocene" begin?