The study of how the distributions and numbers of humans are determined by interactions with conspecific individuals, with members of other species, and with the abiotic environment. Human ecology encompasses both the responses of humans to, and the effects of humans on, the environment. The lineage leading to modern humans arose in Africa millions of years ago. The oldest fossil remains of members of the genus Homo are estimated to be about 2 million years old. The tools associated with human fossils, as well as cave paintings created by early humans, reveal the existence of culture. Cultural learning greatly facilitated the spread of domestic plants and animals and the development of pastoral and agricultural societies. Agriculture, in turn, led to an increasingly sedentary life, greatly expanded food supplies, the development of cities, and the rapid growth of the human population. Human ecology today is the combined result of humans' evolutionary nature and cultural developments.