There is evidence of rock carvings along the Nile terraces and in desert oases. In the 10th millennium BC, a culture of hunter-gatherers and fishers was replaced by a grain-grinding culture. Climate changes and/or overgrazing around 8000 BC began to desiccate the pastoral lands of Egypt, forming the Sahara. Early tribal peoples migrated to the Nile River where they developed a settled agricultural economy and more centralized society