There is evidence of rock carvings along the Nile terraces and in desert oases. In the 10th millennium BCE, a culture of hunter-gatherers and fishers was replaced by a grain-grinding culture. Climate changes or overgrazing around 8000 BCE 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 centralised society.