The developed Neolithic cultures belonging to the phases Pre-Pottery Neolithic A (10,200 BC) and Pre-Pottery Neolithic B (7600 to 6000 BC) appeared in the fertile crescent and from there spread eastwards and westwards. Contemporaneously, a grain-grinding culture using the earliest type of sickle blades had replaced the culture of hunters, fishers, and gathering people using stone tools along the Nile. Geological evidence and computer climate modeling studies also suggest that natural climate changes around 8000 BC began to desiccate the extensive pastoral lands of northern Africa, eventually forming the Sahara. Continued desiccation forced the early ancestors of the Egyptians to settle around the Nile more permanently and to adopt a more sedentary lifestyle. The oldest fully developed neolithic culture in Egypt is Fayum A culture that began around 5500 B.C.