From about 10,500 BCE the arid post-Ice Age Sahara was slowly transformed by increased rainfall; by 3500 BCE the Sahara Desert was a fertile steppe, covered in grass and woodland. Lake Chad was created by rivers, which cascaded from surrounding mountains. The fertile soil made northern and eastern Africa one of the first places where plant cultivation and animal domestication too place, with nomadic hunter gatherers, who exploited an abundance of fish and wild game, beginning to live in settled communities. Animal husbandry and livestock rearing began in the northern Sahara (4500 BCE) before reaching the Nile and northwest Africa. Grain was first cultivated in the Ethiopian Highlands (2000–3000 BCE) where ensete (‘Ethiopian banana’) and tef, a cereal crop, were grown. Knowledge about livestock rearing and agricultural practices spread along trade routes and, by 2000 BCE, much of northern and eastern Africa practised farming. A change to a more arid climate c. 500 BCE desertfied much of the Sahara, with only the Nile flood plain remaining fertile.