The territory of present-day Rwanda has been green and fertile for many thousands of years, even during the last ice age, when part of Nyungwe Forest was fed by the alpine ice sheets of the Rwenzoris. It is not known when the country was first inhabited, but it is thought that humans moved into the area shortly after that ice age, either in the Neolithic period, around ten thousand years ago, or in the long humid period which followed, up to around 3000 BC. The earliest inhabitants of the region are generally thought to have been the Twa, a group of Pygmy forest hunters and gatherers, whose descendants still live in Rwanda today.