Olduvai Gorge or Oldupai Gorge in Tanzania is one of the most important paleoanthropological sites in the world and has been instrumental in furthering the understanding of early human evolution. This site was occupied by Homo habilis approximately 1.9 million years ago, Paranthropus boisei 1.8 million years ago, and Homo erectus 1.2 million years ago. Homo sapiens is dated to have occupied the site 17,000 years ago. Olduvai Gorge is a steep-sided ravine in the Great Rift Valley that stretches through eastern Africa. It is in the eastern Serengeti Plains in Arusha Region, Tanzania and is about 48 km (30 mi) long.