The Olduvai Gorge or Oldupai Gorge is a steep-sided ravine in the Great Rift Valley, which stretches along eastern Africa. Olduvai is in the eastern Serengeti Plains in northern Tanzania and is about 30 miles long. The gorge is named after the Maasai word for the wild sisal plant, Sansevieria ehrenbergiana, commonly called Oldupai.

It is one of the most important prehistoric sites in the world and has been instrumental in furthering understanding of early human development. Excavation work there was pioneered by Louis Leakey in the 1950s and is continued today by his family. Millions of years ago, the site was that of a large lake, the shores of which were covered with successive deposits of volcanic ash. Around 500,000 years ago seismic activity diverted a nearby stream which began to cut down into the sediments, revealing seven main layers in the walls of the gorge.

The stratigraphy is extremely deep and layers of volcanic ashes and stones allow radiometric dating of the embedded artifacts, mostly through potassium-argon dating. The first artifacts in Olduvai (pebble tools and choppers) date to ca. 2 million years ago but fossil remains of human ancestors have been found from as long as 2.5 million years ago.

More than anyone," writes author Roger Lewin, "Louis Leakey established paleoanthropology as a high-profile endeavor. By the time he died 30 years ago this month, his name had become synonymous with the search for human origins." Possessed of boundless enthusiasm and a conviction that Africa was the birthplace of early humans, Leakey made fossils with names such as Zinjanthropus and Homo habilis famous around the world as he pursued the quest for the oldest relative of modern man.

Louis Leakey was born in Kabete, in colonial Kenya, the son of Harry and Mary Leakey, who ran an Anglican mission there. Louis spent much of his youth among Kikuyu children, and his three siblings were often his only European peers. He was, he said, "anything but typically English," and though he would obtain degrees in anthropology from Cambridge University, it was to Africa that he would return, convinced that "Africa is the cradle of mankind."

Leakey and his second wife, Mary, formed an anthropological team. Their finds over more than three decades of work in the field, much of it spent in Tanzania's Olduvai Gorge, helped redraw the human family tree. Although Leakey would attract much criticism in his life for announcing his conclusions before he marshaled his evidence, he remains a seminal figure in the development of the field of paleoanthropology. Leakey, says David Pilbeam, professor of anthropology at Harvard, "had an energizing effect on the field and on the people doing the research."