Oldupai Gorge
The Oldupai Gorge is commonly referred to as "The Cradle of Mankind."
 It is a steep-sided ravine in the Great Rift Valley between Ngorongoro Crater and The Serengeti National Park in Tanzania.

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.

Excavation work was pioneered by Louis and Mary Leakey in the 1950s.
The first artifacts in Olduvai (pebble tools and choppers) date to circa 2 million years ago
 but fossil remains of human ancestors have been found from as long as 2.5 million years ago.

I visited the gorge in 2008 as part of my 52-day, 7-country IntrepidTravel camping-truck trip from Nairobi to Johannesburg.

We had an excellent lecture from the local guide while we were seated in an open-air pavilion overlooking the gorge.

In 1976 Mary Leakey found footprints of early hominids nearby.
Ash from a volcanic eruption and rain had made mud set like cement.
In the small museum here, there was an excellent display including a plaster-cast of the footprints.
The original footprints have been reburied to protect them.