Rift Valley - The Great Rift Valley of Eastern Africa

Was the Rift Valley the Cradle of Humankind—and Why?

Ethiopia, Rift Valley, aerial view
Aerial View of the Great Rift Valley of Eastern Africa. Philippe Bourseiller / Getty Images

The Rift Valley of eastern Africa and Asia (sometimes called the Great Rift Valley [GRV} or East African Rift system [EAR or EARS]) is an enormous geological split in the crust of the earth, thousands of kilometers long, up to 200 kilometers (125 miles) wide, and between a few hundred to thousands of meters deep. First designated as the Great Rift Valley in the late 19th century and visible from space, the valley has also been a great source of hominid fossils, most famously in Tanzania's Olduvai Gorge.

The Rift Valley is the result of an ancient series of faults, rifts, and volcanoes deriving from the shifting of tectonic plates at the junction between the Somalian and the African plates. Scholars recognize two branches of the GRV: the eastern half—which is that piece north of Lake Victoria that runs NE/SW and meets the Red Sea; and the western half—running nearly N/S from Victoria to the Zambezi river in Mozambique. The eastern branch rifts first occurred 30 million years ago, the western 12.6 million years ago. In terms of rift evolution, many parts of the Great Rift Valley are in different stages, from pre-rift in the Limpopo valley, to initial-rift stage at the Malawi rift; to typical-rift stage in the northern Tanganyika rift region; to advanced-rift stage in the Ethiopian rift region; and finally to oceanic-rift stage in the Afar range.

That means the region is still quite tectonically active: see Chorowicz (2005) for much more detail concerning the ages of the different rift regions.

Geography and Topography

The Eastern African Rift Valley is a long valley flanked by uplifted shoulders that step down to the central rift by more or less parallel faults. The main valley is classed as a continental rift, extending from 12 degrees north to 15 degrees south of our planet's​ equator. It extends a length of 3,500 km and intersects major portions of the modern countries of Eritrea, Ethiopia, Somalia, Kenya, Uganda, Tanzania, Malawi, and Mozambique and minor portions of others.

The width of the valley varies between 30 km to 200 km (20-125 mi), with the widest section at the northern end where it links to the Red Sea in the Afar region of Ethiopia. The depth of the valley varies across eastern Africa, but for most of its length it is more than 1 km (3280 feet) deep and at its deepest, in Ethiopia, it is over 3 km (9,800 ft) deep.

The topographical steepness of its shoulders and the depth of the valley have created specialized microclimates and hydrology within its walls. Most rivers are short and small within the valley, but a few follow the rifts for hundreds of kilometers, discharging into deep lake basins. The valley acts as a north-south corridor for the migration of animals and birds and inhibits east/west movements. When glaciers dominated most of Europe and Asia during the Pleistocene, the rift lake basins were havens for animals and plant life, including early hominins.

History of the Rift Valley Studies

Following on the mid- to late-19th-century work of dozens of explorers including the famous David Livingstone, the concept of an East African rift fracture was established by Austrian geologist Eduard Suess, and named the Great Rift Valley of East Africa in 1896 by British geologist John Walter Gregory.

In 1921, Gregory described the GRV as a system of graben basins which included the valleys of the Red and Dead Seas in western Asia, as the Afro-Arabian rift system. Gregory's interpretation of the GRV formation was that two faults had opened up and a central piece dropped down forming the valley (called a graben).

Since Gregory's investigations, scholars have re-interpreted the rift as the result of multiple graben faults organized over a major fault line at the plate juncture. The faults occurred in time from the Paleozoic to Quaternary eras, a time span of some 500 million years. In many areas, there have been repeated rifting events, including at least seven phases of rifting over the past 200 million years.

Paleontology in the Rift Valley

In the 1970s, paleontologist Richard Leakey designated the East African Rift region as the "Cradle of Mankind", and there is no doubt that the earliest hominids—members of the Homo species—arose within its boundaries.

Why that happened is a matter of conjecture, but may have something to do with the steep valley walls and microclimates created within them.

The interior of the rift valley was isolated from the rest of Africa during the Pleistocene ice age and sheltered freshwater lakes located in savannahs. As with other animals, our early ancestors may have found refuge there when the ice covered much of the planet, and then evolved as hominids within its tall shoulders. An interesting study on the genetics of frog species (Freilich and colleagues) showed that the valley's micro-climates and topography are at least in this case a biogeographic barrier that resulted in the splitting of the species into two separate gene pools.

It is the eastern branch (much of Kenya and Ethiopia) where much of the paleontological work has identified hominids. Beginning about 2 million years ago, barriers in the eastern branch eroded away, a time which is coeval (as much as that clock can be called co-eval) with the spread of Homo species outside of Africa.