The Great Rift Valley

3 minute read

The climate of the Early Pliocene differed from that of the Miocene primarily by the appearance of a cooling and drying trend across Africa, where early hominids evolved. It is likely that these climatic changes led to a decrease in the forest cover of large parts of the African continent.

Some of the climatic conditions of the early Pliocene were indirect effects of tectonic activity on the African continent. The most important manifestation of Africa's geology, beginning some twenty million years ago, is the formation of the Great Rift Valley. The Great Rift is an ocean being born, as parts of East Africa pull away from the rest of the continent, leaving a sinking basin in their wake. This geology is not limited to Africa, but stretches into West Asia, causing the Dead Sea to occupy the lowest continental basin on Earth, and continuing to generate earthquake activity as far north as Anatolia.

The first stages of geologic changes in East Africa involved massive uplift, resulting in vast highlands stretching from Ethiopia in the north to Mozambique in the south. The origins of these highlands apparently led to a disruption in the normal atmospheric circulation across Africa, creating a rain shadow on the easternmost edge of the continent, and fragmenting the rain forest that stretched across the equatorial region of Africa during the early Miocene. Later, geologic rifting lowered the center of the vast highland region, creating a second rain shadow and yet more biotic fragmentation. This process of habitat fragmentation has been suggested to be at the root of the evolutionary changes witnessed in late Miocene apes and early hominids. As expressed by French paleoanthropologist Yves Coppens, this hypothesis has been called the "East Side Story," as it placed the origin of the first hominids to the east of the rift, contrasting with other African apes to the west (Coppens, 1984).

The history of uplift and rifting also created a unique environment for the preservation and recovery of ancient fossil remains. Over time, the rifting created new river systems and a series of great lakes along the eastern edge of the continent, including present-day Lake Victoria, Lake Malawi, Lake Tanganyika, and Lake Turkana. These lakes have had complex histories as ancient faults moved and shifted their shores and the rivers that fed them. These movements have left the sediments of ancient lakes and rivers exposed, and the precious fossils that they bear today erode from these sedimentary rock layers. The fossils bear clues about paleoenvironments as well as about the ancient species that they represent, and it is from these clues that we can reconstruct the habitats and adaptations of early hominids.

Although early hominids were highly preserved in the East African rift context, they may not have actually originated there. Recently, discoveries to the west of the rift, including those at Bahr el Ghazal and Toros-Menalla in Chad, have pointed to the possibility that hominids existed in Central and West Africa as well. It may be that the formation of the rift and the consequent changes in East African climate actually had little or nothing to do with hominid origins. In this case, the "East Side Story" would be wrong.

A side effect of the tectonic rifting is volcanism, which has left its own distinct mark on the paleoanthropological record. The largest mountains in East Africa are volcanoes, including Mounts Kenya and Kilimanjaro, the Virungas, and the vast Ngorongoro crater. These mountains and other ancient volcanoes periodically erupted during the Pliocene and Pleistocene, spreading layers of volcanic ash and pumice across broad areas. As these ashfalls accumulated over time, they formed rock layers called tephra, which are interleaved with other sedimentary layers in the East African geology. Because of their volcanic origin, these tephra can be absolutely dated using radiometric methods, providing a nearly unparalleled sequence of sediments in time, a sequence that can be correlated among sites and can provide very precise dates for important fossil specimens (Feibel, 2001).