Of Human Evolution S

C CTONI HYP TE OT HE SE OF HUMAN EVOLUTION S M. Royhan Gani and Nahid DS Gani tum. Tectonics can be seen as the Tearing Apart root cause of Earth’s changing and ometime around 7 million diverse landscapes and seascapes and The African superplume has been years ago, the land in East their associated climates. The cyclic brewing beneath eastern Africa for Africa began to rise in ear- breakup and reassembly of the super- at least the last 45 million years. The nest, and life on Earth took continents Rodinia and Pangea can earliest direct surface evidence of the Sa drastic turn. A creature began to be explained simply by the existence rising hot plume-head is in north- evolve into a form that would eventu- of one or two superplumes (upwell- east Africa’s Afar region, as seen by ally take over the world. ings of abnormally hot rock from the extensive volcanism around 30 mil- Exactly how humans originated core-mantle boundary) in the mantle. lion years ago. Consequent weaken- and evolved is an intrinsic intel- Only two superplumes exist today: ing of the lithosphere, Earth’s rigid lectual question. But one thing has one beneath Africa and the other outer layer, by excessive heating and become fairly clear: Tectonics was beneath the Pacific. Perhaps, it is this mechanical stretching resulted in rift- ultimately responsible for the evolu- African superplume that was ulti- ing in three directions in Ethiopia and tion of humankind. mately responsible for driving human in the eventual creation of the Gulf of Any discussion of the evolution of evolution in East Africa. Aden, the East African Rift and the life starts with Charles Darwin, and Humanity began in Africa. Genetic Red Sea. Rifting causes subsidence human evolution is no exception. and fossil evidence suggest that that results in rift valleys, and Earth’s More than a century ago, Darwin between 33 million and 22 million crust adjusts wto this subsidence by saw a link between the African arid years ago hominoids, or apes — a raising rift shoulders on both sides savanna and human evolution, an group that includes gibbons, orang- of the valley. In East Africa, uplift idea that later became popular as the utans, African apes (chimpanzees and of the narrow rift shoulders (tens of “savanna hypothesis.” However, in gorillas) and hominins (ancient and kilometers wide) were superimposed his original theory of natural selec- modern humans) — began to evolve. on the uplift of wide domes (hun- tion, Darwin downplayed the role of Hominins and African apes (partic- dreds of kilometers wide) linked to the physical environment as a mecha- ularly chimps) are closely related. the plume-head pushing from under- nism for evolution, stating instead These two lineages split off between neath. The mode and tempo of these that natural selection could drive evo- 7 million and 4 million years ago, as uplifts in East Africa were anything lutionary changes in the absence of indicated by DNA and fossil evidence. but simple. any change in the physical environ- Most paleoanthropologists agree that Over the past 30 million years, the ment or climate. this split is the most critical juncture plume has created the East African Over the last century, the impact in human evolution, although fos- Rift, a north-south elongated dynamic of geomorphic and climatic change on sil evidence is rare from this period. topography with isolated volcanic evolution, particularly human evolu- Much was happening in Africa at that peaks such as Mount Kilimanjaro. tion, has gained considerable momen- time, however. Rifting occurred in a few places 22 Geotimes January 2008 www.geotimes.org Mediterranean Sea ONIC H Red Sea T Y Nile River C P Saudi Arabia O Red Sea L. Tana E Hadar T T Blue Nile Chad H Africa Middle Awash Yemen Ethiopian Melka E Plateau Kunture Omo S EASTERN BRANCH Gulf of Aden Turkana L. Albert Afar E L. Edward Depression Main Atlantic Ocean Ethiopian L. Kivu S Rift L.Victoria Turkana Lows Olduvai Kenya Indian Ocean Rift L. Tanganyika Laetoli Indian Ocean L. Rukwa Atlantic Ocean Kenya L. Mweru Dome WESTERN BRANCH L. Malawi Uraha N L. Kariba N Kanapoi {M. Royhan Gani{M. Royhan and Nahid DS Gani 500 km N 500 km Elevation 500 km (m) Lakes (L.) 5100 Rivers 4280 Madagascar Karoo Makapansgat Hominin fossil 2680 Plateau sites 1080 Sterkfontein Major rift faults Tuang The north-south oriented Wall of Africa, about 6000 kilometers long and 600 kilometers wide, characterized by a unique and dynamic physiography with plateaus, volcanoes, mountains, deep rift-valleys, freshwater lakes, and vegetation patchwork ranging from closed woodland to desert grassland. Notably, all the early hominin fossil sites of the world, except Chad, are located inside this narrow wall. Perhaps the unique physiography of the wall was the reason for human evolution the way we are now — walking tall and thinking big. throughout East Africa and along the rift floors and dramatic Global Climatic Shifts propagated both northward relief that runs from 156 meters and southward, from the Afar below sea level — Lake Assal in the Over the last 15 million years, region in northern Ethiopia Afar Depression, the lowest point in Earth has experienced a net cool- down almost to South Africa. Africa — to 5,895 meters above sea ing trend superimposed by repeated The result was the formation level at Kilimanjaro, the highest point cooling-and-warming cycles — the so- of the “Wall of Africa,” which in Africa. Although the Wall of Africa called Milankovitch cycles related to is about 6,000 kilometers long started to form around 30 million Earth’s orbital forcing that controls and 600 kilometers wide, and years ago, recent studies show that the planet’s variable dose of solar rises to more than five kilome- most of the uplift occurred between 7 radiation, with periodicities ranging ters high. million and 2 million years ago, just from 20,000 to 100,000 years. The This belt is the largest and about when hominins split off from great ice ages, associated with a drier longest-lived rift in the world, African apes, developed bipedalism climate, started only between 3 mil- punctuated with freshwater lakes and evolved bigger brains. lion and 2.5 million years ago. This present-day vegetation patchwork of East Africa is related to the massive uplift of the Wall of Africa between 7 million and 2 million years ago, an uplift that wrung moisture out of monsoonal air moving across the region and caused progressive aridification of East Africa. {credit} M. Royhan Gani M. Royhan and Nahid DS Gani{credit} www.geotimes.org January 2008 23 Geotimes Because our early ancestors were primar- Thinking Locally ily herbivores, scientists tend to focus heavily on past vegetation changes in East Africa to Léo Laporte and Adrienne Zihlman of the understand early hominin evolution, as altera- University of California at Santa Cruz were the tions in type and distribution of plants would pioneers in linking East African aridification immediately impact an ape’s food system. Using to local tectonics. In 1983, they argued that carbon isotopes, researchers can determine the African uplift and rifting threw the eastern type of vegetation present in a region at a part of the continent into a rain shadow, caus- particular time in the geologic past, such as ing a spread of the savanna and the end of the whether vegetation was wood-dominated or rainforests that had dominated the landscape. grass-dominated. Recent isotopic studies sug- Many subsequent studies have argued essen- gest that an important global shift from wood- tially the same thing. So far, probably the most dominated to grass-dominated ecosystems that convincing case was made by Pierre Sepulchre occurred between 8 million and 4 million years of the Laboratoire des Sciences du Climat et de ago was likely related to an increase in aridity l’Environnement Gif-sur-Yvette in France and and, to some extent, fires that cleared out large his team in a recent atmospheric and vegeta- swaths of forest. No doubt these climate and tion modeling study. They found that reduction climate-driven vegetation changes, again at of East African present-day topography by just the right time in the geologic past, were critical one kilometer would permit moist air to circu- in hominin evolution. But were these changes late across the region, increasing rainfall. The that led to a progressive increase of aridity and currently dry and grass-dominated landscape consequential expansion of open grassland, a would thus transform into one of tree-domi- phenomenon known as East African aridifica- nated vegetation. If this rain shadow hypothesis tion, primarily controlled by global (climate) or is correct, the uplift history of the Wall of Africa by local (tectonic) processes? is crucial in assessing East African aridification, and thus human evolutionary changes. Again, the East African Rift uplift occurred sometime over the past 30 million years, with likely the most uplift occurring sometime after 7 million years ago. New research on the Ethiopian Plateau — perhaps the most promi- nent part of the wall and next to a significant hominin fossil site, the Afar rift valley — sug- For survival, these gests that the plateau itself uplifted right in Ethiopian women that time period. Analyzing how fast the Blue walk daily for tens Nile carved a spectacularly deep canyon, a true of kilometers on this rough and dry rival of the Grand Canyon of North America, terrain to collect on the plateau, Nahid Gani and her team found water and trade that the Ethiopian Plateau uplifted more than a foods. Perhaps kilometer between 6 million and 3 million years early hominins ago.

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