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

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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 prominent 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. developed bipedal- Other studies have shown a similar trend ism, an energy- elsewhere in the rift valley: The majority of the saving mechanism for traveling long uplift of the Kenya rift-flank, currently rising to distances by walk- 3.4 kilometers high and constituting the central ing upright, for part of the Wall of Africa, occurred between 7 similar reasons, as million and 2 million years ago. The Tanganyika their food resourc- and Malawi rift-shoulders, which reach 5.1 kilo- es were getting meters high and comprise the western branch of widely separated the wall, were mainly uplifted between 5 million because of the spreading savan- and 2 million years ago. And the Karoo Plateau na related to East in South Africa, the southernmost extension of African aridifica- the wall, which currently rises to 3.4 kilometers tion. high, was massively elevated during the past 5 million years. Clearly, the Wall of Africa grew to be a prominent elevated feature over the last 7 million years, thereby playing a primary role in East African aridification by wringing moisture out of monsoonal air moving across the region. M. Royhan GaniM. Royhan and Nahid DS Gani

24 Geotimes  January 2008 www.geotimes.org But, the uniqueness of the Wall of Africa is not so much that it is narrow and elevated, but rather that it is spatially variable with contrasting topography. The wall is breached in several places, such as the Turkana Depression, which has an average elevation of only 500 meters and sits between the Ethiopian Plateau in the north and the Kenya Dome in the south. Various rift- Courtesy NASA, Visible EarthVisible Courtesy NASA, flanks and plateaus, in addition to isolated topographic features like volcanoes, likely reached threshold elevations at various times between 7 million and 2 million years ago, result- ing in an overall aridification of the region that likely varied somewhat from place to place and from time to time. This view is supported by the finding that the expansion of grasses in East Africa was asynchronous and spatially heterogeneous over this time window, a finding that also goes in favor of local tectonic controls, rather than a global change, for the expansion of grass-dominated vegetation. The lakes of the African Rift Valley exhibit wide Hominin Cradle variations in water color as can be seen in this SeaWiFS image. From left to right the lakes are: Lake Upembe, Lake Mwaru, Lake Tanganyika Although fossils of hominoids, from which hominins evolved, are (largest), and Lake Rukwa. This image spans the widely found in Africa and Eurasia, all of the early — greater than 3 southeastern corner of the Democratic Republic million years old — known hominin fossil sites are located along the of the Congo, northeastern Zambia, and south- narrow belt of the East African Rift. The only exception is the oldest ern Tanzania. (6-million- to 7-million-year-old) “hominin” bones of Sahelanthropus tchadensis found farther west from the wall in Chad that, some argue, On the Ethiopian Plateau, the Blue Nile carved a could be the common ancestor of humans and chimpanzees, rather than 1.6-kilometers-deep spectacular canyon, a true hominins. rival of the Grand Canyon of North America. By studying when and how fast the river was cut- Clearly, the physiography of East Africa drastically changed dur- ting down into the plateau, we now know that ing the period when the first hominins evolved — from a near-flat the plateau rose more than a kilometer between forestland to a landscape with plateaus, volcanoes, mountains, deep 6 million and 3 million years ago, playing a rift-valleys, freshwater lakes and vegetation patchwork ranging from significant role in drying out of and creating closed woodland to desert grassland. But, what significance does this savanna mosaic in East Africa, which was once a change bear on hominin evolution? To address this question, we must lush tree-dominated land. first understand what defines a hominin, or for that matter, a human. Paleoanthropologists agree that at least four major characteristics separate the hominins from the hominoids: changes in dentition (the development and shape of teeth), elaboration of culture, increase in brain size and bipedalism. Not all of these traits are informative at this period of hominin evolution, however. Dental changes are often too complex to track successfully, elaboration of culture can be too subjective, and bigger brains appeared late in the game, around 2 million years ago. Thus we are left with bipedalism. Fossil evidence suggests that bipedalism developed between 7 million and 4 million years ago, which was also the time when hominins split off from the knuckle-walking African apes. Most scientists think that being able to walk upright on two legs is the defining hominin character- istic, and, perhaps, its development is the single most important event in human evolution. M. Royhan GaniM. Royhan and Nahid DS Gani

www.geotimes.org January 2008  25 Geotimes Walking Tall

At least a dozen different hypoth- eses have been proposed to explain the evolutionary reasons for developing bipedalism. Perhaps, the sim- plest and hence scientifically most reasonable explanation was offered by Peter Rodman and Henry McHenry of the University of California at Davis. They argued that as hominoid food resources became thinly dispersed due to the ever-expanding savanna, bipedalism evolved as a more energy- efficient way to travel between food resources that were becoming increas- ingly farther away from each other. And the reduction of tree cover is linked with overall aridification of East Africa related to the overall massive uplift of the Wall of Africa. Aridification particularly intensified after 6 million years ago, coinciding with the evolution of bipedalism. The landscapes where early hominins evolved were not all deserts, however. Analyses of some hominin fossil sites suggest that these areas were isolated freshwater ponds sur- rounded by open forests, a place where hominins could return after a prolonged food search. Even the anatomy of some hominin bones, for example, the gorilla-like shoulder blade and chimp-like curved fingers of the 3.3-million-year-old Dikika baby (Australopithecus afarensis), investi- gated by Zeresenay Alemseged at the Max-Planck Institute for Evolutionary Anthropology in Germany and his team, suggests that some early hominins were not fully bipedal, instead spending some part of their time still climbing trees. And interestingly, computer simulations show that a hominin could perform up to 40 per- cent of its daily activities in trees and still benefit from being bipedal. The asynchronous uplift and subsidence of the East African Rift resulted in a distinctly complex topography that was markedly variable both aeri- ally, at a length-scale ranging from one kilometer to 1,000 kilometers, and temporally, at a time-scale ranging from 100 years to a few million years. Freshwater lakes within rift valleys were repeatedly created and

26 Geotimes  January 2008 www.geotimes.org The Afar Depression is a plate tectonic triple junction where the spreading ridg- es that are forming the Red Sea and the Gulf of Aden emerge on land and meet the East African Rift. Many hominin fossils have been found in this area.

destroyed by continuous morpho-tectonic adjustments of drainage divides. These variable, tectonics-driven envi- ronmental stresses are likely responsible — by thinning out food resources in a controlled way — for forcing hominins to branch off from the hominoids and develop energy-saving upright walking. Such stresses could also be responsible, at a later stage, for hominins developing a bigger brain — as a way to cope with these extremely variable and challenging landscapes. NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER Science Team ASTER Science and U.S./Japan NASA/GSFC/METI/ERSDAC/JAROS Perhaps, this topographic complexity even provided bipedal, omnivorous hominins with a tactical advantage, enabling them to hunt and hide in a high-relief terrain, as recently argued by Geoffrey King of the Laboratoire Tectonique, Institut de Physique du Globe Paris in France and Geoff Bailey of the University of York in the United Kingdom. In the quest to better understand human origins, researchers have came a long way from the traditional research of finding and dating hominin fossils, to investigating the global climate drumbeat and finally to con- sidering the tectonic rocking of local climates. However, we are still at a preliminary stage of untangling local climatic signals from global noises. Future studies directed toward more a precise understanding of the mag- nitude and timelines of uplift and subsidence of the East African Rift, and toward modeling the topography of the Wall of Africa in changing the East African complex climate over the last 7 million years, would be certainly useful in answering the tantalizing question of how a hominoid became human over this time window.

Royhan Gani is a research assistant professor and Nahid Gani is a research scientist at the Energy and Geoscience Institute at the University of Utah in Salt Lake City.