Tectonic Hypotheses of Human Evolution

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Jan 11, 2008 - copy of Geotimes! Geotimes Poll: Do you think tectonics or climate played a larger role in human evolution in Africa? Tectonics. Climate. Both.
Geotimes - January 2008 - Tectonic Hypotheses of Human Evolution

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Tectonic Hypotheses of Human Evolution M. Royhan Gani and Nahid DS Gani Sometime around 7 million years ago, the land in East Africa began to rise in earnest, and life on Earth took a drastic turn. A creature began to evolve into a form that would eventually take over the world. Exactly how humans originated and evolved is an intrinsic intellectual question. But one thing has become fairly clear: Tectonics was ultimately responsible for the evolution of humankind. Any discussion of the evolution of life starts with Charles Darwin, and human evolution is no exception. More than a century ago, Darwin saw a link between the African arid savanna and human evolution, an idea that later became popular as the “savanna hypothesis.” However, in his original theory of natural selection, Darwin downplayed the role of the physical environment as a mechanism for evolution, stating instead that M. R natural selection could drive evolutionary changes in the absence of any change in the physical The present-day vegetation of East Africa environment or climate. uplift of the Wall of Africa between 7 milli years ago. Over the last century, the impact of geomorphic and climatic change on evolution, particularly human evolution, has gained considerable moment seen as the root cause of Earth’s changing and diverse landscapes, seascapes and their associa breakup and reassembly of the supercontinents Rodinia and Pangea can be explained simply by two superplumes (upwellings of abnormally hot rock from the core-mantle boundary) in the mantl exist today: one beneath Africa and the other beneath the Pacific. Perhaps, it is this African supe ultimately responsible for driving human evolution in East Africa. Humanity began in Africa. Genetic and fossil evidence suggest that between 33 million and 22 m hominoids, or apes — a group that includes gibbons, orangutans, African apes (chimpanzees an (ancient and modern humans) — began to evolve. Hominins and African apes (particularly chimp These two lineages split off between 7 million and 4 million years ago, as indicated by DNA and f paleoanthropologists agree that this split is the most critical juncture in human evolution, although this period is rare. Much was happening in Africa at that time, however.

Tearing Apart

The African superplume has been brewing beneath eastern Africa for at least the last 45 million y surface evidence of the rising hot plume-head is in northeast Africa’s Afar region, as seen by exte 30 million years ago. Consequent weakening of the lithosphere, Earth’s rigid outer layer, by exce mechanical stretching resulted in rifting in three directions in Ethiopia and in the eventual creation East African Rift and the Red Sea. Rifting causes subsidence that results in rift valleys, and Earth subsidence by raising rift shoulders on both sides of the valley. In East Africa, uplift of the narrow kilometers wide) were superimposed on the uplift of wide domes (hundreds of kilometers wide) lin pushing from underneath. The mode and tempo of these uplifts in East Africa were anything but s

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Over the past 30 million years, the plume has created the East African Rift, a north-south elongat with isolated volcanic peaks such as Mount Kilimanjaro. Rifting in a few places throughout East A both northward and southward, from the Afar region in northern Ethiopia down almost to South A formation of the “Wall of Africa,” which is about 6,000 kilometers long and 600 kilometers wide, a five kilometers high. This belt is the largest and longest-lived rift in the world, punctuated with freshwater lakes along t dramatic relief that runs from 156 meters below sea level — Lake Assal in the Afar Depression, th — to 5,895 meters above sea level at Kilimanjaro, the highest point in Africa. Although the Wall o around 30 million years ago, recent studies show that most of the uplift occurred between 7 millio ago, just about when hominins split off from African apes, developed bipedalism and evolved bigg

Global Climatic Shifts

Over the last 15 million years, Earth has experienced a net cooling trend superimposed by repea cycles — the so-called Milankovitch cycles related to Earth’s orbital forcing that controls the plane solar radiation, with periodicities ranging from 20,000 to 100,000 years. The great ice ages, asso climate, started only between 3 million and 2.5 million years ago. Because our early ancestors were primarily herbivores, scientists tend to focus heavily on past ve East Africa to understand early hominin evolution, as alterations in type and distribution of plants impact an ape’s food system. Using carbon isotopes, researchers can determine the type of vege region at a particular time in the geologic past, such as whether vegetation was wood-dominated Recent isotopic studies suggest that an important global shift from wood-dominated to grass-dom occurred between 8 million and 4 million years ago was likely related to an increase in aridity and that cleared out large swaths of forest. No doubt these climate and climate-driven vegetation cha time in the geologic past, were critical in hominin evolution. But were these changes that led to a aridity and consequential expansion of open grassland, a phenomenon known as East African ar controlled by global (climate) or by local (tectonic) processes?

Thinking Locally

Léo Laporte and Adrienne Zihlman of the U Santa Cruz were the pioneers in linking Ea local tectonics. In 1983, they argued that A threw the eastern part of the continent into a spread of the savanna and the end of the dominated the landscape. Many subseque essentially the same thing. So far, probably case was made by Pierre Sepulchre of the Sciences du Climat et de l’Environnement and his team in a recent atmospheric and v study. They found that reduction of East Af topography by just one kilometer would pe across the region, increasing rainfall. The c dominated landscape would thus transform dominated vegetation. If this rain shadow h uplift history of the Wall of Africa is crucial African aridification, and thus human evolu

M. Royhan Gani and Nahid DS Gani

For survival, these Ethiopian women walk daily for tens of kilometers on rough terrain to collect water and trade food. Perhaps early hominins developed bipedalism, an energysaving mechanism for traveling long distances by walking upright, for similar reasons, as their food resources were getting widely separated

Again, the East African Rift uplift occurred 30 million years, with likely the most uplift o 7 million years ago. New research on the E perhaps the most prominent part of the wa significant hominin fossil site, the Afar Rift V the plateau itself uplifted right in that time p fast the Blue Nile carved a spectacularly de plateau, a true rival of North America's Gra and her team found that the Ethiopian Plate kilometer between 6 million and 3 million ye Other studies have shown a similar trend e valley: The majority of the uplift of the Keny rising to 3.4 kilometers high and constitutin Wall of Africa, occurred between 7 million a The Tanganyika and Malawi rift-shoulders, kilometers high and comprise the western

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mainly uplifted between 5 million and 2 mil Karoo Plateau in South Africa, the southern wall, which currently rises to 3.4 kilometers elevated during the past 5 million years. Cl grew to be a prominent elevated feature over the last 7 million years, thereby playing a primary ro aridification by wringing moisture out of monsoonal air moving across the region. because of the spreading savanna related to East African aridification.

But, the uniqueness of the Wall of Africa is not so much that it is narrow and elevated, but rather variable with contrasting topography. The wall is breached in several places, such as the Turkana an average elevation of only 500 meters and sits between the Ethiopian Plateau in the north and south. Various rift-flanks and plateaus, in addition to isolated topographic features like volcanoes elevations at various times between 7 million and 2 million years ago, resulting in an overall aridif likely varied somewhat from place to place and from time to time. This view is supported by the fi of grasses in East Africa was asynchronous and spatially heterogeneous over this time window, a in favor of local tectonic controls, rather than a global change, for the expansion of grass-dominat

Hominin Cradle

Although fossils of hominoids, from which hominins evolved, are widely found in Africa and Euras greater than 3 million years old — known hominin fossil sites are located along the narrow belt of The only exceptions are the oldest (6-million- to 7-million-year-old) “hominin” bones of Sahelanthr farther west in Chad that, some argue, could be the common ancestor of humans and chimpanze hominins. Clearly, the physiography of East Africa drastically changed during the period when the first homi near-flat forestland to a landscape with plateaus, volcanoes, mountains, deep rift valleys, freshwa patchwork ranging from closed woodland to desert grassland. But, what significance does this ch evolution? To address this question, we must first understand what defines a hominin, or for that Paleoanthropologists agree that at least four major characteristics separate the hominins from the dentition (the development and shape of teeth), elaboration of culture, increase in brain size and 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 characteristic, and, perhaps, its development is the single most important event in human evolution.

Walking Tall

At least a dozen different hypotheses have been proposed to explain the evolutionary reasons for developing bipedalism. Perhaps the simplest and M. R hence scientifically most reasonable explanation was offered by Peter Rodman and Henry The Blue Nile carved a spectacular 1.6-kil McHenry of the University of California at Davis. a true rival of the Grand Canyon of North They argued that as hominoid food resources when and how fast the river cut down into became thinly dispersed due to the everknow that the plateau rose more than a ki expanding savanna, bipedalism evolved as a million and 3 million years ago. more energy-efficient way to travel between food resources that were becoming increasingly farther away from each other. And the reduction of tree cover is linked with overall aridification of East A overall massive uplift of the Wall of Africa. Aridification particularly intensified after 6 million years evolution of bipedalism. The landscapes where early hominins evolved were not all deserts, however. Analyses of some h suggest that these areas were isolated freshwater ponds surrounded by open forests, a place wh return after a prolonged food search. Even the anatomy of some hominin bones, for example, the

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blade and chimp-like curved fingers of the 3.3-million-year-old Dikika baby (Australopithecus afar Zeresenay Alemseged at the Max-Planck Institute for Evolutionary Anthropology in Germany and some early hominins were not fully bipedal, instead spending some part of their time still climbing computer simulations show that a hominin could perform up to 40 percent of its daily activities in from being bipedal. The asynchronous uplift and subsidence of the East African Rift resulted in a distinctly complex to markedly variable both aerially, at a length-scale ranging from one kilometer to 1,000 kilometers, time-scale ranging from 100 years to a few million years. Freshwater lakes within rift valleys were destroyed by continuous morpho-tectonic adjustments of drainage divides. These variable, tecton environmental stresses are likely responsible — by thinning out food resources in a controlled wa to branch off from the hominoids and develop energy-saving upright walking. Such stresses could a later stage, for hominins developing a bigger brain — as a way to cope with these extremely va landscapes. Perhaps, this topographic complexity even provided bipedal, omnivorous hominins w enabling them to hunt and hide in a high-relief terrain, as recently argued by Geoffrey King of the Institut de Physique du Globe Paris in France and Geoff Bailey of the University of York in the Un In the quest to better understand human origins, researchers have come a long way from the trad finding and dating hominin fossils, to investigating the global climate drumbeat and finally to cons rocking of local climates. However, we are still at a preliminary stage of untangling local climatic s noises. Future studies directed toward a more precise understanding of the magnitude and timeli subsidence of the East African Rift, and toward modeling the topography of the Wall of Africa to d played in changing the East African complex climate over the last 7 million years, would be certai the tantalizing question of how a hominoid became human.

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

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