Saturday, December 22, 2007
'Wall Of Africa' Allowed Humanity To Emerge
Finding: The accelerated uplift of mountains and highlands stretching from Ethiopia to South Africa blocked much ocean moisture, converting lush tropical forests into an arid patchwork of woodlands and savannah grasslands that gradually favored human ancestors who came down from the trees and started walking on two feet -- an energy-efficient way to search larger areas for food in an arid environment.
Scientists long have focused on how climate and vegetation allowed human ancestors to evolve in Africa. Now, University of Utah geologists are calling renewed attention to the idea that ground movements formed mountains and valleys, creating environments that favored the emergence of humanity.
Tectonics or the movement of Earth's crust may have been ultimately responsible for the evolution of humankind. This includes the movements of Earth's crust, its ever-shifting tectonic plates and the creation of mountains, valleys and ocean basins. It also includes the 3,700-mile-long stretch of highlands and mountains also known as "the Wall of Africa." It parallels the East African Rift valley, where many fossils of human ancestors were found.
As a topic about the influence on human evolution tectonics has been discussed since at least 1983. But much of the previous discussion of how climate affected human evolution involves global climate changes, such as those caused by cyclic changes in Earth's orbit around the sun, and not local and regional climate changes caused by East Africa's rising landscape.
However, 0ver the last 7 million years the crustal movement or tectonism in East Africa, the landscape drastically changed. That landscape controlled climate on a local to regional scale. That climate change spurred human ancestors to evolve away from the ape line.
Hominins (the new scientific word for humans (Homo) and their ancestors, including Ardipithecus, Paranthropus and Australopithecus) split from apes on the evolutionary tree roughly 7 million to 4 million years ago. The earliest undisputed hominin was Ardipithecus ramidus 4.4 million years ago. The earliest Homo arose 2.5 million years ago, and our species, Homo sapiens, almost 200,000 years ago.
A Force from within the Earth
The geological or tectonic forces shaping Africa begin deep in the Earth, where a "superplume" of hot and molten rock has swelled upward for at least the past 45 million years. This superplume and its branching smaller plumes help push apart the African and Arabian tectonic plates of Earth's crust, forming the Red Sea, Gulf of Aden and the Great Rift Valley that stretches from Syria to southern Africa.
As part of this process, Africa is being split apart along the East African Rift, a valley bounded by elevated "shoulders" a few tens of miles wide and sitting atop "domes" a few hundreds of miles wide and caused by upward bulging of the plume.
The East African Rift runs about 3,700 miles from the Ethiopian Plateau south-southwest to South Africa's Karoo Plateau. It is up to 370 miles wide and includes mountains reaching a maximum elevation of about 19,340 feet at Mount Kilimanjaro.
The rift "is characterized by volcanic peaks, plateaus, valleys and large basins and freshwater lakes," including sites where many fossils of early humans and their ancestors have been found, says Nahid Gani (pronounced nah-heed go-knee), a research scientist. There was some uplift in East Africa as early as 40 million years ago, but "most of these topographic features developed between 7 million and 2 million years ago."
A Wall Rises and New Species Evolve
The Wall of Africa started to form around 30 million years ago, recent studies show most of the uplift occurred between 7 million and 2 million years ago, just about when hominins split off from African apes, developed bipedalism and evolved bigger brains.
Nature built this wall, and then humans could evolve, walk tall and think big.
Is there any characteristic feature of the Wall that drove human evolution?
The answer is the variable landscape and vegetation resulting from uplift of the Wall of Africa, which created a topographic barrier to moisture, mostly from the Indian Ocean and dried the climate. Contrary to those who cite global climate cycles, the climate changes in East Africa were local and resulted from the uplift of different parts of the wall at different times.
The change from forests to a patchwork of woodland and open savannah did not happen everywhere in East Africa at the same time, and the changes also happened in East Africa later than elsewhere in the world.
The Rise of the Wall
Studies of the roughly 300-mile-by-300-mile Ethiopian Plateau, which is the most prominent part of the Wall of Africa indicated the plateau reached its present average elevation of 8,200 feet 25 million years ago. New analysis shows that the rates at which the Blue Nile River cut down into the Ethiopian Plateau, creating a canyon that rivals North America's Grand Canyon.
The conclusion: There were periods of low-to-moderate incision and uplift between 29 million and 10 million years ago, and again between 10 million and 6 million years ago, but the most rapid uplift of the Ethiopian Plateau (by some 3,200 vertical feet) happened 6 million to 3 million years ago.
Other research has shown the Kenyan part of the wall rose mostly between 7 million and 2 million years ago, mountains in Tanganyika and Malawi were uplifted mainly between 5 million and 2 million years ago, and the wall's southernmost end gained most of its elevation during the past 5 million years.
The Time Frame of the Wall development and Human evolution
Clearly, the Wall of Africa grew to be a prominent elevated feature over the last 7 million years, thereby playing a prominent role in East African aridification by wringing moisture out of monsoonal air moving across the region. That period coincides with evolution of human ancestors in the area.
The earliest undisputed evidence of true bipedalism (as opposed to knuckle-dragging by apes) is 4.1 million years ago in Australopithecus anamensis, but some believe the trait existed as early as 6 million to 7 million years ago.
The shaping of varied landscapes by tectonic forces -- lake basins, valleys, mountains, grasslands, woodlands could also be responsible, at a later stage, for hominins developing a bigger brain as a way to cope with these extremely variable and changing landscapes in which they had to find food and survive predators.
For now the lack of more precise timeframes makes it difficult to link specific tectonic events to the development of upright walking, bigger brains and other key steps in human evolution.