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Posted: 2:42 PM- A theoretical consensus among anthropologists holds that early humans diverged from chimpanzees on the evolutionary tree 4 to 7 million years ago when they left lush forests to roam Africa's arid savannas.
University of Utah geologists have devised a tectonic-based theory that supports this idea by linking massive uplift in east Africa with local climate changes that helped jump-start early human evolution.
"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," Royhan and Nahid Gani state in an article in the current edition of Geotimes, a magazine of the American Geological Institute.
"That landscape controlled climate on a local to regional scale. That climate change spurred human ancestors to evolve from apes," said Royhan Gani, a research scientist and professor of civil and environmental engineering with the U.'s Energy and Geoscience Institute.
Most discussion on the role of climate on human evolution has focused on climate change on a global scale, possibly related to shifts in Earth's orbit. The Ganis, however, are building a more regional case to explain early human evolution, one that targets crustal movements that transformed east Africa at the very time human ancestors left wooded areas to explore their world on two feet--that is, became human.
The uplift created the 3,700-mile-long "Wall of Africa," a network of valleys and plateaus stretching from Ethiopia to South Africa, including 19,340-foot Mount Kilimanjaro. It not only changed the landscape, but altered the climate by obstructing wet storm cycles spinning off the Indian Ocean. As the landscape became drier, lush forests gave way to an arid patchwork of grasslands and woodlands.
"Our early ancestors were mostly a herbivorous, tree-dwelling animals," Royhan Gani said. "If they lose the trees, they have to go further to find food. It's more efficient to walk on two feet rather than knuckle walking, known scientifically as quadripedalism."
During their Edenic life in the jungle, hominins - the scientific term for humans, under the genus Homo, and their ancestors--enjoyed their food source hanging in branches around them. Savannah life naturally selected for bigger brains because these pre-humans had to figure out where to find nourishment and to discriminate between types of food. Hunting and gathering in arid grasslands also placed a premium on walking because an upright posture exposes less body surface to the dehydrating effects of the savannah's hot sun, Gani said.
The earliest Homo, appears in the fossil record about 2.5 million years ago, while our species Homo sapiens appears 200,000 years ago.
East Africa's topographical diversity is crucial to understanding the importance of landscape in human evolution, the Ganis stress. If the Wall of Africa created a vast desert, the pre-human hominins would not have evolved into walkers because there would be no place to walk to.
"You need patches of trees and water," Royhan Gani said. "The wall is not continuous highland. It has lowlands, grasslands, freshwater lakes, featuring a variety of microclimates."
The Ganis focused their study on the Ethiopian Plateau, a 300-by-300-mile highland at the headwaters of the Blue Nile, where they analyzed how the tributary to the world's longest river carved a spectacular landscape reminiscent of Utah's Colorado Plateau. This study helped them determine rates of uplift, according to their results published in the September 2007 edition of GSA Today, the journal of the Geological Society of America. The most rapid uplift (about 3,200 feet) began about 6 million years ago, associated with large shield volcanoes that erupted great volumes of basalt, precisely when crucial human evolution began occurring. But more study is required to conclusively credit East African uplift with appearance of bipedal walking and greater intelligence among human ancestors.
"But it all happened within the right time period. Now we need to nail it down," Gani said.
Tagline q=l --- f=zapf dingbats f=interstate-black BRIAN MAFFLY f=nimrod mt it can be contacted at bmaffly@sltrib.com" Target="_BLANK">bmaffly@sltrib.com or 801-257-8605.