On Faculty: Gauging Panama's Influence 3 Million Years Ago Emil Venere ----------------------------------- Homewood News and Information Paleontologist Steven Stanley believes he has found an ironic twist to the human evolution story. Stanley, in a paper published this month in the Journal of Paleontology, cites evidence that the Ice Age and, eventually, the evolution of the human race were triggered when a chance movement of plates in the Earth's crust formed the Isthmus of Panama 3 million years ago. There was a time when the Arctic had no tundra; the region was covered with something resembling evergreen forests. The seasons blended together, without the extremes of today's annual weather cycle. On the North American continent, a subtropical climate reached all the way to present-day Virginia. But the Ice Age ushered in a whole new era of more seasonal weather. This rude global awakening was a product of geological coincidence--a narrow strip of Earth's crust became a barrier between the Atlantic and Pacific oceans in a crucial location, preventing the waters from mixing and eventually leading to an increase in salinity of water in the Atlantic Ocean, Stanley says. Heightened salinity, in turn, changed ocean flow patterns, preventing warm water from reaching the Arctic Ocean, which started a cascade of events that brought about the Ice Age. Meanwhile, thousands of miles away, in tropical Africa, the chain of climatic and ecological events ultimately resulted in the origin of human beings, Stanley theorizes. More arid, and slightly cooler, conditions caused a dramatic shrinking of the rain forests. Humankind's tree-dwelling ancestors, suddenly faced with a dwindling forest habitat produced by climate change, were forced to begin living on the ground, setting the stage for the evolution of larger-brained, upright-walking descendants. Stanley conceived his theory while writing a book about the origin of human beings. The book, Children of the Ice Age: How a Global Catastrophe Allowed Humans to Evolve, will be published in April 1996. His theory is centered on the observation that the present "conveyor belt" of ocean water flow that keeps the Arctic Ocean cold could not have existed without the Isthmus of Panama. The conveyor belt, a huge ribbon of moving water, circulates from the central Pacific Ocean, through the Indian Ocean, around Africa and westward across the Atlantic Ocean, where--blocked from the Pacific by the Isthmus of Panama--it joins the warm ocean current known as the Gulf Stream on a curved path northward. When the conveyor belt reaches the northernmost Atlantic Ocean, most of the warm water descends and turns back to the south, traversing the ocean at great depth, passing back around the tip of Africa to the Pacific Ocean. It then rises in the Pacific, north of the equator, looping back and retracing its path to the North Atlantic. Previous theories have failed to explain why the Arctic suddenly grew colder slightly more than 3 million years ago. Some of those theories suggest that the expansion of glaciers caused the Ice Age, but, in fact, the glaciers were the result of the Arctic freeze, not the cause, Stanley argues. He points out that the Arctic is still frigid today, even though the glaciers have shrunk considerably. Before the Isthmus of Panama and the conveyor existed, warm water from the Atlantic Ocean mixed freely with the Arctic Ocean. The conveyor cut off that source of warm water by routing it deeper and southward, Stanley says. "You suddenly rob the Arctic Ocean of its source of heat. It became what I call the Arctic pond. It just sits there at a high latitude, with the sun's rays striking at a low angle," says Stanley, a professor of paleobiology in the Department of Earth and Planetary Sciences. The Ice Age hasn't actually ended. "We are simply in an interval of glacial recession," Stanley says. The prominence of glaciers waxes and wanes over long periods of time, with changes in the Earth's rotational dynamics, such as the angle of the Earth's tilt on its axis and the shape of its orbit, he says. The Arctic pond touched off the Ice Age, sending frigid air and water southward, and bringing about a dramatic change in global climate. Increases in snowfall and accumulation of sea ice in the polar region also caused more sunlight to be reflected, making it even colder, says Stanley, who studies fossil records to trace the history of life and ecosystems. ----------------------------------------------------------------- Steven Stanley has a knack for upsetting the apple cart of scientific dogma; his Ice Age theory isn't the first time he has proposed something controversial. The paleobiologist isn't timid about wandering outside his chosen field, the evolution of mollusks. His theory explaining why humans evolved from their apelike ancestors has drawn praise as well as skepticism from the scientific community. The most popular idea is that evolution took a decisive turn toward humanity when protohumans climbed out of the trees and began living on the ground after a global cooling turned much of Africa's rain forests into savannas. But, while Stanley agrees with that overall scenario, he has proposed a bold new mechanism to further explain the evolution of humans. The infants of tree-living apes must be able to fend for themselves much faster than human infants; they must be able to cling to their acrobatic mothers and learn how to feed themselves, so their brains are nearly fully developed at birth. Stanley theorizes that when the prehuman apes, known as Australopithecus afarensis, began living on the ground, infants could take more time to develop. Like other primates, a human's brain is about 10 percent of its body weight at birth. But unlike other primates, humans retain that ratio throughout life; for a year after birth, the brain continues to develop as fast as the body does. That makes human infants helpless much longer than other primates. "Our whole development is slowed down," Stanley said. "It's as if we have a gestation period that goes on for a year after birth." Living on the ground made it possible for that extended brain development, he said. --E.V.
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