News Release

The 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. 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.

The chain of climatic and ecological changes ultimately resulted in the origin of human beings thousands of miles away, in tropical Africa, said Steven M. Stanley, a professor of paleobiology in the Johns Hopkins Department of Earth and Planetary Sciences. More arid, and slightly cooler conditions caused a dramatic shrinking of the rain forests. Human kind'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 said.

A scientific paper on the theory is being published in November in the Journal of Paleontology. Stanley conceived his theory while writing a book about the origin of human beings. The book, entitled 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 great 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. 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 conveyer cut off that source of warm water by routing it deeper and southward. "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," Stanley said.

The Arctic pond touched off the Ice Age, sending frigid air and water southward, and bringing about a dramatic change in global climate, said Stanley, who studies fossil records to trace the history of life and ecosystems.

"Once you make the Arctic cold, you send a lot of cold air and cold water back south, and the Gulf Stream gets cooled much more dramatically than it would have before," he said. Increases in snowfall and accumulation of sea ice in the polar region also caused more sunlight to be reflected, making it even colder.

The main driving force for the conveyer is its descent near the Arctic Ocean. The water sinks because it is denser than the surrounding water. One reason it is denser is that it is saltier than average seawater. The increased saltiness is directly related to the Isthmus of Panama.

As the water passes across the Atlantic toward Panama, the dry trade winds that sweep westward on either side of the equator evaporate surface waters in the ocean's conveyer belt. The evaporation causes a concentration of salt. Then, the moisture- laden trade winds pass over the narrow continental region of Central America and dump rainfall into the Pacific Ocean.

The result is an imbalance of salt concentration; the Atlantic waters are left slightly concentrated, and the larger Pacific Ocean is slightly diluted, making the Atlantic a little more saline, on average, than the Pacific.

Stanley pieced together his theory by reviewing the findings of other scientists. Important evidence is found in core samples from the sea floor on either side of the Isthmus of Panama. Single-celled animals called plankton shed their skeletons into the sea. A careful analysis of the shells in core samples reveals a ratio of oxygen isotopes clearly showing an increase of salinity in the Atlantic Ocean, compared with the Pacific Ocean, between 3 million and 4 million years ago.

Therefore, the main driving force of the conveyor belt was established at that time, he said.

Scientists also have documented changes in deep sea flora and animal life in the same time period near the mouth of the Labrador Sea, where the ocean's conveyor belt descends. Fossil records of migrating mollusks, including the type of blue mussels we eat today, indicate that the Arctic Ocean grew much colder at that time.

"Previously, it was quite warm in the Arctic," Stanley said. Instead of frozen tundra, the region was covered with something resembling evergreen forests. And the weather was much milder on the North American continent, where a subtropical climate reached all the way to present-day Virginia. The Ice Age brought much more seasonal weather and colder winters.

"We didn't have cold winters before the Ice Age in mid- latitudes," Stanley said. "Winters were much warmer before the Ice Age, and summers were not as warm as they are today."

The Ice Age hasn't actually ended. "We are simply in an interval of glacial recession," Stanley said. 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 said.